Calculating the pH for a Titration 1. Calculate the millimoles (volume in mL x M

Question

Calculating the pH for a Titration 1. Calculate the millimoles (volume in mL x M) 2. Assume the neutralization reaction goes to of acid and base at the given point. completion and use a "SCAN" chart to determine the stoichiometric quantities left over after neutralization. 3. Use the millimole values in the Henderson- Hasselbalch equation to predict the new pH if you are still in the "buffer" region. 4. If the solution is no longer a "buffer", divide the remaining mmol values by total volume in mL to get M values and use an ICE table. Side 21 of 36 Tro: Chapter 17 hapter 17

Explanation / Answer

Ka of HOCl , a weak acid = 2.9x10-8

Thus pKa = 7.5376

The reaction for the given titration is

NaOCl + HCl -------------> HOCl + NaCl

40x0.1=4 0 0 0 initial mmoles

a) after adding 20 mL of HCl

NaOCl + HCl -------------> HOCl + NaCl

40x0.1=4 0 0 0 initial mmoles

- 20x0.1 - - change

2 0 2 - after reaction

Thus the solution now has a weak acid and its conjugate base , so acts as a buffer whose pH is given by Hendersen equation .

pH = pKa + log [conjugate base]/[acid]

= 7.5376 + log [2/2]

= 7.5376

b) after addition of 30 ml of HCl

NaOCl + HCl -------------> HOCl + NaCl

40x0.1=4 0 0 0 initial mmoles

- 30x0.1 - - change

1 0 3 - after reaction

Now also it is a buffer

pH = 7.5376 + log (1/3))

= 7.0604

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