Chemistry
posted by Connor .
Al(s) + HCl(aq) AlCl3(aq) + H2(g)
Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)
Consider the unbalanced equations above. A 0.200 g sample of a mixture of aluminum and zinc metal is reacted with an excess of hydrochloric acid. Both metals react with hydrochloric acid. The sample produces 168.2 mL of hydrogen gas at STP.
(a) What is the mass percent aluminum in the sample?
(b) What is the mass percent zinc in the sample?

Two equations in two unknowns means two equations solved simultaneously. You should first balance both equations (which you should have done on all of these before posting). Another note: you should find the arrow key on your computer. You don't know what's what if you can't tell the reactants from the products.
2Al(s) +6 HCl(aq)=> 2AlCl3(aq) + 3H2(g)
Zn(s) + 2HCl(aq)=> ZnCl2(aq) + H2(g)
Let X = mass Al metal.
and Y = mass Zn metal.

eqn 1 is
X + Y = 0.200g
The second equation equates mols H2 with mols H2.
mols H2 from Zn is:
Y*(1/atomic mass Zn)*(1 mol H2/1 mol Zn)
mols H2 from Al is:
X(1 mol Al/atomic mass Al)*(3 mol H2/2 mol Al)
eqn 2 is
add mols H2 from X to mols H2 from Y and make it equal to 168.2/22,400
Solve for X and Y to obtain grams Al and Zn.
Then % Al = (grams Al/0.200)*100 = ?
%Zn = (grams Zn/0.200)*100 = ? 
67.8