Hello, Ra,
Please provide the necessary constants when asking a question. In this case, we need the energy content of methane and the specific heat of water.
I'll show how this problem is solved, assuming the following constants:
Specific heat of combustion of methane = 55.5 kJ/g
Specific heat of water = 4.18 J/gC
Find the heat required to bring 750 ml (750 grams) of water from 15C to 100C. Please note that I'll not calculate the heat required for the phase transition from liquid to gas at 100C. This can be added by doing a similar calculation using the specific heat of vaporization of water, which was not provided in the question, so I'll assume the question does not include vaporization of the 100C water.
The heat required for heating 750g of water from 15C to 100C is:
q = cm(T2 - T1)
where q is the heat/energy, c is the specific heat (of water), and T2-T1 is the temperature change.
I chose a value of c = 4.18 J/gC, (the units are convenient).
Water's density is 1g/ml, so we have 750 grams of water.
The temperature change is 85C.
q = (4.18J/gC)*(750g)*(85C)
q = 266475J
or 266 kJ (3 sig figs)
I'll assume a heat of combustion for methane of 55.5 kJ/g. Please check this number with what you were given.
We need 266 kJ. So we'll need (266kJ/55.5kJ/g), or 4.80 grams of methane.
Bob
