The density of a monobasic strong acid (Molar mass 24 . 2gmol ) is...

millimole of $NaOH = 0 . 24 \times 25$
For nuetralisation, equal moles of monoacidic base and monobasic acid must be present:

$∴$ millimole of acid $= 0 . 24 \times 25$

$⇒$ mass of acid $= 0 . 24 \times 25 \times 24 . 2g$

for pure acid,

$V = \dfrac{w}{d}; ( d = 1 . 21kg / L = 1 . 21g / ml )$

$∴ V = \dfrac{0 . 24 \times 25 \times 24 . 2}{1 . 12} \times {10}^{- 3}$

$= 120 \times {10}^{- 3}ml$

$= 12 \times {10}^{- 2}ml$