Question

a) you have a 2.00L container of 0.100M acetic acid (ka=1.80x10^-5) and you need to make a buffer solution of pH=4.90 to calibrate your pH meter. how many grams of sodium acetate (NaC2H3O2 MM=84.03g/mol) must you weigh out into the container of 0.100M acetic acid? Note: assume no significant volume change when Nac2H3O2 is added to the solution. b) in a separate beaker, you place 30 mL of the 0.100 M sodium acetate buffer and add 10 mL of 0.200M NaOH. what would be the pH after addition?

Answer 1

pH of acidic buffer = pka + log(ch3cooNa/CH33COOH)

pH = 4.9

pka of CH3COOH = -logKa = -log(1.8*10^-5) = 4.74

no of mol of CH3COOH = M*V

                      = 0.1*2

                      = 0.2 mol

No of mol of CH3COONa = x mol

4.9 = 4.74+log(x/0.2)

x = 0.289

No of mol of CH3COONa = x = 0.289 mol

amount of CH3COONa = n*Mwt

                   = 0.289*84.03

                   = 24.3 g

after adddition of NaOH

no of mol of buffer taken = M*V

                          = 0.1*30

                          = 3 mmol

No of mol of CH3COONa = x mmol

no of mol of CH3COOH = 3-x mmol

pH = pka+log(CH3COONa/CH3COOH)

4.9 = 4.74+log(x/(3-x))

x = 1.77

CH3COONa = X = 1.77 mmol

CH3COOH = 3-1.77 = 1.33 mmol

no of mol of NaOH added = 0.2*10 = 2.0 mmol

here, total CH3COOH converts into CH3COONa and some excess NAoH is left in the solution.

excess NaOH = 2-1.33 = 0.67 mmol

concentration of EXCESS NaOH = n/v

                             = 0.67/40

                             = 0.01675 M

pOH = -log(OH-)

    = -log0.01675

    = 1.78

pH = 14-pOH

   = 14-1.78

   = 12.22