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## Q. 4.13

When hydrofluoric acid, HF(aq), gets on the skin, it migrates away from the site of exposure and shuts down the microcapillaries that carry blood. The clinical management of burns on the skin from exposure to hydrofluoric acid may include the injection of a soluble calcium salt in the skin near the site of contact to stop the migration of HF(aq). As $F^{-}$ ions from the acid move through the skin, they combine with $Ca^{2+}$ ions and form deposits of insoluble $CaF_{2}$. If 1.00 mL of a 2.24 M aqueous solution of $Ca^{2+}$ is injected, what is the mass of $CaF_{2}$ produced if all the calcium ions react with fluoride ions?

## Verified Solution

Collect and Organize We are given the volume and concentration of a solution of $Ca^{2+}$ ions and asked to determine the maximum amount of $CaF_{2}$ precipitate that could be formed upon reaction with $F^{-}$ ions.

Analyze The net ionic reaction for the process involved is

$Ca^{2+}(aq) + 2 F^{-}(aq) → CaF_{2}(s)$

The problem states that all the calcium ions react, so we calculate the mass of product, assuming that $Ca^{2+}$ ions are the limiting reactant. Solve We can calculate the number of moles of $Ca^{2+}(aq)$ and use the stoichiometric relationships in the net ionic equation to determine the mass of product. We will also need the molar mass of $CaF_{2}$:

$\mathscr{M}_{CaF_{2}}=40.08 g/mol +2(19.00 g/mol) = 78.08 g/mol CaF_{2}$

$1.00 \times 10^{-3} \sout{L Ca^{2+}}\times \frac{2.24 \sout{mol Ca^{2+}}}{1 \sout{L Ca^{2+}}} \times \frac{1 \sout{mol CaF_{2}}}{1 \sout{mol Ca^{2+}}} \times \frac{78.08 g CaF_{2}}{1 \sout{mol Ca^{2+}}}=0.175 g CaF_{2}$

Think About It This problem asks for the mass of $CaF_{2}$ that could be formed, assuming that $Ca^{2+}$ ions were the limiting reactant. However, when this treatment is actually used, it is more likely that the $Ca^{2+}$ ions will be in excess to make sure all the fluoride ions are consumed.