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Answer:

pH = 6.20

Explanation:

The pKa of cacodylic acid is 6.

To solve this question we must use Henderson-Hasselbalch equation:

pH = pKa +log [A⁻] / [HA]

Where pKa is the pKa of the weak acid = 6

And [] could be taken as the moles of A⁻ the conjugate base, and HA, the weak acid.

The moles of the NaOH added to the solution of the weak acid are = Moles A⁻

And moles HA = Initial moles HA - Moles NaOH added

Initial moles HA:

0.0200L * (0.130mol / L) = 0.00260 moles

Moles NaOH:

0.0200L * (0.0800mol / L) = 0.00160 moles = [A⁻]

Moles HA =

0.00260 moles - 0.00160 moles = 0.00100 moles = [HA]

pH = 6 +log [0.00160 moles] / [0.00100 moles]

pH = 6.20

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The pH of the resulting solution is 1.6.

Let cacodylic acid be HA, mixing cacodylic acid and NaOH, the following occurs;

HA(aq) + NaOH(aq) ------> NaA(aq) + H2O(l)

Number of moles of NaOH = 0.0800 molL−1 × 20.0/1000 = 0.0016 moles

Number of moles of HA = 20.0/1000  × 0.130 = 0.0026 moles

We can see that the HA is in excess, Number of moles of excess acid =

0.0026 - 0.0016 = 0.001 moles

Total volume of solution = 20.0 mL + 20.0 mL = 40 mL or 0.004 L

Molarity of excess acid =  0.001 moles/0.004 L = 0.025 M

pH = -log[H^+]

pH = -log[0.025 M]

pH = 1.6

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