Question

1) A 500.00 mL buffer solution contains 0.100 M in acetic acid (HC2H3O2) and 0.250 M sodium acetate (NaC2H3O2). pKa for acetic acid is 4.74. Write the chemical equation(s) and determine

A) the initial pH of the solution?
B) the pH after addition of 0.0050 mol HCl (assume volume changes to be negligible after addition of HCl)?

2)Amphetamine (C9H13N) is a weak base with a pkb of 4.20. Calculate the pH of a solution containing a amphetamine concentration of 265 mg/L.

Answer 1

1) The given solution is a buffer of sodium acetate and aceitc acid:

(A)

Total volume of the solution = 500 mL

Molarity of the acetic acid = 0.1 M

Molarity of the sodium acetate = 0.25 M

The acetic acid + sodium acetate is a buffer solution with the an equilibrium maintained between the acetate ions.

The pH of the solution is measured using the Henderson-Hasselbach equation:

[base] = 0.25 M

[acid] = 0.1 M

pK_a = 4.74

(B)

When acid is added to the solution, the value [acid] changes.

New Value of acid = [Acetic Acid] + [HCl]

Molarity of HCl , [HCl] = no. of moles added / volume of the solution

no. of moles added = 0.005 moles

Volume = 0.5 L

[HCl] = 0.005 / 0.5 = 0.01

New Value = 0.1 + 0.01

[acid] = 0.11

Therefore,

pH:

2)

Amphetamine has a pK_b of 4.20.

Molar mass of amphetamine = 135.21 g / mol

Given: 265 mg / L or 0.265 g / L

Number of moles in 265 mg = 0.265 / 135 = 0.0019

Let x be the number of moles of C₉H₁₄N⁺ and OH^- formed.

Therefore:

0.0019 X X

pK_b = - logK_b

K_b = 6.3 x !0^-5

But:

Therefore = [OH^-] = 3.31 x 10^-4

But: