Chemistry121 [Tyvoll]
Quiz 7 Key
June 25, 2007
Part I. Problem (10 points) (Show all work for credit!)
A recently synthesized sample of a new gaseous compound was weighed and found to have a
mass of 0.551 g. If the gas sample occupied a flask with a volume of 250 mL at 23 0C and a
pressure of 715 mmHg, what was the molar mass of the compound? Show work for credit!
First recognize that this requires PV = nRT … begin by seeing what is given:
P = (715 mmHg)(1 atm/760 mmHg) = 0.941 atm
T = 23 + 273 = 296 K
V = (250 mL)(10-3 L/1 mL) = 0.250 L
R = 0.0821 L atm/mol K
n=?
Now, substitute and calculate number of mols of gas, n:
PV = nRT @ (0.941 atm)( 0.250 L) = n(0.0821 L atm/mol K)(296 K)
n = [(0.941 atm)( 0.250 L)] / [(0.0821 L atm/mol K)(296 K)] = 9.68 x 10-3 mol gas
M = g/mol = 0.551 g / 9.68 x 10-3 mol gas = 5.69 x 101 = 56.9 g/mol
Part II. Multiple Choice (2 points each)
1. A gauge on a tank of compressed helium reads 1850 psi (lb/in2). What is this pressure in mm
Hg? (760 mmHg = 14.70 lb/in2)
1. 9.57 × 104 (mmHg = (1850 lb/in2)(760 mmHg/14.70 lb/in2) = 9.57 × 104 lb/in2)
2. Respiratory therapists use the unit "cm of H2O" to measure small differences in pressure. If
the density of mercury is 13.6 g/cm3 and the density of water is 1.00 g/cm3, how many cm of
H2O is equal to a pressure of 100 mmHg?
3. 1360 cm H2O (cm H2O = (100 mmHg)(13.6 g/cm3/1.00 g/cm3) )
3. A balloon of helium on a mountain where the pressure is 695 mmHg occupied a volume of
1.75 L. What would the volume of this balloon be (in L) if it were taken to sea level (760
mmHg)?
2. 1.60 L (695 mmHg)(1.75 L) = (760 mmHg)V2 )
4. Convert 129°C to K. ( K = 0C + 273 = 129 + 273 = 402 K)
2. 402 K
5. Consider four identical 1.0-L flasks each containing one of the following gases at 0°C and 1
atm pressure. Which sample has the most molecules? (note that conditions are STP)
5. each flask has the same number of molecules