Write charge and mass balances for aqueous ${\mathbf{Ca}}_{\mathbf{3}}{\left({\mathrm{PO}}_4\right)}_{\mathbf{2}}$ if the species are ${\mathbf{Ca}}^{\mathbf{2}\mathbf{+}}\mathbf{,}{\mathbf{CaOH}}^{\mathbf{+}}\mathbf{,}{\mathbf{CaPO}}_{\mathbf{4}}^{\mathbf{-}}\mathbf{,}{\mathbf{PO}}^{\mathbf{3}\mathbf{-}}\mathbf{,}{\mathbf{HPO}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\mathbf{,}{\mathbf{H}}_{\mathbf{2}}{\mathbf{PO}}_{\mathbf{4}}^{\mathbf{-}}\mathbf{,}{\mathbf{H}}_{\mathbf{3}}{\mathbf{PO}}_{\mathbf{4}}$.

Chemical equations for mass and charge are frequently balanced in aqueous solutions. On both sides of the equation, balancing for mass yields the same number and types of atoms. When the net charge on both sides of the equation is zero, the equation is said to be balanced for charge.

Mass balance: All species in a solution containing a certain atom (or group of atoms) must be equal to the amount of that atom (or group) given to the solution.

Given

Aqueous${\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2$

Species:${\mathrm{Ca}}^{2+},{\mathrm{CaOH}}^{+},{\mathrm{CaPO}}_4^{-},{\mathrm{PO}}^{3-},{\mathrm{HPO}}_4^{2-},{\mathrm{H}}_2{\mathrm{PO}}_4^{-},{\mathrm{H}}_3{\mathrm{PO}}_4$

Now charge balance.

$2\left[{\mathrm{Ca}}^{2+}\right]+\left[{\mathrm{CaOH}}^{+}\right]+\left[{\mathrm{H}}^{+}\right]=\left[{\mathrm{CaPO}}_4^{-}\right]+3\left[{\mathrm{PO}}_4^{3-}\right]+2\left[{\mathrm{HPO}}_4^{2-}\right]+\left[{\mathrm{H}}_2{\mathrm{PO}}_4^{-}\right]\left[{\mathrm{OH}}^{-}\right]$

By the definition of charge balancerole="math" localid="1654834648341" $\underset{}{\underset{\mathrm{species}\mathrm{containing}\mathrm{calcium}}{2\left\{\underbrace{\left[{\mathrm{Ca}}^{2+}\right]+\left[{\mathrm{CaOH}}^{+}\right]+\left[{\mathrm{CaPO}}_4^{-}\right]}\right\}}}$

$=\underset{\mathrm{species}\mathrm{containing}\mathrm{phosphate}}{\underbrace{3\left\{\left[{\mathrm{CaPO}}_4^{-}\right]+\left[{\mathrm{PO}}_4^{3-}\right]+2\left[{\mathrm{HPO}}_4^{2-}\right]+\left[{\mathrm{H}}_2{\mathrm{PO}}_4^{-}\right]\left[{\mathrm{H}}_3{\mathrm{PO}}_4\right]\right\}}}$