help - heat of combustion question

[Justanotherhuman]

 0.250 g of black coal was burnt and used to heat 300ml of water. Temperature of water rises by 6.67 degrees. Calculate the heat of combustion of in kJ/g of the coal assuming all energy released was used to heat the water.

Step 1:
q=mcT
= 300 X 4.18 X 6.67
= 8364.18 J

Step 2:
Energy = heat of combustion X mol
STUCK HERE, how do you calculate the mol of "back coal" since you don't know the molar mass?

[sweetcheeks]

The question specifies kJ/g rather than kJ/mole.

[Justanotherhuman]

ohhh I see, how do we work that out?

[sweetcheeks]

ohhh I see, how do we work that out?

The same way you would solve for molar heat of combustion, except you use grams rather than moles and kJ/g rather than kJ/ mole. Energy = Mass x Heat of combustion (kJ/g). Rearrange to solve for heat of combustion.

[huity]

Hey 

You've done step 1 nearly perfectly (good work on not falling for the trap of using the mass of coal instead of the mass of water).
See what is written in the below comment

Continuing with what sweetcheeks wrote, for Step 2:
heat of combustion (kJ/g)
= energy from step 1 (kJ)/ mass of coal (g)
= 8.36418 kJ/ 0.250 g
= 33.5 kJ/g (3 sig figs).

Hope that helps 

[KiNSKi01]

Also you have to convert 300 mL to a mass because you can;t use a measurement of capacity in q=mc delta t

So multiply 300 mL by 0.997 (density in g/mL of water). Basically get the same answer but still