a. A solution of cynaic acid (HOCN) with initial concentration 1.00x10^-2 M has

Question

a. A solution of cynaic acid (HOCN) with initial concentration 1.00x10^-2 M has pH 2.77 Calculate the Ka of the acid
b. A solution of formic acid (HCOOH , Ka = 1.8x10^-4) has pH of 2.7. Calculate the initial concentration of HCOOH Assumption is not valid a. A solution of cynaic acid (HOCN) with initial concentration 1.00x10^-2 M has pH 2.77 Calculate the Ka of the acid
b. A solution of formic acid (HCOOH , Ka = 1.8x10^-4) has pH of 2.7. Calculate the initial concentration of HCOOH Assumption is not valid
b. A solution of formic acid (HCOOH , Ka = 1.8x10^-4) has pH of 2.7. Calculate the initial concentration of HCOOH Assumption is not valid

Explanation / Answer

(a) Let be the dissociation of the weak acid
                            HA <---> H + + A-

initial conc.            c               0         0

change                -c            +c      +c

Equb. conc.         c(1-)          c    c

Dissociation constant , Ka = c x c / ( c(1-)

                                         = c 2 / (1-)

In the case of weak acids is very small so 1- is taken as 1

So Ka = c2

==> = ( Ka / c )

Given c = concentration = 1.00x10-2M

   pH = - log [H+] = 2.77

[H+] = 10-2.77 = 1.698x10-3 M

c = 1.698x10-3 M

   = (1.698x10-3 ) / (1.00x10-2 ) = 0.169

So  Ka = c2 = (1.00x10-2 ) x 0.169 = 1.698x10-3

simillarly do the part (b)

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