Problem: A 1.26 g gas sample occupies 651 mL at 31°C and 1.00 atm.What is the molar mass of the gas?

Molar mass:

$\boxed{\mathbf{Molar}\mathbf{ }\mathbf{Mass}\mathbf{ }\mathbf{\text{(MM)}}\mathbf{=}\frac{\mathbf{g}}{\mathbf{mol}}}$

First, we have to calculate the amount of gas in moles using the ideal gas equation.

$\boxed{\mathbf{P}\mathbf{V}\mathbf{=}\mathbf{n}\mathbf{R}\mathbf{T}}$

P = pressure, atm
V = volume, L
n = moles, mol
R = gas constant = 0.08206 (L·atm)/(mol·K)
T = temperature, K

Isolate n (number of moles of gas): 

$$\begin{gathered} \frac{\mathbf{P}\mathbf{V}}{\mathbf{R}\mathbf{T}}\mathbf{=}\frac{\mathbf{n}\cancel{\mathbf{R}\mathbf{T}}}{\cancel{\mathbf{R}\mathbf{T}}} \\ \boxed{\mathbf{n}\mathbf{=}\frac{\mathbf{P}\mathbf{V}}{\mathbf{R}\mathbf{T}}} \end{gathered}$$

Given:

$\mathbf{V}\mathbf{=}\mathbf{651}\mathbf{ }\cancel{\mathbf{mL}}\mathbf{\times }\frac{{\mathbf{10}}^{\mathbf{-}\mathbf{3}}\mathbf{ }\mathbf{L}}{\mathbf{1}\mathbf{ }\cancel{\mathbf{mL}}}$

V = 0.651 L

P = 1.00 atm
T = 31°C + 273.15 = 304.15 K
R = 0.08206 (L·atm)/(mol·K)           

Calculate for n: