Chemistry $ gas laws?

Mass of H₂ = 20.94 - 20.7 g = 0.24 g
Molar mass of H₂ = (1.0 × 2) g/mol = 2.0 g/mol
No. of moles of H₂ = (0.24 g) / (2.0 g/mol) = 0.12 mol

Under identical conditions (i.e. STP), equal no. of moles of gas occupies equal volumes (i.e. the volume of the flask).
No. of moles of the unknown gas = No. of moles of H₂ = 0.12 mol

Mass of the unknown gas = 29.32 - 20.7 g = 8.62 g
Molar mass of the unknown gas = (8.62 g) / (0.12 mol) = 71.8 g/mol

At standard temperature and pressure, the volume of one mole of a gas is 22.4 liters. To determine mass of hydrogen in the flask, subtract the two masses.

Mass H2 = 20.94 – 20.7 = 0.24 grams
The mass of one mole of hydrogen is 2 grams. Let’s determine the number of moles of hydrogen in the flask.

n = 0.24 ÷ 2 = 0.12 mole
To determine the volume of flask, multiply the number of moles by 22.4.
V = 0.12 * 22.4 = 2.688 liters

To determine the mass of the unknown gas, subtract the mass of the flask from the mass of flask and unknown gas.

Mass = 29.32 – 20.7 = 8.62 grams

This is mass of 2.688 liters of the unknown gas. Let’s use the following equation to determine mass of 22.4 liters of this gas.

8.62 ÷ 2.688 = M ÷ 22.4
M = 193.08 ÷ 2.688

This mass of one mole of this gas is approximately 71.83 grams. I hope this helps you to understand how to solve this type of problem.

If you start with the ideal gas law
.. PV = nRT

for 2 gases
.. P1V1 = n1RT1
.. P2V2 = n2RT2

dividing and rearranging a bit
.. P1V1 / (n1T1) = P2V2 / (n2T2)... . <---- remember this !!!

now we have 2 gases.. H2 and the unknown gas
and we know that molar mass = mass / mole ---> n = mass / mw
where "mw" is that molar mass or molecular weight or whatever else you want to call it
so
.. P1V1 / ((mass1 / mw1) * T1) = P2V2 / ((mass2 / mw2) * T2)

rearranging
.. P1*V1*mw1 / (mass1*T1) = P2*V2*mw2 / (mass2*T2)
.. mw2 = mw1 * (P1/P2) * (V1/V2) * (mass2 / mass1) * (T2 / T1)

and because P1 and T1 weren't mentioned, the only way to solve this is to assume
.. P1=P2
.. T1=T2
.. V1=V2.... the container size was the same
so that all those terms drop out leaving
.. mw2 = mw1 * (mass2 / mass1)

solving
.. mw2 = 2.016g/mol * ((29.32g - 20.7g) / (20.94g - 20.7g)) = 72.4 g/mol

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not sure why the other folks here are using 2 g/mol for the molar mass of H2 then reporting the result to 3 or 4 sig figs. Perhaps they would like to explain