How to Calculate Molecular Weight of an Unknown Gas at STP
What is the molecular weight of the unknown gas?
A 1.20-L container contains 1.10 g of an unknown gas at STP.
Which of the following is the correct molecular weight of the unknown gas?
A) 1.32 g/mole
B) 0.917 g/mole
C) 22.4 g/mole
D) 20.5 g/mole
Answer
The correct molecular weight of the unknown gas is 20.5 g/mol.
To calculate the molecular weight of the unknown gas, we can follow these steps:
Step 1: Calculate Density
The formula for density is: density = mass / volume
Given data:
Volume of gas = 1.20 L
Mass of gas = 1.10 g
Calculate density: 1.10 g / 1.20 L = 0.92 g/L
Step 2: Calculate Molar Mass
Use the formula for density of gas: density = PM/RT
Given: density = 0.92 g/L, pressure (P) = 1 atm, gas constant (R) = 0.0821 atm.L/mol.K, temperature (T) = 273.15 K
Calculate molar mass: 0.92 g/L = 1 atm × M / 0.0821 atm.L/mol.K × 273.15 K
M = 0.92 g/L × 0.0821 atm.L/mol.K × 273.15 K / 1 atm
M = 20.5 g/mol
Therefore, the molecular weight of the unknown gas is 20.5 g/mol.