A certain chemical reaction runs very slow at high pH values and very fast at lo

Question

A certain chemical reaction runs very slow at high pH values and very fast at lower pH values. To study the reaction, a chemist needs to buffer the solution at a basic pH. What would be the pH of a buffer solution that had the following concentrations: 0.42 M NH_4Cl and 0.75 M NH_3 = 1.8 times 10^-5 at 25 degree C. 9.00 4.49 9.51 11.57 To culture a certain bacteria a microbiologist would like to buffer the media at a pH of 3.75. To maximize t efficiency of the system a 1:1 ratio of acid to salt will be used. Which of the following acids would make th choice for the buffer? Acetic acid; K_a = 1.8 times 10^-5 Propanoic acid; K_a = 1.3 times 10^-5 Formic acid; K_a = 1.8 times 10^-4 Nitrous acid; K_a = 4.0 times 10^-4

Explanation / Answer

The Henderson-Hasselbach equation for base is:

pOH = pKb + log [NH4Cl]/[NH3]

Given: Kb = 1.8*10^-5

pKb = -log[Kb]= -log [1.8*10^-5] = 4.74

pOH = 4.74 + log[0.42/0.75] = 4.49

pH + pOH = 14

pH = 14 -pOH = 14-4.49 = 9.51

Ans (3) pH = 9.51

b) Again, as per Henderson-Hasselbach eq for acids:

pH = pKa + log[conjugate base]/[acid]

it is given that the salt and acid are in 1:1 ratio

hence the above equation becomes:

pH = pKa

Thus, pKa = 3.75

pKa = -log[Ka]

Ka = 10^-pKa = 10^-3.75 = 1.8*10^-4

Formic acid would be the choice for buffer

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