[NewSoul]

Hi everyone.  I'm having a bit of an issue.  It could just be that I did the experiment wrong, but I think my work might also be wrong.  I should end up calculating a similar amount of zinc for #1 and #3, but I am getting a massive difference between the two.

This was from a lab, so here are values which may be useful:
Total mass unknown sample: 1.501g
Total volume H₂ gas: 423mL
Volume HCl used: 9.60mL
Atmospheric pressure: 73.06mmHg
Water temperature: 280.4K
Vapor pressure water at 280.4K: 7.72mmHg
Calculated conversion factor: 26,500mL H₂/mol Zn
Reaction: Zn + 2HCl ZnCl₂ + H₂O

1.  Use the conversion factor you calculated earlier to determine the mass of zinc in the unknown sample.
volume H2 * conversion factor * molar mass Zn
.423L H₂*(1mol Zn/26.5L H₂)*(65.41g/mol) = 1.04g Zn

I think I did this one correctly.

2.  Calculate the pressure of H₂ using Dalton's Law.
P_total = P_H2 + P_H2O
73.06mmHg = P_H2 + 7.72mmHg
P_H2 = 65.34mmHg

3.  Calculate the mass of zinc in the unknown sample using #2 and the ideal gas law
PV = nRT
n = PV/RT
n = (65.34mmHg*.423L)/(62.36367LmmHgmol^-1K^-1*280.4K) = 0.001580555mol H2

0.001580555mol H₂*(1mol Zn/1mol H₂)*(65.41g/mol) = 0.103g Zn

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As you can see, 1.04 is nowhere near 0.103.  Where did I go wrong?  If nothing is wrong, it's possible the conversion factor is wrong.  In which case, would you guys mind checking that also if I post it here?  Thanks.

[Borek]

Atmospheric pressure: 73.06mmHg

Without checking anything else - that's about 10% of the normal atmospheric pressure, so this number is incorrect.

[NewSoul]

Atmospheric pressure: 73.06mmHg

Without checking anything else - that's about 10% of the normal atmospheric pressure, so this number is incorrect.

Woah!  Thanks for this!  I never would have caught that.  It appears that on a different paper I wrote 734.06mmHg.  I will try the second and third equations now and see what I get.

*edit*  Okay, so I just did it and ended up with 1.15g Zn.  The error is a lot higher than I would like, but it makes way more sense now!