Question

A sample of oxygen gas was found to effuse at a rate equal to two times that of an unknown gas. what is the molar mass (in g/mol) of the unknown gas?

Answer 1

128 g/mol Using Graham's law of effusion we have the formula: r1/r2 = sqrt(m2/m1) where r1 = rate of effusion for gas 1 r2 = rate of effusion for gas 2 m1 = molar mass of gas 1 m2 = molar mass of gas 2 Since the atomic weight of oxygen is 15.999, the molar mass for O2 = 2 * 15.999 = 31.998 Now let's subsitute the known values into Graham's equation and solve for m2. r1/r2 = sqrt(m2/m1) 2/1 = sqrt(m2/31.998) 4/1 = m2/31.998 127.992 = m2 So the molar mass of the unknown gas is 127.992 g/mol. Rounding to 3 significant figures gives 128 g/mol