Chem 3411 Solution Set 5 - Mindanao State University - Iligan Institute Of Technology - 2010

CHEM 3411, Fall 2010
Solution Set 5
In this solution set, an underline is used to show the last significant digit of numbers. For instance in
x = 2.51693
the 2,5,1, and 6 are all significant. Digits to the right of the underlined digit, the 9 & 3 in the example, are not
significant and would be rounded off at the end of calculations. Carrying these extra digits for intermediate values in
calculations reduces rounding errors and ensures we get the same answer regardless of the order of arithmetic steps.
Numbers without underlines (including final answers) are shown with the proper number of sig figs.
1
Exercise 5.5b pg 201
Given
The vapour pressure of 2-propanol (M
= 60.1 g/ mol) is 50.00 kPa at 338.8
C, but it fell to 49.62 kPa when
w,p
m
= 8.69 g of an involatile organic compound was dissolved in m
= 250 g of 2-propanol.
u
p
In terms of given variables, this is written:
M
= 60.1 g/ mol
w,p
T = 338.8
C
p
= 50.0 kPa
Pressure of pure 2-propanol solvent
p
p
= 49.62 kPa
Pressure of solution containing unknown
p
m
= 8.69 g
u
m
= 250 g
p
Find
Calculate the molar mass M
of this unkown compound.
w,u
Strategy
We’ll start with Raoult’s Law (eq 5.21 on pg 164) which relates the partial pressure p
of a liquid in a solution to its
a
pure vapour pressure p
by its mole fraction in solution χ
.
a
a
p
= χ
p
a
a
a
This allows us to calculate the mole fraction of 2-propanol χ
(in the solution with the dissolved unknown) to the
p
solutions vapour pressure p
and the vapour pressure of pure 2-propanol p
.
p
p
χ
= p
/p
p
p
p
1

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