Acetic Acid (HC2H3O2) is a weak acid and Acetate (C2H3O2-) is its conjugate base
ID: 560282 • Letter: A
Question
Acetic Acid (HC2H3O2) is a weak acid and Acetate (C2H3O2-) is its conjugate base. It has a Ka value of 1.76 x 10-5 Suppose you created a buffer by mixing 25.0 mL of 0.30 M Acetic Acid (HC2H3O2) with 50.0 mL of 0.30 M Acetate (C2H3O2-). Find the equilibrium pH of the solution. [See the example in the notes. It used different volumes and a different acid and Ka, but the method is the same. Find initial concentrations of mixture assuming additive volumes, set up an ICE table, write the Ka expression in terms of the algebraic expressions of the equil. concentrations and solve for x (using the weak acid approximation). Determine the equilibrium H+ concentration from x. Take the -log of it to get pH.]
Explanation / Answer
Ka = 1.76 x 10^-5
pKa = -log Ka = 4.75
millimoles of acetic acid = 25 x 0.30 = 7.5
millimoles of acetate = 50 x 0.30 = 15 .
pH = pKa + log [salt / acid]
= 4.76 + log [acetate / acetic acid]
= 4.76 + log [15 / 7.5]
pH = 5.06