Question

A 200.0mL flask of unknown gas was placed in a hot water bath. The temperature of the bath was found to be 99 degrees C. The pressure of the flask was 733mmHg. The flask with the unknown gas in it had a mass of 126.593g. The same flask weighed 125.623g when it was empty. Determine the mass f the unknown gas in the flask. Determine the Molar Mass of an unknown gas. (Assume that the Ideal Gas Law is obeyed and use R= 0.0821 L atm/mol K SHOW ALL WORK.)

Answer 1

a. Mass of the unknown gas.

The mass of the unknown gas is calculated by subtracting the weight of the empty flask from the mass when there was air in it.

m = 126.593 g - 125.623 g

Simplifying,
m = 0.97 grams

b. Molar mass of the gas
The molar mass of the gas is calculated by the ideal gas law.

PV = (m/M)RT

where P is pressure,V is volume, m is the mass, M is the molar mass, R is the universal gas constant, and T is temperature. Substituting the known values to the equation,

(733 mmHg/760 mmHg/atm)(0.20 L) = (0.97 g/M)(0.0821 L.atm/molK)(99 + 273.15 K)

The value of M in the equation is 153.64 g/mol.

Answer: 153.64 g/mol