A solution of magnesium chloride that is 5.10% magnesium by mass has a density $1.17 \mathrm{g}/\mathrm{mL}$. How many moles of${\mathrm{CI}}^{-}$ ions are in 300 mL of the solution?

Mass is volume times density. Here, the 5.10% magnesium by mass is given. Thus, by this relation of mass volume and density, the number of mass can be given as,

n=$300\times 1.17\times \frac{5.1}{100}\times \frac{1}{24}\times 2$

n=1.49.