<u>Answer:</u> The molar mass of monoprotic acid is 126.1 g/mol
<u>Explanation:</u>
To calculate the molarity of solution, we use the equation:

Molarity of KOH solution = 0.300 M
Volume of solution = 23.00 mL
Putting values in above equation, we get:

The chemical equation for the reaction of monoprotic acid and KOH follows:

By Stoichiometry of the reaction:
1 mole of KOH reacts with 1 mole of HA
So, 0.0069 moles of KOH will react with =
moles of HA
To calculate the molar mass for given number of moles, we use the equation:

Given mass of HA = 0.870 g
Moles of monoprotic acid = 0.0069 mol
Putting values in above equation, we get:

Hence, the molar mass of monoprotic acid is 126.1 g/mol