Acids and Bases: Buffers

How are buffers made?

A buffer must contain a weak acid and its conjugate base. There are several ways a solution containing these two components can be made:

Buffers can be made from weak acids or base and their salts.
For example, if 12.21 grams of solid sodium benzoate are dissolved in 1.00 L 0.100 M benzoic acid (C₆H₅COOH, pK_a = 4.19) solution, a buffer with a pH of 4.19 will result:

Buffers can be made from two salts that provide a conjugate acid-base pair.
A buffer can be made by dissolving 15.0 grams NaH₂PO₄ and 17.7 grams Na₂HPO₄ in water and diluting to 1.00 liter.

What will be the pH of this buffer (molecular weight of NaH₂PO₄ is 120.0 g/mol, molecular weight of Na₂HPO₄ is 142.0 g/mol)?

H₃PO₄ + H₂O H₂PO₄^- + H₃O⁺   K_a1 = 7.1 x 10^-3
H₂PO₄^- + H₂O HPO₄^2- + H₃O⁺   K_a2 = 6.3 x 10^-8
HPO₄^2- + H₂O PO₄^3- + H₃O⁺   K_a3 = 7.1 x 10^-13

2.14	7.10	12.15

Buffers can be made by adding a strong acid or base to a weak acid or base.
If a strong acid such as HCl is added to NH₃ (a weak base), NH₄⁺ (the conjugate acid of NH₃) is produced:

H⁺ + NH₃ NH₄⁺

If 50.0 mL 0.10 M HCl is added to 100.0 mL 0.10 M NH₃, what are the resulting concentrations of NH₃ and NH₄⁺?

[NH3] = 0.10 M [NH4+] = 0 M

[NH3] = 0.10 M [NH4+] = 0.10 M

[NH3] = 0.10 M [NH4+] = 0.05 M

[NH3] = 0.05 M [NH4+] = 0.05 M