Recall that at the ** equivalence point** of a titration:

$\overline{){\mathbf{moles}}{\mathbf{}}{\mathbf{acid}}{\mathbf{=}}{\mathbf{moles}}{\mathbf{}}{\mathbf{base}}}$

Also, recall that:

**moles = molarity × volume****moles = molarity × molar mass**

**For acids:** Multiply by the number of *H ^{+} ions*

**For bases:** Multiply by the number of *OH ^{–}, H^{–}, O^{2–}, or NH_{2}^{–} ions*

This means:

$\overline{){\left(\mathbf{MV}\right)}_{{\mathbf{acid}}}{\mathbf{=}}{\mathbf{2}}\mathbf{\left(}{\mathbf{moles}}_{\mathbf{base}}\mathbf{\right)}}$

For Group 2A metal hydroxide → There are 2 OH^{–}

- M(OH)
_{2}

Solving for MM_{base}:

$\left(\mathbf{2}\mathbf{.}\mathbf{50}\mathbf{}\frac{\mathbf{mol}\mathbf{}\mathbf{HCl}}{\overline{)\mathbf{L}}}\right)\mathbf{(}\mathbf{56}\mathbf{.}\mathbf{9}\mathbf{}\overline{)\mathbf{mL}}\mathbf{\times}\frac{\mathbf{}{\mathbf{10}}^{\mathbf{-}\mathbf{3}}\mathbf{}\overline{)\mathbf{L}}}{\overline{)\mathbf{mL}}}\mathbf{)}\mathbf{=}\mathbf{2}\mathbf{\left(}\frac{\mathbf{8}\mathbf{.}\mathbf{65}\mathbf{}\mathbf{g}}{\mathbf{MM}}\mathbf{\right)}$

$\mathbf{MM}\mathbf{=}\frac{\mathbf{2}(8.65g)}{\mathbf{0}\mathbf{.}\mathbf{14225}\mathbf{}\mathbf{mol}}$** = 121.6 g/mol**

**Molar mass of metal hydroxide = molar mass of metal + 2(molar mass O) + 2(molar mass H)**

An 8.65-g sample of an unknown group 2A metal hydroxide is dissolved in 85.0 mL of water. An acid-base indicator is added and the resulting solution is titrated with 2.50 M HCl (aq) solution. The indicator changes color signaling that the equivalence point has been reached after 56.9 mL of the hydrochloric acid solution has been added.

What is the identity of the metal cation: Ca^{2+}, Sr^{2+}, Ba^{2+}?

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