High School

A 50.0 mL sample of a 1.00 M solution of a diprotic acid \( \text{H}_2\text{A} \) ( \( \text{K}_{a1} = 1.0 \times 10^{-6} \) and \( \text{K}_{a2} = 1.0 \times 10^{-10} \) ) is titrated with 2.00 M NaOH.

What is the minimum volume of 2.00 M NaOH needed to reach a pH of 10.00?

Answer :

A 50. 0 ml sample of a 1.00 M solution of a diprotic acid is titrated with 2 M NaOH. The minimum volume of 2.00 m NaOH needed to reach a pH of 10 is 37.5 mL.

The molarity = 1 M

The volume = 50 mL

The moles of the diprotic acid = molarity × volume in L

The moles of the diprotic acid = 1 × 0.050

The moles of the diprotic acid = 0.050 mol

At the equivalence point, the moles of the diprotic acid is equals to the moles of the base = 0.050 mol.

At the 1st half equivalence:

pH = pka1 = 6

At the 2nd half equivalence :

pH = pka2 = 10

When the half reaction will occurs when we add 0.025moles of NaOH :

The total moles of NaOH = 0.075 moles

The volume of NaOH = 0.075 / 2

= 0.0375 L

= 37.5 mL

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