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The initial rate of hydrolysis of methyl...

The initial rate of hydrolysis of methyl acetate (1M) by a weak acid `(HA,1M)` is `1//100th` of that of a strong acid `(HX, 1M),` at `25^(@)C`. The `K_(a)(HA)` is

A

`1xx10^(-4)`

B

`1xx10^(-5)`

C

`1xx10^(-6)`

D

`1xx10^(-3)`

Text Solution

Verified by Experts

The correct Answer is:
A
Rate w.r.t. weak acid, `R_1=k [H^+]_(WA)` [ ester]
Rate w.r.t. strong acid, `R_2=k[H^+]_(SA)` [ ester ]
`:." "R_1/R_2=([H^+]_(WA))/([H^+]_(SA))=1/100" (Given)"`
`:." "[H^+]_(WA)=([H^+]_(SA))/(100)=1/100xx1M=0.01M`
`=10^(-2)M`
`HAhArrH^++A^-`
Initial `"C mol L"^-1`
At eqm. ` C-Calpha" "Calpha" "Calpha`
`=C(1-alpha)`
`[H^+]=Calpha" or "10?^-2=1xxalpha" or "alpha=10^-2`
`K_a= (Calpha.Calpha)/(C(1-alpha))=(Calpha^2)/(1-alpha)~~Calpha^2`
`=1xx(10^-2)=10"^-4`
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Knowledge Check

  • The initial ratio of hydrolysis of methyl acetate (1 M) by a weak acid (HA, 1M) is 1//100^(th) of that of a strong acid (HX, 1M) at 25^(@)C . The K_(a) of HA is

    A
    `1 xx 10^(-4)`
    B
    `1 xx 10^(-5)`
    C
    `1 xx 10^(-6)`
    D
    `1 xx 10^(-3)`

  • pH of 1M HA (weak acid) is 2. Hence van't Hoff factor is -

    A
    1.2
    B
    1.02
    C
    1.1
    D
    1.01

  • The dissociation constant of a weak monobasic acid K_(a) is 1xx 10^(-5) . The pH of 0.1 M of that acid would be

    A
    5
    B
    1
    C
    2
    D
    3

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