A solution is prepared by mixing 500 mL of 0.150 M CH_3 COOH and 500.0 mL of 0.2

Question

A solution is prepared by mixing 500 mL of 0.150 M CH_3 COOH and 500.0 mL of 0.20 M CH_3 COONa. K_a for CH_3 COOH is 1.8 times 10^5. a. Calculate the pH of this buffer. b. What is the pH after 1.5 mL of 1.0 M HCl is added to 25.0 mL of this buffer. c. What is the pH of the solution after 1.5 mL of 1.0 M NaOH is added to 25.0 mL of this buffer. a. Will a precipitate of magnesium fluoride form when 300.0 mL of 1.2 times 10^3 M MgCl_2 are added to 500.0 mL of 1.5 times 10^3 M NaF? (K_sp (MgF_2) = 6.9 times 10^-9) I r

Explanation / Answer

a. no of mole of CH3COOH = 0.15*500/1000 = 0.075 mole

   no of mole of CH3COONa = 0.2*500/1000 = 0.1 mole

pH of acidic buffer = pka + log(salt/acid)

       pka = -logka = -log(1.8*10^-5) = 4.74

   pH = 4.74+log(0.1/0.075)

      = 4.865

b. no of mole of HCl added = 1.5*1/1000 = 0.0015 mole

   pH of acidic buffer after addition of acid = pka + log(salt-HCl/acid+HCl)

            = 4.74+log((0.1*25/500-0.0015)/(0.075*25/500+0.0015))

            = 4.564

c. no of mole of NaOH added = 1.5*1/1000 = 0.0015 mole

   pH of acidic buffer after addition of base = pka + log(salt+NaOH/acid-NaOH)

            = 4.74+log((0.1*25/500+0.0015)/(0.075*25/500-0.0015))

            = 5.2

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