Question

The rate of effusion of an unknown gas is 8.2 mL/min. Under the same conditions, the rate of effusion of helium (He) gas is 24.6 mL/min. What is the molar mass of the unknown gas

Answer 1

According to Graham's law, the ratio of rates of effusion of Helium and unknown gas can be given as follows:

Rate(He)/Rate(unknown) = [M(unknown)/M(He)]^1/2

Where, M(unknown) is the molar mass of unknown gas and M(He) is the molar mass of helium gas.

Substitute given values in above expression,

(24.6 mL/min) / (8.2 mL/min) = [M(unknown)/4.00]^1/2

3.0 = [M(unknown)/4.00 g/mol]^1/2

Squaring both sides,

9.0 = M(unknown)/(4.00 g/mol)

M(unknown) = 9.0 x 4.0g/mol

M(unknown) = 36.0g/mol

Thus, molar mass of unknown gas is 36.0 g/mol