use the Henderson-Hasselbalch equation, ie
pH = pka + log ([salt]/[acid])
here [salt] is 0.75*1 ie 0.75 (as disassociation is only 75%)
[acid] is 0.2
hence
pH = -log(2.4*10-4) + log ( 0.75/0.2)
pH = 4 - log (2.4) + log (37.5)
pH = 4 - 0.380 + 0.574
pH = 4.194
Moderation Team
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