10 mLof H,A (weak diprotic acid) solutions is titrated against 0.1M NaOH. pH of the solution is plotted against volume of strong base added and following observation is made. If pH of the solution at `1^(st)` equivalence point is `pH_(1)` and at `2^(nd)` equivalnee point is `pH_(2)`, Cal the value `(pH_(2),-pH_(1))` at `25^(@)`C. Given for `H_(2),A, p^(Kal) =4.6 &p^(Ka2) =8`

Answer

4

Explanation

` H_2A +NaOH to NaHA +H_2O " at " I^(st) " end point,"`
` pH = (p^(K_(a_1))+p^(K_a_2))/(2) =( 8+ 4.6)/( 2) = 6.3`
` N_1V_1 =N_2V_2 `
` N_1xx 10=0 .1 xx 20 rArr N_1 =0.2`
at ` 2^(nd) ` end point , `Na_2 A` salt will be formed
`{:(NaHA+ ,NaOH to ,Na_2 A + H_2O), (2m " moles " ,2m " moles",0),(-,-, 2 m " moles "):}`
` [Na_2 A]=(2)/(50) =0.04 M`