Question

In: Chemistry

Calculate the pH of buffers based on the recipe: 13.16 g citric acid monohydrate 171 mL...

Calculate the pH of buffers based on the recipe:

13.16 g citric acid monohydrate

171 mL 1M NaOH

829 mL H2O

---------------------------

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

Solutions

Expert Solution

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 pKa values.

C6O7H4 -------------> C6O7H-3 + H+   pKa1

C6O7H-3 ----------------> C6O7H-2 + H+ pKa2

C6O7H-2 -----------------> C6O7H- + H+ pKa3

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 = pKa + log (A- / HA)

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

pH = pKa + log (0.0272 X 105 )

pH = pKa + 3.434

pH = 3.128 + 3.434

pH = 6.562


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