Question

1. Calculate the pH of a 0.1M solution of HClO3. Ka = 1x10^10.

2. Calculate the pH of a buffer solution prepared by mixing equal volumes of 0.2M NaHC2O2 and 0.1M Na2C2O2. Ka for H2C2O2 = 6.4x10^-5.

3. Calculate the pH of a solution obtained by dissolving 0.3M of CH3NH3OCN in 2L of water. Kb for CH3NH2 = 5x10^-4. Ka for HOCN = 3.5x10^-4.

Answer 1

1. Calculate the pH of a 0.1M solution of HClO3. Ka = 1x10^10.

solution :

HClO3 is strong acid it is known from its Ka value

[H+] = 0.1 M

pH = -log[H+] = -log (0.1)

pH = 1

2. Calculate the pH of a buffer solution prepared by mixing equal volumes of 0.2M NaHC2O2 and 0.1M Na2C2O2. Ka for H2C2O2 = 6.4x10^-5.

solution :

pKa = -log Ka = -log (6.4x10^-5.)= 4.194

pH = pKa + log [Na2C2O2]/[NaHC2O2]

pH = 4.194 + log (0.1 /0.2)

pH = 3.89

3. Calculate the pH of a solution obtained by dissolving 0.3M of CH3NH3OCN in 2L of water. Kb for CH3NH2 = 5x10^-4. Ka for HOCN = 3.5x10^-4.

solution : it is weak acid weak base salt . so pH depends on pKa and pKb

pKb = -log (5 x 10^-4) = 3.30

pKa = -log (3.5 x10^-4) = 3.46

pKa > pKb the the solution dominates basic character

pH = 7 + 1/2 [pKa -pKb]

pH = 7 + 1/2 (3.46-3.30)

pH = 7.08