In the titration of a solutionof weak acid, HX withNaOH ,the pH is 5.8after 10 mL of NaOH solution has been added and 6.40 after 20.0mLof the NaOH has been added. What is the ionisation constant of the acid HX ?

In the titration of a solutionof weak acid, HX withNaOH ,the pH is 5.8

1.	In the titration of a solutionof weak acid, HX withNaOH ,the pH is 5.8after 10 mL of NaOH solution has been added and 6.40 after 20.0mLof the NaOH has been added. What is the ionisation constant of the acid HX ?
Answer» `7. 94 xx 10 ^(-7) ` ` 7. 94 xx 10 ^(-6) ` ` 7. 94 xx 10 ^(-8) ` ` 7. 94 xx 10 ^(-9) ` Solution :`P^(H) =pK_a +log ""(S)/(A) ` ` 5.8= pKa + log ""(10 xx B)/( A- 10B) ...(1) ` ` 6.4 = pKa +log "" ( 20 xx B)/( A - 20 B ) ...(2) ` ` 0.6 = log ""( 20 B)/( A- 20B) xx (A-10B)/(10B) ` ` 0.6 = log ""(2(A-10 B))/( (A-20B)) =log 4` ` rArr (A- 10B )/( A- 20 B)=2 rArr A =30 B ` substituting in ..........(1) ` 5.8= PKa+ log ""( 10 B)/( 3 0 B - 10 B) ` ` 5.8 = PKa+log ""(10B) /( 30 B -10 B) ` `5.8= pKa +log ""( 1)/(2) ` ` pKa =5.8 -log "" (1)/(2) = 5.8+0.3 =6.1` ` -log KA =6.1 , log Ka = -7 +0.7 = 7 . 9 4 xx 10 ^(-7)`