Slide – Exercise 15 – Metals


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Chemical Equations that Crystal Zinc Dissolved in Dilute Sulfuric Acid

A crystal of zinc weighing 31.4 grams was dissolved in dilute sulfuric acid forming zinc sulfate and hydrogen gas, which was collected. The gas was dried and then completely burned to give 8.67 grams of water.
Write chemical equation for these reactions. Now this is a hard calculation. It looks like it’s fairly straightforward. But there’s a lot to this question. The reason why it’s hard is because it’s multi-step. This is one of those ones where it’s good to pay attention now because this is something that if you learn how to do now, you will have no problem in titration, which is considered one of the more painful topics in chemistry.
Chemical equations. The first one is zinc dissolves within the H2SO4 to release hydrogen and zinc sulfate. A simple equation.
Then the hydrogen gas is burned with oxygen to produce H2O.
So deduce the mass of the zinc crystal. So we had a lump of zinc. But we don’t actually know how much of it was actually zinc. So we want to know the mass of the zinc inside that crystal.
So we start with this reaction and this reaction. So the mass of the water is the only mass that we’re given. So we calculate the number of moles of that. So 8.67 divided by the molar mass of water, which is this. It gives you 0.4795 moles. OK?
Now from the second chemical equation, the number of moles of water equals the number of moles of hydrogen. So the number of moles of hydrogen is also 0.4795 moles.
Then you might be wondering, “Well, why in the heck would I bother wanting to know how much hydrogen comes out?” Well, if you look at that equation, what links the two equations? The amount of hydrogen because the hydrogen that comes out of here is burned here. So the hydrogen that’s consumed here must be the same as the hydrogen from here.
That’s why we want to have the number of moles of hydrogen there. OK? From the first chemical equation then, the number of moles of hydrogen equals the number of moles of zinc, which is again 0.4795.
So the mass of the zinc is just the moles of the zinc times the molar mass, which is 0.4795 times 65.38, which is 31.35 grams. OK? So you can see that throughout this question, we’ve been – the reason why it’s difficult is because there are all these linking parts that you have to still work your way through.
If we told you all the steps, then that would be easy. But the way to do this question is just to work back and calculate things that you know until – and then link things that are supposed to be linked and then you get your answer.
So that’s this question. It’s a very, very typical style of question that they ask, lots of link to parts and so it’s just up to you to sort of filter your way through all the information.