The temperature of 500.0 g of water contained in a calorimeter increased from 20.0 °C to 50.0 °C when 5.00 g of fuel was burned to completion. What is the ΔH of this combustion reaction? (The specific heat of water is 4.184 J/g°C and the molecular weight of the fuel is 40.0 g/mol.)

The ΔH of this combustion reaction is - 502 kJ/mol

Explanation:

Step 1: Data given

Mass of water = 500.0 grams

Mass of fuel = 5.00 grams

Initial temperature = 20.0 °C

Final temperature = 50.0°C

The specific heat of water is 4.184 J/g°C

Molecular weight of the fuel is 40.0 g/mol

Step 2: Calculate the heat transfer

q = m*c * ΔT

⇒ with m = the mass of water = 500 grams

⇒ with c = the specific heat of water = 4.184 J/g°C

⇒ with ΔT = the change in temperature = T2 - T1 = 50.0°C - 20.0°C = 30.0°C

q = 500 * 4.184 * 30

q = 62760 J

Step 3: Calculate moles

moles = mass / molar mass

Moles = 5 grams / 40 g/mol

moles = 0.125 moles

Step 4: Calculate ΔH of this combustion reaction

ΔH = 62760 / 0.125 mol

ΔH = 502080 J/mol = 502 kJ / mol (negative because it's an exothermic reaction)

ΔH = - 502 kJ/mol