Page 255 - Schaum's Outline of Theory and Problems of Applied Physics

240 KINETIC THEORY OF MATTER [CHAP. 20

The molecular mass of UF 6 is
1(U) = (1)(238.03) u = 238.03 u
6(F) = (6)(19.00) u = 114.00 u
352.03 u = 352.03 g/mol
so the mass of UF 6 is

Mass of UF 6 = (moles of UF 6 )(molecular mass of UF 6 ) = (2.52 mol)(352.03 g/mol) = 887 g

SOLVED PROBLEM 20.15

Find the density in grams per liter of ethylene (C 2 H 4 ) at STP.
At STP 1 mol of any gas occupies 22.4 L. The molecular mass of C 2 H 4 is

2(C) = (2)(12.01) u = 24.02 u
4(H) = (4)(1.008) u = 4.03 u
28.05 u = 28.05 g/mol

One mole of C 2 H 4 therefore has a density at STP of
m 28.05 g
d = = = 1.25 g/L
V 22.4L
SOLVED PROBLEM 20.16
What is the density of oxygen at 20 C and 5 atm of pressure?
◦
It is simplest here to use the ideal gas law to find the mass of1LofO 2 under the specified conditions. The
◦
number of moles in1LofO 2 at T = 20 C = 293 K and p = 5 atm is, from pV = nRT ,
pV (5 atm)(1L)
n = = = 0.208 mol
RT [0.0821 atm·L/(mol·K)](293 K)
Since the molecular mass of O 2 is (2)(16.00) u = 32.00 u = 32.00 g/mol, the mass here is

Mass of O 2 = (moles of O 2 )(molecular mass of O 2 ) = (0.208 mol)(32.00 g/mol) = 6.66 g
Hence the density of the gas is

m 6.66 g
g = = = 6.66 g/L
V 1L
3
3
3
There are 10 grams per kilogram and 10 liters per cubic meter, which means that the density in SI units is 6.66 kg/m .
SOLVED PROBLEM 20.17
A sample of an unknown gas has a mass of 28.1 g and occupies 4.8 L at STP. What is its molecular mass?
Since 1 mol of any gas occupies 22.4 L at STP, this sample must consist of

4.8L
= 0.214 mol
22.4 L/mol
Hence
mass of sample 28.1g
Molecular mass = = = 131 g/mol = 131 u
moles of sample 0.214 mol