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[Music]
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hello my name is Jose Rosales and for
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today's experiment we're gonna going to
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be measuring measuring the molecular
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weights of an unknown by its freezing
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point depression so first we have to
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define what a solution consists of so a
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solution consists of two things it
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consists of the solute and a solvent so
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an example of a solute in a solvent is
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basically salt and water
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so sometimes solutions can have
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different properties one of the two
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properties is for example freezing point
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depression another property is the
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boiling point elevation
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today we're going going to be measuring
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the freezing point depression of a
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solute to solve for its molecular
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weights and taking into account that
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we're going to be freezing as a
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particular type of compound which is
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water and we have to note that the
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freezing temperature of water is when
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the solid and the liquid are basically
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are in equilibrium with each other so
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coming from this from the freezing point
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of water now you actually you guys are
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actually gonna make a graph of how water
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is affected through by basically
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decreasing the temperature over a
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certain amount of time so in this graph
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it shows the decrease in temperature
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over time and you notice that there is a
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dip in the graph which basically
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corresponds to its super cooling effect
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super cooling effect is basically
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tholian water and it depicts that as you
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decrease the temperature of your
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solution and you go beyond or under it's
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freezing points basically still stays as
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a liquid however once it starts to
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crystallize or return it to a solid
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that's when it reaches it's freezing
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points notice that the freezing point of
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water is exactly zero degrees Celsius as
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demonstrated on the chart
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the next step is to basically first
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solve for the freezing point depression
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of your solution so the way to calculate
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for that is that first we have to find
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the freezing point of pure solvent water
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so the way we do that is that uh we're
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going to be doing an experiment where we
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freeze water up into a points and then
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we're going to measure it as freezing
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points and then to get the freezing
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point depression of the actual solution
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we have to subtract that value minus the
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freezing point of the actual solution so
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after we solve for the freezing point
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depression we will basically plug it
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into another equation that basically
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creates or solves for the freezing point
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depression so freezing point depression
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is equal to the molality times the
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freezing point depression constant of
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water times the vant Hoff factor or I
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solutes so in order for us to solve for
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this we have to define each value that
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is inside the equation so to define what
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molality is first we have to consider
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what molality is so basically molality
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is a unit of measurement where it
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includes the moles of the solid solute
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and the kilograms of the solvents or you
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would have to do to solve for molarity
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is divide the values of the moles of the
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solute by the kilograms of the solvents
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for the freezing point depression
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constant of water it is it is a constant
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and the value of it is one point eight
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five three degrees Celsius per Mille L
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so now next we had to define the vant
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Hoff factor which is I so the vant Hoff
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factor or I is basically equal to the
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number of ions of found in your solution
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say for example you dissolve sodium
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chloride in water so sodium chloride
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with dissolve in water and produce two
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ions one ion of sodium and another ion
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of chloride so basically the vant Hoff
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factor there would be equal to two
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next if we were to put another salt in
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there say for example calcium chloride
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that would produce three ions in
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solution because remember that calcium
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chloride dissolves into calcium ions and
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two chloride ions calcium has a having a
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positive two charge and chloride having
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a minus one charge so therefore you
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would have to have two chloride ions to
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balance out the equation so therefore
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you the vant Hoff factor of calcium
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chloride would be three in this
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experiment we're going to be measuring
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the vant Hoff factor or the freezing
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point suppression of us of a non
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ionizable salt solutes which is for
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example glucose since glucose dissolves
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in water but does not produce ions the
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vant Hoff factor that will be equal to
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what and in the midst of all that once
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you solve for each one of the components
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of the equation you have to rearrange
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the equation to basically solve for
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molarity first because to measure or to
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figure out or to determine the molecular
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weight of an unknown compound you will
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first have to figure out how many moles
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are in solution and then figure out how
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many grams of the solvent that you have
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so basically we go back to the finding
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well molality is remember that molality
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is the moles of the solute divided by
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the kilograms of the solvent so if we
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rearrange the equation we can basically
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define moles of the solute as to be
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equal to the molarity times the
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kilograms of the solvent once we figure
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out the moles of the solutes
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we can then plug it in to the molecular
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weights equation so before we start the
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experiments make sure to wear your
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safety equipment at all times
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for example safety gloves a lab coat
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safety glasses and remember to always
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wear closed toed shoes and cover up your
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ankles wearing long pants and for people
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who have long hair
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please remember to tie your hair back
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so these are the materials that we're
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going to use for today's experiment
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we're going to be using the micro lab
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hardware we're also going to be using
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the thermistor to measure the
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temperature we're going to be using the
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Styrofoam cup to conceal the temperature
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inside with the ice and water we're also
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gonna be using these propylene tubes to
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measure the temperature of the water
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inside the test tube to see the change
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in temperature also gonna be using the
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unknown which you have it always caps
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before you use it we're also going to be
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using the salts to keep the freezing
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points of the solution below zero so we
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can measure it afterwards to create the
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super cooling graph so to start the
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experiments of first we have to get a
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batch of ice with water so after we get
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the batch of ice with water we're going
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to dump it into this Styrofoam cup after
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adding the ice to the Styrofoam cup
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we're going to be adding salts
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and remember the salt is to keep the
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freezing points of water or to reach the
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freezing point of water and below it as
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well so make sure to mix the solution
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very well so once you've done your salts
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of an ice water solution you would have
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to first fill a test tube of up to 12 mo
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litres of water and that's to put inside
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your Styrofoam cup because after that
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we're going to be measuring the
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temperature change of the water inside
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them too and how it drops according to
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how much ice and water with the salt was
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in it so basically we're gonna be doing
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that and then afterwards to basically bc
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or it gets the molecular weights of your
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known first you have to measure how much
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of your known you placed inside your
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test tube before before hands you have
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to get a weighing scale look at the
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Wayne Skylar basically have to tear the
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weighing scale with the test tube inside
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and then afterwards you would have to
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place about 0.5 grams or 500 milligrams
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unknown solid
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we'll add you place it inside this tomb
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and then you measure approximately 500
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milligrams and now you're ready for the
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next step of experiments so this part of
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experiments is going to consist of
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setting up here of micro lab software
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system and as well as measuring that
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cool experiment itself so first off in
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the computer you have to open up the
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micro level icon after up you the open
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up the micro lab icon you have to click
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on micro lab experiments so after that
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of this graph shows two data sources so
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basically you have to add two sensors in
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order to depict the transition of
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lowering the temperature over time
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basically first you have to input your
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thermistor in the micro lab components
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so basically you can choose any ports
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but in my preference put it in part a
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because with the thermistor you're going
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to be measuring the change in
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temperature inside the little test tube
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there so after you placed your
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thermistor in the apparatus you would
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have to add that sensor to the breath so
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you click on add sensor on the left-hand
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side and then this window pops up so
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first you have to choose your sensor so
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remember that we're measuring
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temperatures so are you gonna click on
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is the thermistor and then you're going
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to choose an input since I put it in
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slot a I would select a and then uh
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reports or reports the value of the
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temperature and that slots
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and then you're gonna click on use
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factory calibration because these
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machines already come calibrated so once
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we already put in the thermistor sensor
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or the temperature sensor the next thing
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we have to add is the time sensitive so
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click on add sensor one more time and
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then this time you're going to use a
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different sensor time center and then it
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shows these three little slots you don't
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have to have put anything there all you
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all you would have to do is just click
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on one of them perfectly it was one and
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then you're going to set and options so
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in here you can do an automatic
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operation or a program control with the
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keyboard or we're just going to use the
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automatic operation and then the units
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or the timer is going to be set in
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seconds so then you click on finish and
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now you have your temperature sensor as
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well as your time sensor so after that
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happy after you added the sensors you
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would have to click and drag to the
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chart so they can display the values a
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record the values as time goes on so
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this is the chart that's going to be
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demonstrating the change of temperature
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over time and as well you can click and
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drag the sensors down to the lower lower
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left-hand side of the screen so you can
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report the numerical values of each one
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of them
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now we have to put in the thermistor
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inside the test tube so after you place
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the thermistor inside has to you would
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quickly place on start so that it can
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start reporting the values for time so
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notice that that is our speed decreasing
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over time but you can actually shape the
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swirl the ice water solution so that it
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can go down
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remember--the so as you can see
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throughout time it's recording the
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values of the temperature over certain
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amount of time so after you place the
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pope inside the test tube and it started
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to measure the temperature decrease over
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time you're basically going to get a
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breath like this
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Jovian so notice that there is in fact
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the dip in the graph which is really
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demonstrates two super cooling effect of
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water and then it starts to rise once it
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crystallizes in solution so that's where
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you can determine that the freezing
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point is in fact zero degree Celsius
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because if you go across to that toe you
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see in fact that it is two degrees
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Celsius which is the freezing point of
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water so for the next step of
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experiments you're going to do the same
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procedure as we did with the pure
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solvents now the solute that you
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introduce it to this test tube so in the
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situs test tube a at about 12
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milliliters of water and I shook it to
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fully dissolve so that we can go on and
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measure the same way as we did with the
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pure solvents but this is unknown
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so once you have gathered all the data
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from your experiments by gathering the
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data from the freezing point of water as
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well as the freezing points of your
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unknown you're going to subtract the
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values to figure out the freezing point
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depression of your compound once you
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have a preset for depression you can
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plug it into the formula we saw earlier
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this software the molality of the
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unknown solutes once you have your
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molality we're going to solve for moles
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and then from there you're going to
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divide the mass that you put in the in
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the flask in v2 and then divide that
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through the moles that you get from the
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molality of the inversion and that
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concludes this experiment thank you
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[Music]
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[Music]
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