Question

Calculate the fraction of pentachlorophenol (PCP) as a neutral species in a raindrop (pH = 4.0) and in lake water (pH = 8.0). For PCP, pKia = 4.75.

Answer 1

PCP <-> H+ + PCP-

so.

easier to get with Henderson Haselbach equations

pH = pKa + log(A-/HA)

i.e. PCP vs PCP -

so

at pH = 4., raindrop

4 = 4.75 + log(PCP- / PCP )

10^(4-4.75) = ratio

0.1778 = -PCP / PCP

0.1778 PCP = -PCP

assume 1 mol of PCP so

-PCP = 0.1778

fraction of NEUTRAL PCP = 1/(1+0.1778) = 0.8490

for

lake water

pH = 8

8 = 4.75 + log(PCP- / PCP )

10^(8-4.75) = ratio

1778.279 = (PCP- / PCP )

1778.279 PCP = PCP-

assume 1 mol of PCP

PCP- = 1778.279

so

PCP = 1/(1778.279 +1) = 0.0005620

Neutral fraction PCP = 0.0005620