# Chemistry help A level

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#1
I know this question has already been asked by someone on tsr like two years ago but it never got an answer and I'm really stuck. If anyone could help me out I'd be so grateful! (It's part of the 2017 june paper for edexcel)

In the mark scheme it says I have to times by 2 for the molar mass but idk why

The student prepared an aqueous solution by dissolving 1.15 g of Mg(NO2)2•xH2O in distilled water, making up the solution to 250.0 cm3 in a volumetric flask and shaking the mixture.
The student titrated this solution against 25.0 cm3 portions of an acidified solution of 0.0200 mol dm−3 potassium manganate(VII), KMnO4(aq).

Q: In the titration reaction, 2 mol MnO4− react with 5 mol NO2−.
Calculate the number of moles of NO2−, in the 250 cm3 of solution prepared by the student and hence the value of x in Mg(NO2)2•xH2O.

Mark scheme below:
Last edited by AelinGalanythius; 3 weeks ago
0
3 weeks ago
#2
It’s because of that 2 after the brackets

Mg(NO2)2

You’ve worked out the number of moles of NO2-

You would then divide by 2 to get The number of moles of Mg(NO2)2

They have written it all together so

Molar mass = mass / n

Molar mass = mass/ (n of NO2- /2)

It’s the same as

Molar Mass = 2 x (mass/n of NO2-)
1
#3
(Original post by GabiAbi84)
It’s because of that 2 after the brackets

Mg(NO2)2

You’ve worked out the number of moles of NO2-

You would then divide by 2 to get The number of moles of Mg(NO2)2

They have written it all together so

Molar mass = mass / n

Molar mass = mass/ (n of NO2- /2)

It’s the same as

Molar Mass = 2 x (mass/n of NO2-)
Oooooh! That makes so much sense. Thank you so much!
0
3 weeks ago
#4
(Original post by AelinGalanythius)
Oooooh! That makes so much sense. Thank you so much!
There’s probably a more technical way to explain it but that’s the reason for it
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#5
(Original post by GabiAbi84)
There’s probably a more technical way to explain it but that’s the reason for it
I just didn't put together that dividing moles by 2 was the same as multipling mr by 2 if you know what I mean
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