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 11.470 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 HCl. Upon completion of the reaction, a total of 9.77 L of hydrogen gas was collected at a pressure of 724 torr and a temperature of 27.0 °C. Additionally, 2.169 g of unreacted copper is recovered. Calculate the mass of hydrogen gas formed from the reaction.

- Calculate the mass of aluminum in the alloy sample

- what is the mass percent composition of the alloy (percentage of the Cu, Al and Zn)

Answer 1

Total mass of the sample= 11.47 gm unreacted copper= 2.169 gm

Mass of aluminium and zinc = 11.47- 2.169=9.301 gm

let x= mass of aluminium, therefore mass of zinc= 9.301-x

Moles of aluminium = mass/atomic weight= x/27   and moles of zinc = (9.301-x)/65

The reaction of Al and HCl gives

2Al+ 6HCl ----> 2AlCl3 + 3H2  

2 moles of aluminium gives 3 moles of H2

0.037x moles of aluminium gives 3*0.037x/2 moles of Hydrogen = 0.0555x moles of hydrogen

Zn+2HCl ---> ZnCl2 +H2

1mole of Zinc produces 1 mole of hydrogen

(9.301-x)/65 moles of zinc produces 0.0154*(9.301-x) moles of Hydrogen

total moles of hydrogen = 0.0555x+0.143- 0.0154x =0.143+0.0401x

Volume of hydrogen colleted= 9.77L and pressure =724 Torr =724/760 atm=0.953 atm and T= 27 deg.c =27+273.15= 300.15 K and R=0.08206 L.atm/mole.K

n= number of moles of hydrogen= PV/RT= 0.953*9.7/ (0.08206*300.15)=0.375 moles

Mass of hydrogen =0.375*2= 0.75 gms

but 0.143+0.0401x=0.375

0.0401x= 0.375-0.143

x=5.78 gm of   aluminium and mass of zinc = 9.301-5.785=3.516 gms

Masses : Al =5.785 gm, Zinc = 3.516gm and Copper= 2.169 gm

mass percentages : Al = 100*5.785/11.47=50.44 % Zinc = 100*3.516/11.47

=30.653% and Copper= 18.91%.