How to Calculate Molecular Weight of an Unknown Gas at STP

03 Aug 2021

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

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:

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

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.