[fumanchu]

I had a question on my homework and get a different answer than what is supposed to be correct.  The question is:

A Zn sample which has a mass of 40.0g and contains 10% inert material reacts with an HCl solution that contains 20.g of pure HCl.  How much ZnC2l will be produced by the system? (Zn + 2HCl ----> ZnCl2 + H2)

When I did the the problem I thought Zn was the limiting reactant (.55 mole) and I thought the amount of ZnCl2 would be 49.22g.  But the answer provided was 37.4 g.  Can someone get the answer that was provided?  I am not sure what I am doing wrong.  Thanks in advance.

[kingchemist]

Moles HCl = 20.0/36.5 (I'm using simple atomic masses H=1, Cl=35.5, Zn = 65 - you use the values expected in your course)
             = 0.548 moles
40.0 g - 10% = 36.0g pure Zn
Moles Zn = 36.0/65 = 0.554 mole

  Zn   +   2HCl   ----> ZnCl2  + H2
1 mol      2 mol
Zinc is clearly in excess as 0.548 moles of HCl can only react with 0.274 moles of Zn
   Zn   +   2HCl   ----> ZnCl2  + H2
              2 mol          1 mol 
           0.548 mole     0.284 mole

1 mole ZnCl2 has a mass of approx 65+71= 136g, so 0.284 moles has a mass of approx. 38.624g

hey

[fumanchu]

Thank you very much!
hey