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Sample Problems
1. How many grams of ethanol would occupy 750 mL at 790.5 torr pressure at 100°C? (molar mass of ethanol is 46.0 g mol^(-1).
Answer:
Solution:
M = mRT/PV
= (3.2845g)( 0.08206 L atm mol^(-1) K^(-1) )(373 K)
(0.987 atm)(0.815 L)
0.08206 L atm / mol K
Given:
R = 0.8206 L atm mol-1 K-1
P = (790.5 torr)(1atm/760 torr) = 1.04 atm
V = (750mL)(1L/1000mL) = 0.750 L
T = (100+273) = 373 K
2. The molar mass of a liquid is determined by the Dumas method. The volume of the flask used was 815 mL. The mass of the volume before the experiment was 45.4642 g. After the experiment, the mass was found to be 48.7487 g. The atmospheric pressure at the time was 750.3 torr. What is the molar mass of the liquid? (A boiling water bath was used to vaporize the liquid at 100 C.)
Prepared By:
Carreon, Nico P.
Plaza, Sally B.
Punzal, Ma. Christine P.
Tuba Ang, Cathleen
Vidal, Patricia Kaye M.
CH31FA1
-French chemist who pioneered in organic chemistry, particularly organic analysis.
-Dumas greatly improved the method of determining the vapour density of substances (and thus their relative molecular masses), and he developed a combustion method for determining nitrogen in organic compounds.
Dumas Method
-A method to determine the molar mass of a compound.
-In this approach, the sample is added to a small flask, the flask is heated and as the sample evaporates, the air is swept out of the container. Then flask is cooled again, and the mass of liquid which condenses must be equal to the mass of vapor that filled the flask in the previous step.
Derivation:
The combined gas law is given by the equation;
P V = n R T
where:
P = Pressure
V = Volume
n = number of moles
R = gas law constant
T = Temperature
n = m
M
where:
m = mass of sample
M = molar mass (substance)