AP CHEMISTRY

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Commercial brass, an alloy of Zn and Cu, reacts with hydrochloric acid as follows.
Zn(s) + 2 HCl(aq) ZnCl2(aq) + H2(g)
(Cu does not react with HCl.) When 0.5093 g of a certain brass alloy is reacted with excess HCl, 0.0953 g ZnCl2 is eventually isolated.
(a) What is the composition of the brass by mass?
%Zn
%Cu

I have done this problem and got 32.42% Zn, 9.032% Zn, and 4.57% Zn and they were all wrong. I also got 35.15%Cu and that was wrong also.

• AP CHEMISTRY -

If you will show your work I will find the error.

• AP CHEMISTRY -

I found the mass of ZnCl2 to be 136.29g
I started out:

.0953/136.3= 6.9e-4

then I did:
6.9e-4 x 65.39= .046 and multiplied that by a hundred

• AP CHEMISTRY -

Two things wrong here.
1. You are rounding off incorrectly. First, you are allowed three significant figures and 0.0953/136.3 = 6.99E-4. Actually your are throwing away part of the answer.
2. Then 6.99E-4 x 65.39 = grams Zn and for that I have 0.0457g.
3. At this point you left out the last step. % Zn = (grams Zn/g sample) = (0.0457/0.5093)*100 = 8.977% which I would round to three s.f. as 8.98%
Then % Cu = 100% - %Zn

• AP CHEMISTRY -

i understand where my mistake was. when i went to find the %Cu i did:

100%-8.98%= 91.02 (rounding to three sig figs would give me 91.0)

• AP CHEMISTRY -

I'm not privy to your on-line database but the USUAL thing wrong when they don't agree with your answer is the number of significant figures.
First, I assume you have a set of atomic masses and/or a periodic table that the database uses to calculate these things. Check those to make sure ZnCl2 and Zn are the same as you are using. I have a web site I use for these calculations and it gives me the same numbers you have used.
Second, I note that the grams for sample is to 4 places but the ZnCl2 is to 3 so that gives just 3 places to use for the answer. Check the numbers in your problem to make SURE you copied it correctly in your post.
If all of these check out ok make a note to talk to your prof tomorrow. From what you have posted, however, these are the right numbers assuming the database used the same values for molar masses.

• AP CHEMISTRY -

Okay, it needed four sig figs.
the next part of the question is:

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

What does this mean?

• AP CHEMISTRY -

a.
If that is 0.0953 and not 0.09530g for ZnCl2, then you are allowed only 3 places.
b.
which result? Result for Zn or result for Cu. I can't think of another way without changing the procedure for Zn but for Cu you can do it another way this way.
g Zn = 0.0457
g sample = 0.5093
g Cu = 0.5093-0.0457 = ?
% Cu = (g Cu/g sample)*100 = ?

Of course this assumes everyone answered the Cu part by %Cu = 100=%Zn. Some students will always do it as I've shown the alternative in which case the shorter route would the the "other" way. :-)

Let me point out something else.
If you do it my alternative route, then four s.f. is the way to go; i.e.,
0.5093-0.0457 = 0.4636 (4 places are allowed doing it this way for Cu), then
(0.4636/0.5093)*100 = 91.0269 which I would round to 91.03% TO FOUR S.F. and my statement still is right---when the databases don't agree USUALLY it is a matter of s.f. :-).

• AP CHEMISTRY -

Thank you so much!

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