Chemistry: Alloy Analysis

Question

A sample of an alloy containing only zinc and aluminum had a mass of 0.5937 g. When this sample was reacted with excess hydrochloric acid, 315.23 mL of hydrogen gas, collected over water at a barometric pressure of 757.1 torr and a temperature of 21.3 degrees C, was generated.

a.) determine the number of moles of hydrogen gas generated.

b.) Determine the percentage of aluminum in the alloy.

Answer 1

a) First, the vapor pressure of water at 21 °C is 18.7 torr (see source below), so the partial pressure of H2 gas collected over the water is 757.1 - 18.7 = 738.4 torr. The number of moles of H2 is then n = PV/RT:

n = (738.4 torr) (0.31523 L) / (62.3637 L*torr/K*mol) (273.15 + 21.3 K)

n = 0.0127 moles of H2 generated.

b) First, calculate the number of moles of H2 expected if the sample were pure Al and pure Zn; then extrapolate between the two.

2 Al + 6 HCl → 2 AlCl3 + 3 H2

(3/2) (0.5937 g) / (26.98 g/mole) = 0.0335 moles of H2 expected from pure Al

Zn + 2 HCl → ZnCl2 + H2

(1/1) (0.5937 g) / (65.38 g/mole) = 0.00908 moles of H2 expected from pure Zn

Since 0.0127 moles of H2 were actually obtained, the fraction of Al in the alloy is:

(0.0127 - 0.00908) / (0.0335 - 0.00908) = 0.1472 or 14.72% Al.