Calculate the number of magnesium ions present in 0.51 g of magnesium chloride.

Molar mass of MgCl₂ = 24 + 2$\times$35.5 = 95 g mol^-1
95 g of MgCl₂ contains = 6.022$\times$10²³ molecules of MgCl₂
0.51 g of MgCl₂ contains = $\frac{6.022\times {10}^{23}}{95}\times 0.51$

                                             = 0.0323 $\times$10²³ molecules
Number of ions of magnesium in MgCl₂ = 1 ion
Therefore number of Mg²⁺ ions in 0.51 g of MgCl₂ = 1$\times$0.0323$\times$10²³ magnesium ions.