gas density and dalton's law

1. Calculate the density of bromine gas at 50.0 °C and 785.0 torr.

PV=nRT
Consider 1 liter sample
(785/760)(1) = n(0.08206)(273+50)
n = 0.03897

Mass of 0.03897 mole bromine
= (0.03897)(79.9x2)
6.227 g

Density = 6.227 g / liter

2. Mustard gas, C4H8SCl2, was used in World War I. It is very toxic and can cause blisters all over the body. Calculate its density at STP and predict what precautions people can take if exposed to this gas. (Hint: compare its density to air.)

Molar volume at STP = 22.4 liter
Molar mass of C4H8SCl2 = 12x4 + 1x8 + 32 + 35.5x2 = 159
Density of the gas = 159/22.4 = 7.098 g / liter

Density of the gas is higher than that of air. It will sink in air. In order to avoid contact with the gas, people can run upstair in a building or run up hill.

3. If a gas has density of 0.761 g/L at STP, what is the molar mass of gas?

PV = nRT
Let No. of mole of 1 liter gas at STP be n
(1)(1) = n(0.08206)(273)
n = 0.0446

Molar mass = 0.76/0.0446 = 17.0 g / mole

4. A chemist analyzed a gas and found it contained 80.0% C and 20.0% H. He also observed that 500 ml of the gas at 760 torr and 0 °C weighed 0.6695 g. What is the empirical formula of the gaseous compound? What is its molecular mass and molecular formula?

CarbonHydrogen
%8020
mass8020
no. of mole80/1220/1
6.66720
ratio6.667/6.66720/6.667
12.9999
13

empirical formula CH₃
empirical formula mass = 12+3=15

From
500 ml of the gas at 760 torr and 0 °C weighed 0.6695 g.

PV=nRT

(760/760)(500/1000) = (0.6695/Mr)(0.08206)(273)
where Mr is the molar mass of the gas.
Mr = 30.00

Molar mass is twice empirical formula mass
Molecular formula is C₂H₆。


2013-10-22 11:14:01 補充：
......................Carbon.......Hydrogen
%...................80...............20
mass............80...............20
no. of mole..80/12..........20/1
......................6.667..........20
ratio..............6.667/6.667.....20/6.667
......................1.............3