A 75 mL solution that is 0.10 M in HC_2H_3O_2 and 0.10 M in NaC_2H_3O_2 has a pH

Question

A 75 mL solution that is 0.10 M in HC_2H_3O_2 and 0.10 M in NaC_2H_3O_2 has a pH of 4.74. Which of the following actions will change the pH of this solution? (I) Adding 15 mL of 0.10 M HCI; (II) Adding 0.010 mol of NaC_2H_3O_2; (III) Diluting the solution from 75 mL to 125 mL (A) I only (B) II only (C) I and II only (D) I, II and III If Fe(OH)2 saturated solution has a pH of 8.75, what is the molar solubility and Ksp? If the pH is increased to 10.00, how does this effect the molar solubility? If Base B has 0.16 M and Kb value of 7.15 times 10^-5, find the pH.

Explanation / Answer

1) the initial pH = 4.74

[Acid] = 0.1

Moles of acid = 0.1 X 75 = 7.5 millimoles

[salt] = 0.1

Moles of salt = 0.1 X 75 = 7.5 millimoles

I ) on adding 15mL of 0.1 M HCl

The moles added= 15 X 0.1 = 1.5 millimoles

So it will increase the concentration of acid and will decrease the concentration of salt by the same moles

[acid] = 7.5+1.5 = 9 milimoles / 90 mL

[salt] = 7.5 - 1.5 = 6 millimoles / 90mL

pH = pKa + log [salt] / [acid] = 4.74 + log 6/9 = 4.74 - 0.18 = 4.56 (slightly changed)

II ) on adding salt the pH will increase

III) On diluting the solution there will be no change on the pH

2) Fe(OH)2 --> Fe+2 + 2OH-

Ksp = [Fe+2] [ OH-]^2

pH = 8.75 , pOH = 14 - 8.75 = 5.25

OH- = 5.62 X 10^-6

[Fe+2] = 5.62 X 10^-6 / 2 = 2.81 X 10^-5

solubility = 2.81 X 10^-5

Ksp = 2.81 X 10^-5 X (5.62 X 10^-6)^2 = 88.75 X 10^-17 = 8.875 X 10^-16

If the pH is increased, the OH- concentration also increases,

The Ksp will remain the same

So the solubility will decrease.

3) The relation between OH- and Kb for weak base is

[OH-] = (Kb X concentration )1/2

[OH-] = (7.15 X 10^-5 X 0.16)1/2

[OH-] = 3.38 X 10^-3

pOH = -log[OH-] = 2.47

pH = 14- pOH = 11.53

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