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Calculate the ph at the equivalence point for the titration of 0. 22 m hcn with 0. 22 m naoh. (ka = 4. 9 × 10^–10 for HCN).

Sagot :

The pH at the equivalence point for the titration of 0. 22 m HCN with 0. 22 m NaOH is 11.17

Calculation,

Concentration of NaCN = 0. 22 m/ 2 = 0.11 M ( at equal volumes of acid and base will be used).

The equilibrium is ,

HCN +[tex]H_{2} O[/tex]  → [tex]H^{+} + CN^{-}[/tex]

C(1-x)              Cx      Cx

Where x , is the degree of hydrolysis and

[tex]K_{h}[/tex] = C[tex]x^{2}[/tex]/(1-x)

We know that [tex]K_{h}[/tex] = [tex]K_{w}/K_{a}[/tex] = 1 ×[tex]10^{-14}[/tex]/4. 9 ×[tex]10^{-10}[/tex] =  2.04×[tex]10^{-5}[/tex]

[tex]K_{h}[/tex] =  C[tex]x^{2}[/tex] =   2.04×[tex]10^{-5}[/tex]  = 0.11 M×[tex]x^{2}[/tex]

[tex]x^{2}[/tex] =  2.04×[tex]10^{-5}[/tex]/0.11 M

x = 1.36×[tex]10^{-2}[/tex]

[tex][OH^{-} ][/tex] = Cx =  1.36×[tex]10^{-2}[/tex] × 0.11 M = 0.15×[tex]10^{-2}[/tex]

[tex][H^{+} ][/tex] =  1 ×[tex]10^{-14}[/tex]/ 0.15×[tex]10^{-2}[/tex] = 6.66×[tex]10^{-12}[/tex]

pH = -㏒[tex][H^{+} ][/tex] = -㏒6.66×[tex]10^{-12}[/tex] = 11.17

The pH at the equivalence point for the titration is  11.17.

To learn more about  equivalence point

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