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Chemical calculations - Chemistry Past Paper - Help needed

Hi, so I was doing a past paper questions and I kept getting stuck on these type of questions (the past papers are in the links below).
I'm stuck on May 2013- Q8d) and May 2012- Q7e)ii)

https://drive.google.com/open?id=0B_OpTg5bRN9JaTdMQThmazg3M0k
https://drive.google.com/open?id=0B_OpTg5bRN9JY29Xd2VxZnFmRVk

I'm not sure what equation i need to use to work it out because there's so many to do with volume and gases...? I have looked at the mark schemes for it, but i still don't understand the thinking process behind it.

Especially May 2012- Q7e)ii) It's given the mass of Mg needed to produce it, so I thought you must have needed to use it to work it out, but according to the mark scheme, you end up working out the moles of acid instead?

So, if someone could explain it to me, that'd be great. Thank you :smile:

Reply 1

charco

Study Forum Helper

Original post by 1DMinHoFTHongKi

Hi, so I was doing a past paper questions and I kept getting stuck on these type of questions (the past papers are in the links below).
I'm stuck on May 2013- Q8d) and May 2012- Q7e)ii)

https://drive.google.com/open?id=0B_OpTg5bRN9JaTdMQThmazg 3M0k
https://drive.google.com/open?id=0B_OpTg5bRN9JY29Xd2VxZnF mRVk

I'm not sure what equation i need to use to work it out because there's so many to do with volume and gases...? I have looked at the mark schemes for it, but i still don't understand the thinking process behind it.

Especially May 2012- Q7e)ii) It's given the mass of Mg needed to produce it, so I thought you must have needed to use it to work it out, but according to the mark scheme, you end up working out the moles of acid instead?

So, if someone could explain it to me, that'd be great. Thank you :smile:

[video]https://youtu.be/HKK0HD-2Igc[/video]

Reply 2

charco

Study Forum Helper

Original post by 1DMinHoFTHongKi

Especially May 2012- Q7e)ii) It's given the mass of Mg needed to produce it, so I thought you must have needed to use it to work it out, but according to the mark scheme, you end up working out the moles of acid instead?

So, if someone could explain it to me, that'd be great. Thank you :smile:

Using the values from experiment A, you see that with ten times the amount of magnesium the acid is now the limiting reagent and the magnesium is in excess.

Use the quantities of acid from A to show that the maximum volume of gas that could be produced is 120 ml