Question

Calculate the pH of the solution that results from each of the following mixtures.

Part A

160.0 mL of 0.25 M  HF with 220.0 mL of 0.30 M  NaF

Express your answer using two decimal places.

Part B

170.0 mL of 0.10 M C2H5NH2 with 265.0 mL of 0.20 M C2H5NH3Cl

Express your answer using two decimal places.

Answer 1

A)

mixture ofHF and NaF act as acidic buffer

pH = pKa + log [NaF] / [HF]

[NaF] = (220 x 0.3 /220 + 160) = 0.174 M

[HF] = (160 x 0.25 / 160 + 220) = 0.143 M

pKa of HF = 3.17 standard value

pH = 3.17 + log [0.174] / [0.143]

pH = 3.25

B) mixture of C2H5NH2 and C2H5NH3Cl act as basic buffer

pOH =pKb + log [C2H5NH3Cl] / [C2H5NH2]

[C2H5NH3Cl] = (265 x 0.2 / 265 + 170) = 53 / 435 = 0.122 M

[C2H5NH2] = (170 x 0.1 / 170 + 265) = 0.039 M

pKb of C2H2NH2 = 3.3 standard value

pOH = 3.3 + log [0.122] / [0.039]

pOH = 3.79

pH = 14 - 3.79

pH = 10.21