# Homework Help: Formula of a mixed copper salt

1. Mar 3, 2009

### wcbryant87

1. The problem statement, all variables and given/known data

Cu2+OH-, and Br- form a mixed salt formula Cux(OH)yBrz. In an experiment to determine the values of x, y, and z in the formula of this compound, .760 g CuBr2 (223.4 g/mol) was dissolved in water and reacted with 10.22 ml 0.555M NaOH. Assume that in this reaction, all of the Cu2+ from the CuBr2 and all of the OH- from the NaOH were incorporated into the product but that not all of the Br- was used.

Calculate the number of moles of Cu2+ and OH- in the product.

2. Relevant equations

3. The attempt at a solution
10.22mL * 1/1000L * .555M NaOH = .00567 mols OH-
I think that is right...but it's finding the moles of Cu that is stumping me.

.760g CuBr2 * 2H2O * 1/259.4 = .00293 mol CuBr2

Now I don't know how to get from that to mols of just Cu. I don't have a volume or anything.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Mar 3, 2009

### alxm

2H2O? Two water molecules? Where did they come from?

If you have a dozen bicycles, how many tires do you have? Two dozen, right?
If you have a dozen molecules of CuBr2, how many atoms of Cu do you have? Two dozen.

A mol is just a quantity. Like dozens.

So if you have a mol of CuBr2 molecules, how many moles of Cu do you have?

3. Mar 4, 2009

### wcbryant87

The 2H2O is because it's a hydrate.

You kind of lost me on the dozen analogy though. If you have a dozen CuBr2 molecules wouldn't that just be a dozen Cu atoms?

4. Mar 4, 2009

### Staff: Mentor

Yes, that's probably what alxm meant.

Why do you think CuBr2 is hydrated? Judging from the molar mass given it is not.

5. Mar 5, 2009

### alxm

Do you know that? Typically these problems would include that information if it was needed, e.g. by writing CuBr2*2H2O.

Whoops, my mind must've slipped. Yes, a dozen Cu and two dozen Br.
(Typical.. screwing up a perfectly simple analogy)