Question

In blood, pH is buffered to a constant value of 7.4 During exercise, your muscles generate lactic acid, HC3H50, (K = 1.4x10-4). Will the lactic acid be mostly protonated, mostly deprotonated, or in roughly equal concentrations? a) It will be mostly protonated (lactic acid) b) It will be mostly deprotonated (lactate) c) It will exist in roughly equal concentrations of lactic acid and lactate d) The lactic acid/lactate ratio cannot be determined from this information OLISCHIC SCI SCHC COLCHONCO HSU 010 A

Answer 1

In blood, pH is buffered to a constant value of 7.4 During exercise, your muscle generate Lactic acid, HG₂ H 5 Oz (ka= 1.48 10-4). Will the Lactic acid be mostly protonated, mostly depretanated er in oroughly equal concentration Correct answer = (b): It will be mostly deprotonated Clactate). solution : Far must solving know this type of problem; you Henderson - Hasselbalch equation. pH = pka + [A ] [HA] - i Where pka of any general acid [A] = deprotonated form [HA] = protonated form of A of Acid Acid A A so for solving this question, first we calculated pka of acid (Lactic acid) using this formula ka = 1.4 X 10-4

(1) pka = -log ka pka : -log(1.4 x 10-4) Ipka = 3.85 since pH of blood = 7.4 Now we used equation (i) to calculated the ratio of [A]/[HA] pH = pka + log but the value of pH and pka. (where 7.4 = 3.85 + log [A] [HA] = HC3 H5Ogh [A] = C3 HSO3 log [A]. 7.4 - 3.85 log SA 355 s log y = x 1 I then y=10* [A]. 103.55 THA) [[A] = 103.55, [HAJI HA = Lactic Acid L A = Lactate [HA] [HA] [HA] s where No

This means the concentration of bo deprotonated Acid (CA']) is 103-55 times to concentrated of protonated acid farm. so deprotonated form of lactic acid is mainly exists in pH = 7.4. correct option = (b) = It will be mostly deprotonated (Lactate) so I
summary:

For solving this type of problem, you must know Henderson-Hasselbalch equation

pH = pKa + log ( [deprotonated form of acid] /[protonated form of acid] )

Since pKa and pH is given. So we calculate ratio of deprotonated to protonated form of acid. Then we tell which form of acid mainly exist at given pH.

Correct answer = b)

Important point:

pH=pKa : both form acid have equal concentration

pH>pKa : deprotonated form of acid mainly exist.

pH<pKa : protonated form of acid mainly exist.