Calculate the molar mass of a gas that has a density of 1.250 g/L at STP.

Question

Quizplus · Accepted Answer

At STP (Standard Temperature and Pressure), the temperature is 273.15 K and the pressure is 1 atm. The molar volume of an ideal gas at these conditions is 22.4 L/mol. 
Using the ideal gas law: PV = nRT
We can rearrange it to solve for the molar mass (M):
M = (mRT/PV) 
Where: 
m = mass of gas (in grams) 
R = universal gas constant = 0.0821 L·atm/mol·K 
P = pressure = 1 atm 
V = volume = 22.4 L/mol 
T = temperature = 273.15 K

Since the density of the gas is given instead of its mass, we can use the density formula to find the mass:
density = mass/volume 
Mass = density x volume = 1.250 g/L x 22.4 L/mol = 28.00 g/mol

Plugging these values into the equation for molar mass: 
M = (28.00 g/mol x 0.0821 L·atm/mol·K x 273.15 K)/(1 atm x 22.4 L/mol) 
M = 28.02 g/mol

Therefore, the correct answer is C.

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