# Problem: 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+?

###### FREE Expert Solution

Recall that at the equivalence point of a titration:

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 MMbase:

= 102.6 g/mol

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

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###### Problem Details

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+?