The enthalpy of combustion of benzoic acid is –26.4 kJ/g. What is the enthalpy of

combustion expressed in joules per kilogram?

-26.4 kJ/g x (1000 J/kJ) x (1000g/kg)

Could you explain how or why ?

I don't know that there is any explaining to do. To change kJ to J you multiply by 1000. There are 1000 g in a kg. Look at the dimensions. kJ in numerator cancel with kJ in the denominator to leave J/g after the first term. The second term cancels g and leave kg.

To convert the enthalpy of combustion from kilojoules per gram (kJ/g) to joules per kilogram (J/kg), we need to use conversion factors.

1 kilojoule (kJ) = 1000 joules (J)
1 gram (g) = 0.001 kilograms (kg)

Given that the enthalpy of combustion of benzoic acid is -26.4 kJ/g, we can now calculate the enthalpy of combustion in J/kg:

Step 1: Convert kJ to J:
-26.4 kJ/g x 1000 J/kJ = -26,400 J/g

Step 2: Convert grams to kilograms:
-26,400 J/g x 0.001 kg/g = -26.4 J/kg

Therefore, the enthalpy of combustion of benzoic acid is -26.4 J/kg.