Homework Help: Chemistry

Posted by ChemLover on Friday, August 12, 2011 at 7:51pm.

A sample of zinc metal reacts completly with an excess of hydrochloric acid:

Zn(s) + 2HCl(aq) --> ZnCl2(aq) + H2(g)

The hydrogen gas produced is collected over water at 25.0 °C. The volume of the gas is 78.0 L & the pressure is 0.980 atm. Calculate the amount of zinc metal in grams consumed in the reaction. (vapor pressure of water at 25 °C = 23.8mmHg)

okay so for this problem i first used PV=nRT and i found moles of H2 which was 3.13 mol H2 and then i converted that into 3.13 mol Zn because i mol of Zn = 1 mol of H2

after i took the mm of Zn (65.38 g/mol)
and multiplied it by 3.13 mol Zn to get grams.

and i got 205 g. The actual answer in my notes is 198 g Zn..i did it a couple of times but i cant seem to get this answer..and i missing a step..thanks for the help

Chemistry - DrBob222, Friday, August 12, 2011 at 10:14pm
I ran a quick calcn for n and obtained 3.0246 (I know that's too many significant figures). I suspect you did not subtract the vapor pressure of water at 25 C. You should use as pressure 0.980-vp H2O at 25C. I looked that up on the web and the site said 23.76 mm but you should check it out. If this is not your problem let me know and I'll look further.

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