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Nymcha421's picture
Joined: Dec 2007

i just don't get how to do problems with them... for example:

When 1.535 g of methanol (CH3OH) was burned in a constant-volume bomb calorimeter, the water temperature rose from 20.27 oC to 26.87 oC. If the mass of water surrounding the calorimeter was exactly 1000 g and the heat capacity of the bomb calorimeter was 1.75 kJ/oC, calculate the molar heat of combustion of CH3OH. The specific heat of water is 4.184 J/g·oC.
A) -8.17 x 105 kJ/mol
B) -817 kJ/mol
C) 1.88 kJ/mol
D) 817 kJ/mol
E) None of the above.

A 1.300 g sample of benzoic acid (C7H6O2) was burned in a bomb calorimeter. The heat capacity of the entire apparatus, including the bomb, pail, thermometer, and water, was found to be 11,145 J/K. As a result of the reaction, the temperature of the calorimeter and water increased 4.627 K. What is the molar heat of combustion of benzoic acid?
A) 4.84 x 106 kJ/mol
B) -2.96 kJ/mol
C) -4844 kJ/mol
D) 549.1 kJ/mol
E) 51.57 kJ/mol

I know the answers are B and C but I don't know how to get there. does anyone else understand these?

Ferris Bueller's picture
Joined: Mar 2008

to start with, you find the temperature that the water increased by. this is how much heat energy the burning of the ch3oh produces.

i don't quite remember where to go from there but i will get back to you, i promise.

it has something to do with finding how much heat energy goes into the various parts and some converting between various units, i think.

all in all, it's just another brick in the wall...

I took: AP US Hist, AP Stats, AP Chem
I am taking: AP Calc, AP English, AP Phsics

Nymcha421's picture
Joined: Dec 2007

i finally figured it out, but thanks ferris!

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