Heat of Combustion (2 Viewers)

[Reikira]

The heat of combustion of ethanol is 1360kj/mol. What mass of ethanol need to be burnt to raise the temp of 350g of water through 77C if 50% of the heat is released by ethanol is lost to the surroundings? The specific heat capacity of water is 4.2 J K g.

Thanks.

 

[b3kh1t]

The heat of combustion of ethanol is 1360kj/mol. What mass of ethanol need to be burnt to raise the temp of 350g of water through 77C if 50% of the heat is released by ethanol is lost to the surroundings? The specific heat capacity of water is 4.2 J K g.

Thanks.

Ok well first find the change in heat using mc(change in temperature) = 350x4.2x77 = 113190 J
therefore 113.19 KJ is required and ethanol produces 1360 KJ per mole although only half is used to heat it, so 680 KJ/mol is ethanols ability to heat the water.
113.19/680 = 0.166 moles
therefore 0.166 moles of ethanol is required, or 7.7 g of ethanol must be burnt.

 

[Reikira]

Thanks, ive got another question.

How do u know when u use (4.18 x 10^3) and when to use 4.18.

 

[b3kh1t]

Thanks, ive got another question.

How do u know when u use (4.18 x 10^3) and when to use 4.18.

you use the 4.18x10^3 if you want you answer to be in Kilojoules straight away, however when you use 4.18 your answer will be in Joules

 

[Reikira]

Thanks again man, you've been a great help.

 

[b3kh1t]

That's aiight anytime best of luck tomorrow

 

[nichorowitz]

yo.
the answer is actually 7.62 kg?

actually did something stupid

 

[nichorowitz]

yeah 7.6 g is right

 

[Reikira]

Might be just some rounding off error that gets carried on throughout the calculation

 

[b3kh1t]

nah forget to change the joules to kilojoules, right?? lol alls good just be careful tomorrow

 

[someth1ng]

you use the 4.18x10^3 if you want you answer to be in Kilojoules straight away, however when you use 4.18 your answer will be in Joules

Wouldn't it be the other way around since if you multiply by 10^3 giving kJ, wouldn't it be the same amount say X*10^3 kJ which is 1000000 times more?