A materials scientist has created an alloy containing aluminum, copper, and zinc

Question

A materials scientist has created an alloy containing aluminum, copper, and zinc, and wants to determine the percent composition of the alloy. The scientist takes a 12.564 g sample of the alloy and reacts it with concentrated HCl. The reaction converts all of the aluminum and zinc in the alloy to aluminum chloride and zinc chloride in addition to producing hydrogen gas. The copper does not react with the HCI. Upor completion of the reaction, a total of 10.1 L of hydrogen gas was collected at a pressure of 727 torr and a temperature of 27.0 . Additionally, 2.837 g of unreacted copper is recovered. Calculate the mass of hydrogen gas formed from the reaction Number Calculate the mass of aluminum in the alloy sample. Number

Explanation / Answer

Volume of H2 gas V = 10.1 L

Pressure P = 727 mmHg x 1atm/760mmHg = 0.956 atm

Temperature T = 27 + 273 = 300K

Moles of H2 gas n = PV/RT

=( 0.956 x 10.1)/ (0.0821 x 300)

= 0.3922 moles

Mass of H2 gas = moles x molecular weight

= 0.3922 moles x 2.016g/mol

= 0.7908 g

The balanced chemical reaction

x Al + y Zn + z Cu + (3x + 2y) HCl ---> x AlCl3 + y ZnCl2 + z Cu(s) + (1.5x + y) H2

Mass of Cl in HCl = 0.7908 g H2 x (35.45g Cl / 1.008g H)

= 27.811 g

Chlorine balance from the stoichiometry of the reaction

(3x + 2y) * 35.45 = 27.811

3x + 2y = 0.7845

Mass of alloy = 12.564 g

Cu recovered = 2.837 g

Mass of Al + mass of zinc = Mass of alloy - Cu recovered

= 12.564 - 2.837 = 9.727 g

Now balance Al and Zn simultaneously

x * (26.98g / mol) + y * (65.38g / mol) = 9.727 g

3x + 2y = 0.7845

Solve the equations, we get

x = 0.224

y = 0.056

Mass of Al = moles x molecular weight

= 0.224 mol x (26.98g / mol)

= 6.04352 g

Mass of Zn = 0.056 mol x(65.38g / mol)

= 3.66128 g

mass of Cu = 2.837 g

% Cu = mass of Cu*100/total mass

= 2.837*100/12.564 = 22.58%

% Al = 6.04352*100/12.564 = 48.20%

% Zn = 3.66128*100/12.564 = 29.14%

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