Question

Part A

At a pH of 7.40, what is the ratio of the molar concentrations of PO43− to HPO42−? Express your answer using two significant figures.

Part B

At a pH of 7.40, what is the ratio of the molar concentrations of PO43− to HPO42−?

Express your answer using two significant figures.

Part C

At a pH of 7.40, what is the ratio of the molar concentrations of H2PO4− to H3PO4?

Express your answer using two significant figures.

Part D

At a pH of 7.40, what is the ratio of the molar concentrations of CO32− to HCO3−?

Express your answer using two significant figures.

Part E

At a pH of 7.40, what is the ratio of the molar concentrations of HCO3− to H2CO3?

Express your answer using two significant figures.

Part F

Normal metabolic processes produce significant amounts of acid, sometimes up to 10 mol/day, in the human body. Thus it is desirable that the buffer system for blood contain more conjugate base than acid.

Which conjugate acid-base pair is most responsible for maintaining the pH of blood?

[HPO42−]/[H2PO4−]

[H2PO4−]/[H3PO4]

[HCO3−]/[H2CO3]

[CO32−]/[HCO3−]

[PO43−]/[HPO42−]

Answer #1

To calculate the relations of conjugate pairs of acid / base, the Henderson-Hasselbach equation is used:

pH = pKa + Log [Base] / [Acid]

It is cleared of the form:

[Base] / [Acid] = 10 ^ pH-pKa

and it is calculated:

A) [PO4-3] / [HPO4-2] = 10 ^ (7.4-12.32) = 1.20x10 ^ -5

B) [PO4-3] / [HPO4-2] = 10 ^ (7.4-12.32) = 1.2x10 ^ -5

C) [H2PO4 -] / [H3PO4] = 10 ^ (7.4-2.12) = 1.9x10 ^ 5

D) [CO3-2] / [HCO3-] = 10 ^ (7.4-10.32) = 1.2x10 ^ -3

E) [HCO3 -] / [H2CO3] = 10 ^ (7.4-6.37) = 1.1x10 ^ 1

F) The buffer that maintains the pH of the most important blood is the one formed by: [HCO3 -] / [H2CO3].

Calculate the pH and the equilibrium concentrations of H2PO4-,
HPO42- and PO43- in a 0.0353 M aqueous phosphoric acid solution.
For H3PO4, Ka1 = 7.5×10-3, Ka2 = 6.2×10-8, and Ka3 = 3.6×10-13
pH = ?
[H2PO4-] = ?M
[HPO42-] = ?M
[PO43-] = ?M

The pH of blood plasma is 7.40. Assuming the principal buffer
system is HCO3-/H2CO3, calculate the ratio [HCO3-]/[H2CO3]. Is this
buffer more effective against an added acid or an added base?

The Henderson-Hasselbalch equation relates the pH of a buffer
solution to the pKa of its conjugate acid and the ratio of
the concentrations of the conjugate base and acid. The equation is
important in laboratory work that makes use of buffered solutions,
in industrial processes where pH needs to be controlled, and in
medicine, where understanding the Henderson-Hasselbalch equation is
critical for the control of blood pH.
Part A
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HPO42-
6.2×10-8
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CO32-
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6.4×10-5
4.19
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HPO42-
6.2×10-8
7.21
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CO32-
4.8×10-11
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Conjugate Base
Ka
pKa
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C2O42-
6.4 x 10-5
4.19
H2PO4-
HPO42-
6.2 x 10-8
7.21
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CO32-
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HPO42-
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CO32-
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