Find the pH of a 0.25 M solution of sodium concentration all species present in

Question

Find the pH of a 0.25 M solution of sodium concentration all species present in the solution. K_a for A buffer is made by mixing 500.0 ml of 0.100 M nitrous acid HNO_2(aq) and 500.0 mL of 0.100 M aqueous potassium nitrite, KNO_2(aq). Nitrous acid is weak acid with K_a = a) What is the pH of the buffer solution? b) The lab technician mixing the buffer in part a) is trying to adjust the pH to what is needed for a particular experiment. He first adds 10.0 mL of 1.0 M HCl(aq) to the buffer and then adds 2.0 mL of 1.0 M NaOH (aq). What is the pH of the buffer solution at that point?

Explanation / Answer

1 a)

NaF concentration = 0.25 M

Ka of HF = 6.3 x 10^-4

[Na+] = 0.25 M

F- + H2O ------------------------> HF + OH-

0.25                                            0        0

0.25-x                                        x           x

Kb = [HF][OH-]/[F-]

1.0 x 10^-14 / 6.3 x 10^-4 = x^2 / 0.25-x

1.58 x 10^-11 = x^2 / 0.25-x

x^2 + 1.58 x 10^-11 x - 3.97 x 10^-12 =0

x = 1.99 x 10^-6

[F-] = 0.25M

[OH-] = 1.99 x 10^-6 M

[HF] = 1.99 x 10^-6 M

[H3O+] = 5.02 x 10^-9 M

pH = -log [H3O+]

pH = 8.30

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