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April 25, 2015

April 25, 2015

Posted by **ana** on Sunday, September 9, 2007 at 8:00pm.

Commercial brass,an alloy of Zn and Cu,reacts with Hydrochloric acid as follows:

Zn+2HCL-->ZnCl2+H2

(Cu does not reacts with HCL)When.0565g of a certain brass alloy is reacted with excess HCL,.0985g ZnCl2 is eventually isolated.

a.What is the composition of the brass by mass?

b.How could this result be checked without changing the above procedure?

I have tried converting .0985g to moles of Zn but then I do not know what to do? To be honest I don't get the question. Thanks in advance for your help.

- ap chemistry please BOB HELP ME -
**bobpursley**, Sunday, September 9, 2007 at 8:05pmYou need to determine the percent composition of Zn in ZnCl2 based on atomic masses and the formula. Then, you can get the grams of Zn recovered.

- ap chemistry please BOB HELP ME -
**DrBob222**, Sunday, September 9, 2007 at 8:30pmThis doesn't change Bob Pursley's answer but let me try to help so you will "get the question." The idea in any analytical scheme is to start with a known mass of substance and convert it to something else that is known. You started with brass and converted the Zn in the brass to ZnCl2. Then the ZnCl2 was weighed. Grams ZnCl2 can be converted to mols Zn and to grams Zn and (g Zn/g sample)*100 = %Zn. The idea is to work backwards from what you can weigh to determine how much of a material you MUST have started with. For the second part, think about this as a hint:

What else is produced in addition to the ZnCl2 and how might you use that to determine the amount of Zn in the brass alloy?

- ap chemistry please BOB HELP ME -
**Julissa**, Monday, September 10, 2007 at 12:32amI'm sorry to be such a big bother but I just want to make sure I did it the right way.

(.0985gZnCl2)(1molZnCl2)/(136.28gZnCl2)

I got(7.23times10withthe exponent-4 mols)

Then I converted moles of ZnCl2 to mol of Zn using themole to mole ratio. Then I converted moles of Zn to grams, and then I didvided the mass of Zn by the mass of ZnCl and times it by a hundred. And I got .03468 (which I think is a very small number), then I multiply the mass in the reactant side .5065g by the percentage which was (3.4times10exponent-4) (igot this by dividing .03468 by 100).

My final answer was (1.7568times10 exponent-4) I will really appreciate If you tell If I'm right, I'm a little unconfident in regard to the answer because is such a small number. Thank you again in advance. Also thank you very much both bobpursleyand Drbob222, your answers where as you might already know very helpfull.

- ap chemistry please BOB HELP ME -
**Anonymous**, Sunday, September 16, 2012 at 9:17pmuse stoich to convert into grams of Zn then find percentage like normal