A 0.250g smaple of magnesium-aluminum alloy dissolves completely in an excess of HCl(aq). When the liberated H2(g) is collected over water at 29C at 752torr, the volume is found to be 297mL. The vapor pressure of water at 29C is 30.0torr.

Question

A 0.250g smaple of magnesium-aluminum alloy dissolves completely in an excess of HCl(aq). When the liberated H2(g) is collected over water at 29C at 752torr, the volume is found to be 297mL. The vapor pressure of water at 29C is 30.0torr.
What is the mass percentage of aluminum in this alloy?

*already calculated the moles of H2 which was 1.14x10^-2

Thanks!

DrBob222 · Answer

I'll assume your value for n is correct.
You need to solve for grams Al and grams Mg.
Let X = grams Al
then Y = grams Mg
==================
X + Y = 0.250
X(2*molar mass Al/3*molar mass H2) + Y(molar mass Mg/molar mass H2) = moles H2.
Solve the two equations simultaneously for X and Y.
Then %Al = (grams Al/grams sample)*100 = ?