Can AQA a level chemistry give you these type of questions in the exam??

A mixture of anhydrous sodium carbonate and sodium hydrogencarbonate of mass 10.000 g was heated until it
reached a constant mass of 8.708 g. Calculate the composition of the mixture in grams of each component.
Sodium hydrogencarbonate thermally decomposes to form sodium carbonate.

this question is weird and i was wondering if it is common to find these tricky questions in the actual exams

Reply 1

TypicalNerd

Original post

by Rio.nr

A mixture of anhydrous sodium carbonate and sodium hydrogencarbonate of mass 10.000 g was heated until it
reached a constant mass of 8.708 g. Calculate the composition of the mixture in grams of each component.
Sodium hydrogencarbonate thermally decomposes to form sodium carbonate.
this question is weird and i was wondering if it is common to find these tricky questions in the actual exams

It's possible but unlikely. There's a trick for these types of questions in the event they are asked - always begin by writing equations for the reactions taking place - in this case, there is only one reaction occurring, as it tells you that the sodium hydrogencarbonate decomposes to form sodium carbonate (which does not decompose further, implying the sodium carbonate already in the mixture must also not decompose either):

2NaHCO₃(s) --> Na₂CO₃(s) + H₂O(g) + CO₂(g)

So the mass lost will be the water and CO₂, which implies their combined mass is 10.000 g - 8.708 g = 1.292 g.

Considering that the relative masses of water and CO₂ are 18 and 44, respectively and that both substances form in a 1:1 mole ratio, we can say that (1 x 18)/(1 x 18 + 1 x 44) = 18/62 of the mass lost is lost as water and that (1 x 44)/(1 x 18 + 1 x 44) = 44/62 of the mass is lost as CO₂, so we lose 0.375 g of water and 0.917 g of CO₂.

Using either one of the two masses I have calculated, can you work out how you would finish solving this problem?

Reply 2

TypicalNerd

If you want a more typical example of a calculation involving mixtures of chemicals reacting, an example came up on a past Edexcel IAL paper (June 2020 unit 5, Q23(a)). The question and a full solution are provided here: https://www.thestudentroom.co.uk/showthread.php?t=7489167&p=99626475

If you are concerned about questions involving mixtures, I would recommend following the link, attempting the question first and then looking at the solution in the spoiler.

To my knowledge, no such questions of this type have yet come up on an AQA past paper, but the material required to solve such problems is 100% examinable and it is worth looking into them.

(edited 1 month ago)

Reply 3

Rio.nr

Original post

by TypicalNerd

If you want a more typical example of a calculation involving mixtures of chemicals reacting, an example came up on a past Edexcel IAL paper (June 2020 unit 5, Q23(a)). The question and a full solution are provided here: https://www.thestudentroom.co.uk/showthread.php?t=7489167&p=99626475
If you are concerned about questions involving mixtures, I would recommend following the link, attempting the question first and then looking at the solution in the spoiler.
To my knowledge, no such questions of this type have yet come up on an AQA past paper, but the material required to solve such problems is 100% examinable and it is worth looking into them.

Thank you so much for replying and helping me out ! I get how you got the masses aswell by using the mr etc.

Yeah so ill probably just practice these questions more and check out the link

Thanks again.

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