Mass% of Magnesium-Aluminum Alloy in HCl

[ahhppull]

Homework Statement

A 0.250-g sample of a magnesium-aluminum alloy dissolves completely in an excess of HCl(aq). When the liberated H2(g) is collected over water at 29C and 752 torr, the volume is found to be 313 mL. The vapor pressure of water at 29C is 30.0 torr.

Homework Equations

PV = nRT
Al = 26.98 g/mol
Mg = 24.31 g/mol

The Attempt at a Solution

I'm unsure how to do this...but here's what I did.

1) First I converted everything.

752 torr = 0.989 atm
313 mL = 0.313 L
29C = 302K

2) Then I used the forumla PV = nRT to find out how many moles of water there was.

3) Then I multiplied the moles by the MM of Al and Mg.

4) Finally I did .3038x + .3371(1-x) = 250 and got a whole number above 100% which means that my answer was wrong.

I haven't used any the information about the vapor pressure of water because I don't know what to do with it...

 

[Borek]

Start with the reaction equations. Water doesn't tell you anything about alloy composition.

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