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Help with chemistry question please!

Magnesium will also displace copper from copper (II) sulphate solution.If an excess of magnesium is added to 100 cm3 of1.0 moldm-3 copper(II) sulphate, the temperature increases by 46.3 oC.
a) Calculate the molar enthalpy change for the reaction

b) Calculate the minimum quantityof magnesium required to ensure it is in excess.

c) Calculate the temperature change if only 0.8g of magnesium is added.

So I got -1935.34 for a, which is wrong and i got completely wrong answers for b and c!!! Can someone explain how to work them out please?

Here are my working for a)
q= mcΔT
q= 100 x 4.18 x 46.3
q= 19353.4 J
q= 19.3534 KJ
moles= conc x vol
moles= 0.100 x 0.100 = 0.0100moles
So enthalpy change = 19.3534/0.0100
= -1935.34 kJmol-1

Can someone please tell we where I went wrong here with the calculation above?
I'm not going to even bother showing my workings for b and c because they are completely wrong and I'm not certain how to work them out anyway (I had an 'educational' guess).

Any help would be greatly appreciated... + REP :wink: :h:
(edited 7 years ago)
Original post by RedRosesBloom
Magnesium will also displace copper from copper (II) sulphate solution.If an excess of magnesium is added to 100 cm3 of1.0 moldm-3 copper(II) sulphate, the temperature increases by 46.3 oC.
a) Calculate the molar enthalpy change for the reaction

b) Calculate the minimum quantityof magnesium required to ensure it is in excess.

c) Calculate the temperature change if only 0.8g of magnesium is added.

So I got -1935.34 for a, which is wrong and i got completely wrong answers for b and c!!! Can someone explain how to work them out please?

Here are my working for a)
q= mcΔT
q= 100 x 4.18 x 46.3
q= 19353.4 J
q= 19.3534 KJ
moles= conc x vol
moles= 0.100 x 0.100 = 0.0100moles
So enthalpy change = 19.3534/0.0100
= -1935.34 kJmol-1

Can someone please tell we where I went wrong here with the calculation above?
I'm not going to even bother showing my workings for b and c because they are completely wrong and I'm not certain how to work them out anyway (I had an 'educational' guess).

Any help would be greatly appreciated... + REP :wink: :h:


In the question it says the solution is 100 cm^3 and 1 M so that would give 0.1 moles not 0.01
Original post by RedRosesBloom
Magnesium will also displace copper from copper (II) sulphate solution.If an excess of magnesium is added to 100 cm3 of1.0 moldm-3 copper(II) sulphate, the temperature increases by 46.3 oC.
a) Calculate the molar enthalpy change for the reaction

b) Calculate the minimum quantityof magnesium required to ensure it is in excess.

c) Calculate the temperature change if only 0.8g of magnesium is added.

So I got -1935.34 for a, which is wrong and i got completely wrong answers for b and c!!! Can someone explain how to work them out please?

Here are my working for a)
q= mcΔT
q= 100 x 4.18 x 46.3
q= 19353.4 J
q= 19.3534 KJ
moles= conc x vol
moles= 0.100 x 0.100 = 0.0100moles
So enthalpy change = 19.3534/0.0100
= -1935.34 kJmol-1

Can someone please tell we where I went wrong here with the calculation above?
I'm not going to even bother showing my workings for b and c because they are completely wrong and I'm not certain how to work them out anyway (I had an 'educational' guess).

Any help would be greatly appreciated... + REP :wink: :h:


All you did was misread the question! You used 0.1 moldm^-3 instead of 1.0 moldm^-3 :tongue:
Original post by langlitz
In the question it says the solution is 100 cm^3 and 1 M so that would give 0.1 moles not 0.01


Original post by rosemondtan
All you did was misread the question! You used 0.1 moldm^-3 instead of 1.0 moldm^-3 :tongue:


Oops, didn't realise, thank you! I hope I don't do this in the exam, because I always seem to end up misreading the questions *facepalm* :P
Original post by RedRosesBloom
Oops, didn't realise, thank you! I hope I don't do this in the exam, because I always seem to end up misreading the questions *facepalm* :P


Stay calm and you will be fine :smile:
Original post by rosemondtan
Stay calm and you will be fine :smile:


Easier said than done, although I will try. I think I will need to read the question several times, just to make sure xD
whats the answer to b
Reply 7
Original post by RedRosesBloom
Magnesium will also displace copper from copper (II) sulphate solution.If an excess of magnesium is added to 100 cm3 of1.0 moldm-3 copper(II) sulphate, the temperature increases by 46.3 oC.
a) Calculate the molar enthalpy change for the reaction

b) Calculate the minimum quantityof magnesium required to ensure it is in excess.

c) Calculate the temperature change if only 0.8g of magnesium is added.

So I got -1935.34 for a, which is wrong and i got completely wrong answers for b and c!!! Can someone explain how to work them out please?

Here are my working for a)
q= mcΔT
q= 100 x 4.18 x 46.3
q= 19353.4 J
q= 19.3534 KJ
moles= conc x vol
moles= 0.100 x 0.100 = 0.0100moles
So enthalpy change = 19.3534/0.0100
= -1935.34 kJmol-1

Can someone please tell we where I went wrong here with the calculation above?
I'm not going to even bother showing my workings for b and c because they are completely wrong and I'm not certain how to work them out anyway (I had an 'educational' guess).

Any help would be greatly appreciated... + REP :wink: :h:

What is the answer to parts b and c??
Reply 8
Original post by OHIfe
What is the answer to parts b and c??

I think b is done by multiplying 0.1 by 24.3. Because 24.3 is the Mr of Magnesium, and Magnesium has a mole of 0.1 due to the equation having a 1: 1 ratio between CuSO4 and Mg. So to find the mass, just use Mass = Mr x mol which gives 2.43g. But I don't understand how to do part c. Someone help me out please!!

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