Question

1. An aqueous buffer solution of volume 100 cm³ consists of 0.20 M CH₃COOH and 0.20 M NaCH₃CO₂.

           pKa value of CH₃COOH is 4.75.

   (a) Predict the pH value of this solution. 3 sig. fig

   (b) Predict the pH value after adding 5.92 mmol NaOH (note, not mM. Volume is not 1 L) to this buffer solution assuming volume is not changed. 3 sig. fig.

Answer 1

a ) pH of the solution is 4.75 ( upto three significant digits ) .

The detail calculation is shown below .

b) pH of the solution is 5.01 ( upto three signuficant figures ) .

is Concentration of weak and, CH₂ Cool [CH₂COOH = 0.20M. > CH₂ coz & Nat is Na CH ₂ CO2 (conjugate base of CH₂ CH2H) :. Concentration of CH₂ CDI I CH₂ coz = 0.20 M. pka value of clip cool is 4.75. PH Henderson - Hesselbalch equation pka log [ conjugate base I weak acid 4.75 + log ( 0.20 ) » pH = PH 4.75 to 4.75 pH of this solution is 4.75

The detail calculation is shown below .

Volume of the solution, v= 100 cm3 = OIL (IL=1000 cm3) Number of moles of Nach added is - n = 5.92 mmol = 5.929103 mol (Imole Imole = 1000 mmole) No. of moles Volume (2) (5.92x103 ) moles Naoh Molarity of 0.1 L 0.059 M NaOH reeacts with weak anid, alg cool to form Na CAZ CO2 <H₂ cook + Nach Chly coona. inito 0.20M 0.059M 0.20M. -0.059M +0.059M. change: -0.059m Equilibrium: 0.1A1M 0.259M. According Henderson - Hesselbalch equation - PH pkat + log [ CH₂ CH2 Na] I CH₂ CO2H] » pH = 4.75 + 0.259 log 2. 0.141 pH TA 11 5.01.