(a) The total carbon dioxide content (H_2CO_3 + HCO_3^-) in a blood sample is de
ID: 514527 • Letter: #
Question
(a) The total carbon dioxide content (H_2CO_3 + HCO_3^-) in a blood sample is determined by acidifying the sample and measuring the volume of CO_2 evolved with a Van Slyke manometric apparatus. The total concentration was determined to be 28.5 mmol/L. The blood pH at 37 degree C was determined to be 7.48. What are the concentrations of H_2CO_3 and HCO_3^- in the blood? H_2CO_3 H^+ + HCO_3^- pKa_1 = 6.1 (b) What weights of NaH_2PO_4 and Na_2HPO_4 would be required to prepare 1 L of a buffer solution of pH 7.45 that has an ionic strength of 0.100? Note that pKa values for H_3PO_4 are 1.96, 7.12, and 12.32.Explanation / Answer
Answer to Part A
H2CO3 <==> HCO3- + H+
Using Henderson Hesselbalch equation:
pH = pKa + log [HCO3-] / [H2CO3]
7.48 = 6.1 + log [HCO3-] / [H2CO3]
log [HCO3-] / [H2CO3] = 1.38
[HCO3-] / [H2CO3] = 101.38 = 23.98
[HCO3-] + [H2CO3] = 28.5 mmol L-1 (Given)
[HCO3-] = 23.98 X [H2CO3]
28.5 - [H2CO3] = 23.98 X [H2CO3]
28.5 = 24.98 X [H2CO3]
[H2CO3] = 1.14 mmol L-1
[HCO3-] + 1.14 = 28.5
[HCO3-] = 27.36 mmol
Answer to Part B
NaH2PO4 and Na2HPO4 will be present around second equivalence point
H2PO4- <==> HPO4-2 + H+
Using Henderson Hesselbalch equation:
pH = pKa + log [HPO4-2] / [H2PO4-]
7.45 = 7.12 + log [HPO4-2] / [H2PO4-]
log [HPO4-2] / [H2PO4-] = 0.33
[HPO4-2] / [H2PO4-] = 100.33
= 2.14
[HPO4-2] = 2.14[H2PO4-]
Total ionic strength = 0.1
[HPO4-2] + [H2PO4-] = 0.1
2.14[H2PO4-] + [H2PO4-] = 0.1
3.14[H2PO4-] = 0.1
[H2PO4-] = 0.032 M
[HPO4-2] + 0.032 = 0.1
[HPO4-2] = 0.068 M
mass of NaH2PO4 = number of moles X Molar mass
= molarity X Volume X Molar mass
= 0.032 M X 1 L X 120 g mol-1
= 3.84 g
mass of Na2HPO4 = number of moles X Molar mass
= molarity X Volume X Molar mass
= 0.068 M X 1 L X 142 g mol-1
= 9.66 g
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