Discuss how to prepare 250.0 mL of an ammonium-ammonia buffer (p_Kb, = 4.70, pH

Question

Discuss how to prepare 250.0 mL of an ammonium-ammonia buffer (p_Kb, = 4.70, pH = 9 and total concentration of 0.05 M) using 0.10 M ammonium chloride and 0.10 M aqueous ammonia. NH^+_4 (aq) + H_2O(l) NH_3(aq) + H_3O^+(aq) Assume that the normal blood buffer contains 0.00080 M carbonic acid and 0.0085 M hydrogen carbonate; the pK_a = 6.35 for carbonic acid and the volume of blood in the body is 7.00 L. The blood pH, due to disruption, is now 7.20. What is the ratio of [HCO_3^-]/[H_2CO_3] now that the blood has been challenged? How many moles of hydrogen carbonate must be added to the blood to bring the carbonic acid/hydrogen carbonate ratio back to a normal pH = 7.4? How many milliliters of 0.10 M hydrogen carbonate must be added to the blood? H_2CO_3(aq) + H_2O(l) HCO^-_3 (aq) + H_3O^+(aq)

Explanation / Answer

5)

total buffer moles = 250 x 0.05 / 1000 = 0.0125

pH = 9

pOH = 5

pKb = 4.70

pOH = pKb + log [salt / base]

5 = 4.70 + log [NH4Cl / NH3]

[NH4Cl / NH3] = 1.995

[NH4Cl + NH3] = 0.0125

1.995 NH3 + NH3 = 0.0125

moles of NH3 = 4.17 x 10^-3

moles of NH4Cl = 8.33 x 10^-3

concentration of NH3 = 0.1

concentratiion of NH4Cl = 0.1

volume of ammonia = 41.7 mL

volume of NH4Cl = 83.26 mL

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