A student investigated the diffusion of ammonia gas, NH3, and hydrogen chloride gas, HCl. The damp red litmus paper for NH3 changed colour after 30 seconds. How long does it take for the damp blue litmus paper to change colour for HCl?

43.96secs

Explanation:

M1 = molar mass of NH3 = 14 + (3x1) = 14 + 3 = 17g/mol

t1 = time for NH3 to diffuse = 30secs

M2 = molar mass of HCl = 1 + 35.5 = 36.5g/mol

t2 = time for HCl to diffuse = ?

From Graham's law of diffusion:

t2/t1 = √ (M2/M1)

t2/30 = √ (36.5/17)

t2/30 = 1.465

Cross multiply

t2 = 30 x 1.465

t2 = 43.96secs

Therefore it will take 43.96secs for the damp blue litmus paper to change colour for HCl.