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# EVEN MOAR enthalpy problems watch

1. (Original post by samb1234)
Find the number of moles of Mg, then times that by the enthalpy change to get energy change
so mol=mass/mr
=0.8/24.3=0.0329mol

0.0329*193.5=6.37kJ
2. (Original post by EricPiphany)
So now you are using 0.8g. (0.8/2.43)*46.3 will give you the new temp. increase.
this gives me 15.8°C but the answer is 15.2 °C
3. (Original post by thefatone)
this gives me 15.8°C but the answer is 15.2 °C
I think you might have typed it into your calculator wrong.
4. (Original post by EricPiphany)
I think you might have typed it into your calculator wrong.
got it now but why do this (0.8/2.43)*46.3 is there a formula to tell me to use this or what?
5. (Original post by thefatone)
got it now but why do this (0.8/2.43)*46.3 is there a formula to tell me to use this or what?
No, there is no formula, I'm just using proportionality.

Continuing down this path will get you the right result.

(Original post by thefatone)
so mol=mass/mr
=0.8/24.3=0.0329mol

0.0329*193.5=6.37kJ
6. (Original post by thefatone)
got it now but why do this (0.8/2.43)*46.3 is there a formula to tell me to use this or what?
Because you have (0.8/2.43) of the number of moles originally reacting now. So therefore since delta T is proportional to Q, which itself is proportional to the number of moles reacting delta T is proportional to number of moles reacting
7. (Original post by EricPiphany)
No, there is no formula, I'm just using proportionality.

Continuing down this path will get you the right result.
what do i do?

6371J=m*4.18*delta T ????

if i put 50 as m then i get an answer 2 times too big which is 30.4
if i put 100 as m then i get the right answer as 15.2 but why?
8. (Original post by thefatone)
what do i do?

6371J=m*4.18*delta T ????

if i put 50 as m then i get an answer 2 times too big which is 30.4
if i put 100 as m then i get the right answer as 15.2 but why?
100, because you have 100g of solution
9. (Original post by EricPiphany)
100, because you have 100g of solution
oh yea ok

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