To standardize a NaOH solution

CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases

Problem

To standardize a NaOH solution, a student dissolves

0.2544\,\text{g}

of solid KHP (molar mass

204.2\,\text{g mol}^{-1}

) and titrates it to a phenolphthalein end point. Because phenolphthalein changes color slightly after the equivalence point for this weak-acid/strong-base titration, the student adds

0.15\,\text{mL}

of excess NaOH beyond stoichiometric neutralization. What qualitative effect does this have on the molarity of NaOH calculated from the titration? (a) The calculated molarity is higher than the true molarity because the excess base is assumed to have reacted, increasing the moles of NaOH in the calculation. (b) The calculated molarity is lower than the true molarity because the extra

0.15\,\text{mL}

of NaOH makes the measured volume larger than it should be. (c) The calculated molarity is unaffected because only the precisely weighed mass of KHP appears in the calculation. (d) The calculated molarity could be higher or lower; the direction depends on the acid–dissociation constant of KHP.

Related theory

Browse the problem library