Question

Calculate the pH of buffers based on the recipe:

13.16 g citric acid monohydrate

171 mL 1M NaOH

829 mL H2O

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Compound	MW	pKa if ionic strength =0
Citric acid monohydrate	210.14 g/mol	3.128, 4.761, 6.396

Answer 1

Citric acid is found in ctrus fruits and it is also part of biochemistry citric acid cycle. it is a triprotic acid with three carboxylic acid groups that is why it has three pK_a values.

C₆O₇H₄ -------------> C₆O₇H^-₃ + H⁺   pK_a1

C₆O₇H^-_{3 ---------------->} C₆O₇H^-₂ + H⁺ pK_a2

C₆O₇H^-₂ -----------------> C₆O₇H^- + H⁺ pK_a3

To calculate the pH of acidic buffer solutions.

1) No of moles of acid = Weight / MW

No of moles of acid = 13.16/210.14 = 0.0626 mol

829 ml of water is the final volume so divide the no of moles of acid with this volume and get value of HA.

HA = 0.0626/829ml of water

HA = 7.55 X 10^-5

Now we calculate the A^- concentration with the help of NaOH

A^- = 1M X 171 ml of NaOH/ 829 ml of water

A^- = 0.206

Now according to Handerson - Hasselbach equation

pH = pK_a + log (A^- / HA)

pH = pK_a + log (0.206 / 7.55 X 10^-5 )

pH = pK_a + log (0.0272 X 10⁵ )

pH = pK_a + 3.434

pH = 3.128 + 3.434

pH = 6.562