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 alkali metal (Group 1A) hydroxide → There is 1 OH^{–}

**MOH**

This means:

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

Solving for MM_{base}:

$\left(\mathbf{2}\mathbf{.}\mathbf{50}\mathbf{}\frac{\mathbf{mol}\mathbf{}\mathbf{HCl}}{\overline{)\mathbf{L}}}\right)\mathbf{(}\mathbf{17}\mathbf{.}\mathbf{0}\mathbf{}\overline{)\mathbf{mL}}\mathbf{\times}\frac{{\mathbf{10}}^{\mathbf{-}\mathbf{3}}\mathbf{}\overline{)\mathbf{L}}}{\overline{)\mathbf{mL}}}\mathbf{)}\mathbf{=}\mathbf{\left(}\frac{\mathbf{4}\mathbf{.}\mathbf{36}\mathbf{}\mathbf{g}}{\mathbf{MM}}\mathbf{\right)}$

$\mathbf{MM}\mathbf{=}\frac{\mathbf{4}\mathbf{.}\mathbf{36}\mathbf{}\mathbf{g}}{\mathbf{0}\mathbf{.}\mathbf{0425}\mathbf{}\mathbf{mol}}$** = 102.6 g/mol**

**Molar mass of metal hydroxide = molar mass of metal + molar mass O + molar mass H**

A 4.36-g sample of an unknown alkali metal hydroxide is dissolved in 100.0 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 17.0 mL of the hydrochloric acid solution has been added.*

*What is the identity of the alkali metal cation: Li ^{+}, Na^{+}, K^{+}, Rb^{+}, or Cs^{+}?*

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