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Source: an IB Chemistry past paper.

I looked it up and found out that brass is an alloy of copper and zinc - however, I don't see zinc in any of the equations, and therefore any brass being separated into copper and zinc, which I had originally thought would be the method to determine the percentage of copper in brass (by looking at the coefficients of the compounds in the resulting chemical equation).

My question is how do these reactions help determine the percentage of copper in brass?

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  • $\begingroup$ As this is a homework question, please share your thoughts and attempts towards the solution. It'll make us certain that ‎we aren't doing your homework for you. $\endgroup$ – user15489 Aug 9 '15 at 11:30
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Well the question would had more to it. The question would have probably gone something like this. A sample of brass was dissolved with excess nitric acid. The $\ce{Cu^{2+}}$ ions are then reduced with excess iodide ions. Iodine is then titrated with thiosulfate (giving its concentration and volume used). Find the percentage of copper in brass.

To do this question, calculate the amount of moles of thiosulfate used, using its concentration and volume. Then using equation 3, you can see the number of moles of iodine is half the number moles of thiosulfate. Then using equation 2, you can see that the number of moles of $\ce{Cu^{2+}}$ ions are twice as the number of moles of iodine and hence are equal to the number of moles of iodine. Finally, using equation 1, the number of moles of copper is equal to the number of moles of $\ce{Cu^{2+}}$ ions and hence are equal to the number of moles of thiosulfate.

Now that we know the number of moles of copper, you can multiply by its molar mass to get its mass and then find the percentage of copper in brass.

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