[Kalesin]

Ok, so I'll post a few tasks I don't really understand and try to figure them out with your help   Note that english is my second language, and there might be slight definition mistakes.

1. Partly oxidized Zn powder contains 0.5% oxygen. Find the procental mass of metalic Zn.

Ok, the way I understand it, there is powder made of Zn and ZnO, and oxygen weights 0.5% of the mass of all that powder. So if we for example say we have 1 mole of ZnO in the powder:

Zn = 65g
O2 = 16g

16g = 0.5%

ZnO = 81 g

16g - 0.5%
81g - x%
16x = 40.5
x = 2.53125(%)

So ZnO is 2.53125% of the total mass of the powder, and the powder consists of ZnO and Zn, so

Zn weights 100 - 2.53125 = 97.46875 (%)

Is this right? Did I understand the question correctly?

I'll keep posting exercises that are not clear to me on thsi thread if it's ok, because there are a lot more.

I have no idea how to do this next task at all.

2. When heating a mixture of Aluminum and malachite, the mass of the mixture didn't change, find the procental composition of the mixture.

According to wikipedia, the formula of malachite is  Cu2CO3(OH)2, though this still gives me no clues how to proceed with the task.

1. (question) Is NaOH more likely to react with Zn or Si? Why?

[DevaDevil]

So ZnO is 2.53125% of the total mass of the powder, and the powder consists of ZnO and Zn, so
Zn weights 100 - 2.53125 = 97.46875 (%)
Is this right? Did I understand the question correctly?

yes

2. When heating a mixture of Aluminum and malachite, the mass of the mixture didn't change, find the procental composition of the mixture.

According to wikipedia, the formula of malachite is  Cu2CO3(OH)2, though this still gives me no clues how to proceed with the task.

too little info. Is this all in the mixture? what is the starting composition?
the only thing I can think of is that if mass doesn't change, this means no carbon dioxide or water evolves.

1. (question) Is NaOH more likely to react with Zn or Si? Why?

aqueous NaOH I presume? Strong bases form complexes with silicon and will react with metallic silicon very strongly.

[Kalesin]

Ok, about task nr. 2, I asked my teacher, and he gave me a hint to check out what happens when you heat up malachite, so what does happen?

Cu2CO3(OH)2 (malachite)

[Kalesin]

Ok, about task nr. 2, I asked my teacher, and he gave me a hint to check out what happens when you heat up malachite, so what does happen?

Cu2CO3(OH)2 (malachite)

[DevaDevil]

when heating malachite you will eventually end up with CuO, it loses its water and carbon dioxide.

Now if there is no mass change, this can only mean that the magnesium reacts with those.

[Kalesin]

Sorry for double post, accident   So

Cu2CO3(OH)2 -> 2CuO + CO2 + H2O

Magnesium? The tasks says Aluminum.

How does aluminum react with CO2 and H2O?

[DevaDevil]

if the mass doesn't change, it means magnesium has to react with both, at the temperatures malachite decomposes, water would be gaseous as well.

and as for 1 or 2 products.. I do not know, not an expert on magnesium compounds. Someone else may know better.

[Kalesin]

(question 2) If there was 1 mole of H2CO3 and it collapsed to CO2 and H2O will there be 1 mole each of these too?

[DevaDevil]

(question 2) If there was 1 mole of H2CO3 and it collapsed to CO2 and H2O will there be 1 mole each of these too?

this is basic stoechiometry... yes, there will be 1 mole of each.

[Kalesin]

I will feed you snacks daily sir 

3. We heat up a mixture of Fe and S. We melt what we get in HCl. When it completely reacts, 8,96 litres of gas come out, and when we burn those 8,96 litres we get 0.2 moles of other gas.

So the way I see it.

1. Fe + S -> FeS
2. FeS + 2HCl -> FeCl2 + H2S(gas)

n(H2S) = 8.96/22.4 = 0.4 moles

3. (BURNING H2S) 2H2S + 3O2 -> 2H2O + 2SO2(gas)

n(SO2) = 0.2 moles

I can't figure this out, how can we burn 0.4 moles of gas and get 0.2 moles of gas if they react 1:1? Does my task have a mistake? Am I doing it completely wrong?

[fledarmus]

What happens if your initial Fe/S mixture isn't 1:1 by moles?

[Kalesin]

If there is a leftover of S I can get additional H2S gas, but still, when it burns, how come of 0.4 moles of gas we get only 0.2 moles? If you know where is my specific mistake, please be straight forward, I'm not looking for someone to do my homework, it's summer, I just want to learn  

[DevaDevil]

this is an interesting question.
You are completely right in your reaction equations, those are the most likely reactions to happen. The only way I can see hydrogen sulfide burning to give less moles of gas than go in is if the S₂O₄ dimer is formed, which is highly unlikely.

The initial mixture of Fe and S does indeed not matter, as the story goes on about the gas that was formed. This is not dependent on the initial ratio as the first quantity given is after the gas has already been formed.

[Kalesin]

Ok, so I can post how to do it tomorrow after I get back from my teacher  Tahnks for all your help so far!

[fledarmus]

How do you get H2S from leftover sulfur? If I recall correctly, only the iron sulfide will react with the hydrochloric acid, not the elemental sulfur.

But I was actually thinking in terms of leftover iron. That would also react with HCl, but it would form hydrogen gas and iron chloride. If the assumption shown in your equation 3 is correct, that hydrogen when it burned would be forming water, which you are not counting as a gas.