Question

Determine the pH (to two decimal places) of the following solutions. If the 5% approximation is valid, use the assumption to compute pH.

8.72×10^-3 M sodium hydrogen sulfate

pKa hydrogen sulfate ion is 1.92

8.73×10^-3 M trifluoroacetic acid

pKa trifluoroacetic acid is 0.20

8.20×10^-3 M ethylamine

pKa ethylammonium ion is 10.87

2.51×10^-1 M sodium nitrite

pKa nitrous acid is 3.37

Answer 1

sodium hydrogen sulfate:

P^H = 1/2P^ka-1/2 log[C]

P^H = 1/2x 1.92- 1/2log[8.72x10^-3]

P^H = 0.96 - 1/2[log8.72-3]

P^H = 0.96- 1/2 [0.9405-3]

P^H = 0.96+1.029

P^H =1.98

Trifluoroacetic acid:

pKa trifluoroacetic acid is 0.20

concentration 8.73×10-3 M

P^H = 1/2P^ka-1/2 log[C]

P^H = 1/2x 0.20- 1/2log[8.73x10^-3]

P^H = 0.1 - 1/2[log8.73-3]

P^H = 0.1- 1/2 [0.9410-3]

P^H = 0.1+1.029

P^H = 1.129

Ethylamine:

pKa ethylammonium ion is 10.87

concentration 8.20×10-3 M

P^H = 1/2P^ka-1/2 log[C]

P^H = 1/2x 10.87- 1/2log[8.20x10^-3]

P^H = 0.1 - 1/2[log8.20-3]

P^H = 0.1- 1/2 [0.9138-3]

P^H = 0.1+1.043

P^H = 1.143

sodium nitrite:

pKa nitrous acid is 3.37

concentration 2.51×10-1 M

P^H = 1/2P^ka-1/2 log[C]

P^H = 1/2x 3.37- 1/2log[2.51x10^-1]

P^H = 1.685 - 1/2[log2.51 -1]

P^H =1.685- 1/2 [0.3996-1]

P^H = 1.685+0.3002

P^H = 1.985