Question

(a) Carbonate buffers are important in regulating the pH of blood at 7.40. what is the concentration ratio of CO2 (usually written H2CO3) to HCO3-in blood at pH = 7.40? H2CO3(aq)-HCO3-(aq) + H+(aq) Ka = 4.3 × 10-7 [H2CO3] / [HCO3-]- (b) Phosphate buffers are important in regulating the pH of intracellular fluids at pH values generally between 7.1 and 7.2. What is the concentration ratio of H2Po4 to HPO2 in intracellular fluid at pH = 7.15? H2PO4(aq)HPO42(aq)H(aq) Ka = 6.2 × 10-8

Answer 1

Answer:

a) H₂CO₃ (aq) $\rightleftharpoons$ HCO₃^-(aq) + H⁺ (aq)

   Acid                  Conjugate base

pH of the buffer is given by Henderson-Hasselbalch equation as,

pH = pKa + log([Conjugate base]/[Acid])

For H₂CO₃ and HCO₃^- buffer,

pH = pKa (H₂CO₃) + log([HCO₃^-]/[H₂CO₃]) ..... (1)

Given data: pH of blood = 7.40,

Ka = 4.3*10^-7. $\Rightarrow$ pKa = -logKa = -log(4.3*10^-7) = 6.37

Placing these values in eq.(1)

7.40 = 6.37 + log([HCO₃^-]/[H₂CO₃])

log([HCO₃^-]/[H₂CO₃]) = 7.40-6.37

log([HCO₃^-]/[H₂CO₃]) = 1.03

[HCO₃^-]/[H₂CO₃] = 10^1.03.

[HCO₃^-]/[H₂CO₃] = 10.7

On reciprocal,

[H₂CO₃] / [HCO₃^-] = 1 / 10.7

[H₂CO₃] / [HCO₃^-] = 0.09

Is the required ratio.

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b) H₂PO₄^- (aq) $\rightleftharpoons$ HPO₄^2- (aq) + H⁺ (aq)

   Acid                  Conjugate base

pH of the buffer is given by Henderson-Hasselbalch equation as,

pH = pKa + log([Conjugate base]/[Acid])

For H₂PO₄^- (aq) and HPO₄^2- (aq)

pH = pKa (H₂CO₃) + log([HPO₄^2-]/[H₂PO₄^-]) ..... (1)

Given data: pH of blood = 7.15,

Ka = 6.2*10^-8. $\Rightarrow$ pKa = -logKa = -log(6.2*10^-8) = 7.21

Placing these values in eq.(1)

7.15 = 7.21 + log([HPO₄^2-]/[H₂PO₄^-])

log([HPO₄^2-]/[H₂PO₄^-]) = 7.15-7.21

log([HPO₄^2-]/[H₂PO₄^-]) = -0.06

[HPO₄^2-]/[H₂PO₄^-] = 10^-0.06.

[HPO₄^2-]/[H₂PO₄^-] = 0.87

On reciprocal,

[H₂PO₄^2-]/[HPO₄^2-] = 1/0.87

[H₂PO₄^2-]/[HPO₄^2-] = 1.15

Is the required ratio.

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