Chemistry
10th Edition
ISBN: 9781305957404
Author: Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher: Cengage Learning
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Calculate the molar mass of the unknown solid using equations 2.2 through 2.5. Show all your work.
I don’t know how to find the value for Kf either.
Please help
Transcribed Image Text:(mm)x (kg solvent) = moles solute
(2.4)
mass solute
moles solute
= molecular mass of solute
(2.5)
In this experiment, you will determine the molar mass of the unknown by
measuring the freezing-point depression observed in a solution in which the solvent is
lauric acid.
Procedure and Collection of Data
You will watch a You Tube video in which an experiment briefly described in the
introduction is carried out. The video can be found by clicking on the following link:
https://www.youtube.com/watch?v=10DrYceYwzA
Record the data in the table below. Note that the temperature at the end of each
experiment is the freezing point of the substance. There is no need to consider all of
the time and temperature data that was collected along the way.
Data Table:
Mass of lauric acid (g)
Mass of residual lauric acid in weighboat (g)
5.0624 q
43.98°
0.74116q
0.00714
0.7389
Mass of lauric acid in test tube (g)
190 tuld Freezing point of lauric acid (°C)
villsio
Mass of unknown (g)
Mass of residual unknown in weighboat (g)
Mass of unknown in test tube (g)
Freezing point of mixture (°C)
38.12°c
Data Analysis
Calculate the molar mass of the unknown solid using equations 2.2 through 2.5. Show
all of your work.
Transcribed Image Text:General Chemistry II
CHM 122
Laboratory 2
Determination of Molar Mass by Freezing-Point Depression
Purpose
The purpose of this experiment is to determine the molar mass of an unknown
compound by measuring the freezing-point depression observed in a solution of the
compound.
Introduction c
The vapor pressure of a liquid is determined by the ability of solvent molecules to
move from the liquid phase to the gaseous phase. In a solution, however, some of the
spaces at the surface of the solution are occupied by the solute particles. As a result,
fewer solvent particles are in positions from which they can move into another phase,
and therefore the solution's vapor pressure is lower than that of the pure solvent. This
means that the solution will have to be heated to a higher temperature before its vapor
pressure reaches that of the atmosphere, therefore resulting in an elevated boiling
point. Similarly, since the solution has a lower vapor pressure, the solution will have to
be cooled to a lower temperature for the solution to freeze.
The extent of freezing-point depression is proportional to the amount of solute
present in a given mass of solvent. The concentration unit employed is molality,
symbolized by a lowercase m. Molality is defined as the number of moles of solute per
kilogram of solvent.
moles solute
(2.1)
m =
kg solvent
The difference between the freezing point of the solution and that of the pure
We can
solvent is called the freezing point depression and is given the symbol Atr.
express the relationship between the difference in freezing point and the molality of the
solution in Equation 2.2:
At, = k, x m
(2.2)
where kf is the freezing-point depression constant in units of °C-kg/mol.
Equation 2.3 shows that by measuring the freezing-point depression of a
solution, you can determine the solution's molality. Knowing the experimentally
determined molality and knowing how the solution was prepared (i.e., what mass of
solute was combined with what mass of solvent), you can obtain the molar mass of
solute according to the following equations.
At
(2.3)
mexpil
k,
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