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 10.652 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 8.94 L of hydrogen gas was collected at a pressure of 726 torr and a temperature of 27.0 °C. Additionally, 2.165 g of unreacted copper is recovered. Calculate the mass of hydrogen gas formed from the reaction.__g Calculate the mass of aluminum in the alloy sample. __g What is the mass percent composition of the alloy? __% Cu __%Al __%Zn

Answer 1

Zn + 2HCl ---> ZnCl2 + H2

Al + 3HCl --> AlCl3 + 3/2 H2

H2 moles got   we calculate form PV = nRT equation where P = 726/760 atm = 0.955 atm

0.955 x 8.94 n x 0.08206 x 300       ( T = 27C = 27+273 = 300 K)

n = 0.3468 = moles of H2

mass of H2 produced = moles of H2 x molar mass of H2 = 0.3468 x 2 = 0.6936 g

copper mass = 2.165 g ,

hence Al + zinc mass = 10.652-2.165 = 8.487 g

let Al moles be n(Al) , zinc moles be n(Zn)

H2 moles produced = 3/2 n(Al) + 2n(Zn) = 0.3468 ............(1)

mass of AL = moles of AL x molar mass of Al = 27n(Al)

mass of Zn = 65.38 n(Zn)

mass of alloy without copper is

27n(Al) + 65.38n(Zn) = 8.487 ....(2)

by (1) ( 2) , n(Al) = 0.129335 , Zn moles = 0.0764

Al mass = 0.129335 x 27 = 3.49 g , Zn mass= 0.0764 x 65.38 = 4.955 g

now mass of AL = ( AL mass x 100) / ( alloy mass) = ( 100 x 3.49/10.652) = 32.76 %

Zn % = ( 100 x 4.955/10.652) = 46.5 %

Cu % = 100-46.5-32.76 = 20.74 %