During periods of intense exercise, the body converts pyruvate into lactate. Lac
ID: 787700 • Letter: D
Question
During periods of intense exercise, the body converts pyruvate into
lactate. Lactate, in the form of lactic acid, is responsible for the burning
sensation that occurs in overworked muscles.
10.00 mL blood samples were taken from a subject before and after
exercise. The samples were titrated by using 2.50 mM KOH. Titration data
for the samples are shown below. Assume that the only reactive species in
each sample is lactic acid (C3H6O3 = 90.0783 g/mol). Note: 1 mM = 1
millimolar = 1 x 10-3 M
KOH Titration Before exercise After exercise
Initial buret reading 4.75 mL 11.95 mL
Final buret reading 11.95 mL 49.35 mL
a. How much lactic acid in milligrams does the 10.00 mL blood sample
from the resting state contain?
b. How much lactic acid in milligrams does the 10.00 mL blood sample
from the post-exercise state contain?
Explanation / Answer
Let HA represent lactic acid
HA + KOH => KA + H2O
(a) Volume of KOH = 11.95 - 4.75 = 7.20 mL
Moles of HA = moles of KOH = volume x concentration of KOH
= 7.20/1000 x 2.50 x 10^(-3) = 1.8 x 10^(-5) mol
Mass of HA = moles x molar mass of HA
= 1.8 x 10^(-5) x 90.0783
= 0.00162 g = 1.62 mg
(b) Volume of KOH = 49.35 - 11.95 = 37.4 mL
Moles of HA = moles of KOH = volume x concentration of KOH
= 37.4/1000 x 2.50 x 10^(-3) = 9.35 x 10^(-5) mol
Mass of HA = moles x molar mass of HA
= 9.35 x 10^(-5) x 90.0783
= 0.00842 g = 8.42 mg
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