In: Chemistry
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 |
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