How much energy is released when 5.28g of acetone freezes?

The enthalpy of fusion (ΔHf) of acetone is 97.99 J / g. Means 97.99 J of energy is required to change solid acetone into liquid acetone. If you want to solidify acetone from liquid then the same amount of energy must be withdrawn (exothermic) from the system, Hence the energy required to be withdrawn from system to solidify acetone from liquid state is - 97.99 Jg⁻¹.

Hence,

For 1 g of Acetone = 97.99 J energy was withdrawn

Then,

For 5.28 g of Acetone = X J wnergy must be withdrawn

Solving for X,

X = (5.28 g * 97.99 J) : 1 g

X = 517.38 J

As the reaction is exotherimic, a negative sign is used. So,

X = - 517.38 J

Or,

X = 0.517 kJ