Question

A)

Calculate the pH of a solution that is 0.270 M in sodium formate (HCOONa) and 0.130 M in formic acid (HCOOH).

Express your answer to two decimal places.

B)

Calculate the pH of a solution that is 0.510 M in pyridine (C5H5N) and 0.470 M in pyridinium chloride (C5H5NHCl).

Express your answer to two decimal places.

C)

Calculate the pH of a solution that is made by combining 55 mL of 0.060 M hydrofluoric acid with 125 mLof 0.110 M sodium fluoride.

Express your answer to two decimal places.

Answer 1

From Henderson Hasselbalch equation

pH = pKa + log[ salt]/[acid]

A). For acetic aced

Ka =1.8 *10^-4

pKa = -log Ka = -log(1.8*10^-4) = 3.74

So, pH = 3.74 + log (0.270/0.130) = 3.74 +0.317 = 4.1

B). pKb for this = 8.77

pOH = pK_b + log[C₅H₅NHCl]/ [C₅H₅N]

pOH = 8.77 + log (0.470/ 0.510) = 8.73

Now, pH = 14 - pOH = 14- 8.73 = 5.3

C) Moles of HF = Molarity * VOlume = 0.060*0.055L =0.0033 moles

Moles of NaF = 0.110M*0.125L = 0.01375 moles

Total volume = 125+55 =180mL = 0.180 L

[HF] = Moles/ Volume = 0.0033 moles/ 0.180L =0.0183 M

[NaF] = 0.01375/0.180L = 0.0764 M

pKa = 3.14

pH = 3.14+ log (0.0764/0.0183) = 3.8