Chemical and Ionic Equilibrium - Result Question 136

####78. 20mL of 0.2M sodium hydroxide is added to 50mL of 0.2M acetic acid solution to give 70mL of the solution. What is the pH of this solution?

Calculate the additional volume of 0.2MNaOH required to make the pH of the solution 4.74.

(Ionisation constant of CH3COOH=1.8×105 ).

(1982,3M)

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Solution:

  1. mmol of NaOH=20×0.2=4

mmol of acetic acid =50×0.2=10

After neutralisation, buffer solution is formed which contain 6 mmolCH3COOH and 4mmolCH3COONa.

pH=pKa+log[CH3COONa][CH3COOH] =log(1.8×105)+log46=4.56

Now, let xmmol of NaOH is further added so that pH of the resulting buffer solution is 4.74 .

Now, the buffer solution contains (4+x)mmolCH3COONa and (6x)mmol of CH3COOH.

4.74=log(1.8×105)+log4+x6x 4+x6x=1 x=1.0mmol=0.2×V V=5.0mmolNaOH.



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