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.

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.

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

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

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

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)

both those answers were wrong on the on the online grader

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?

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. :-).

Thank you so much!

To determine the composition of the brass alloy, we need to use stoichiometry and the given information.

First, let's find the moles of ZnCl2 produced. We know that the molar mass of ZnCl2 is 136.29 g/mol.

Given:
Mass of ZnCl2 = 0.0953 g
Molar mass of ZnCl2 = 136.29 g/mol

Using the formula:
moles = mass / molar mass

moles of ZnCl2 = 0.0953 g / 136.29 g/mol ≈ 0.0007 mol (rounded to 4 decimal places for accuracy)

Next, we need to convert the moles of ZnCl2 to moles of Zn, using the stoichiometry of the balanced equation.

According to the balanced equation:
1 mole of Zn reacts with 1 mole of ZnCl2

moles of Zn = 0.0007 mol

Therefore, there are also 0.0007 moles of Zn in the original brass alloy.

Now, let's find the mass of the brass alloy.

Given:
Mass of Zn in the brass alloy = 0.5093 g

To find the mass percentage of Zn in the brass alloy, we use the formula:

%Zn = (mass of Zn / mass of alloy) × 100

%Zn = (0.5093 g / mass of alloy) × 100

Since the moles of Zn in the alloy are equal to the moles of ZnCl2 produced, we can use the molar masses to relate the mass of Zn to the mass of the alloy.

Molar mass of Zn = 65.38 g/mol
Molar mass of Cu = 63.55 g/mol

Using the formula:
mass of alloy = (moles of Zn + moles of Cu) × (molar mass of Zn + molar mass of Cu)

mass of alloy = (0.0007 mol + moles of Cu) × (65.38 g/mol + 63.55 g/mol)

Now we have two equations:

Equation 1:
%Zn = (0.5093 g / mass of alloy) × 100

Equation 2:
mass of alloy = (0.0007 mol + moles of Cu) × (65.38 g/mol + 63.55 g/mol)

We can solve these two equations simultaneously to find the values of %Zn and %Cu.

Plug Equation 2 into Equation 1:
%Zn = (0.5093 g / [(0.0007 mol + moles of Cu) × (65.38 g/mol + 63.55 g/mol)]) × 100

From here, you can solve the equation to find the values of %Zn and %Cu in the brass alloy.

Please note that I see discrepancies in the values you've provided, so it's possible that there may be calculation errors in your calculations. Double-check your calculations to ensure accuracy.