Question

Calculate the fraction of glycine carboxyl group that is unprotonated at pH = 2.8? (carboxyl pka is 2.3)

Answer 1

pH = pKa + log {RCOO-/RCOOH}
2.8 = 2.3 + log {RCOO-/RCOOH}
log {RCOO-/RCOOH} = 0.5
RCOO-/RCOOH = 3.16
RCOO - = 3.16* RCOOH
fraction = RCOO- / (RCOO- + RCOOH)
                 = 3.16* RCOOH / (3.16* RCOOH + RCOOH)
                = 3.16/4.16
                = 0.76
Answer: 0.76