Mg atomic mass is taken as 24. The first and second ionisation enthalpies of $Mg$ are 738 and $1451 kJ mo l^{- 1}$ . Calculate the total energy, in kJ, needed to convert 4.8 g of gaseous $Mg$ atoms completely into gaseous $Mg X^{2 +}$ ions. Give the answer to the nearest integer.

Numerical answer type, answer in kJ, tolerance 0.5.

Answer: 438 kJ

Moles of Mg = 4.8/24 = 0.20 mol. Energy per mole to form

Mg X^{2 +}

= 738 + 1451 = 2189 kJ. Total = 0.20 x 2189 = 437.8, i.e. about 438 kJ.