A specific pH problem in EK Chem book

[yellowcocopuffs]

Carbonic acid has Ka of 4.3 x 10^-7. What is the pH when 1 mole of NaHCO3 is dissolved in 1L of water?.

Answer is 10.2


But WHY? The explanation given does NOT make sense.

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[venacontracta]

HCO3- is the conjugate base of carbonic acid

Use the given Ka to find the Kb of HCO3-

then use the Kb along with the equilibrium expression for dissociation of the conjugate base to find the concentration of OH- when 1 mole of NaHCO3 is dissolved in 1L of water

then, use [OH-] to find pOH

from pH + pOH = 14, you then know pH.

 

[collegestud2013]

Kb = 1.0x10^-14/4.3x10^-7 = x^2/(1 - x)

x = [OH-] = 1.52x10^-4 M

pOH = -log(1.52x10^-4) = 3.8

pH = 14 - pOH = 14 -3.8 = 10.2

 

[ilovemcat]

Or you can make it much easier for yourself by:

- Finding kb value first (kb times ka = kw or 1x10^-14 therefore kb = kw / ka).
- Find molarity of HCO3- (1 M)

Then use this equation:

pOH = (1/2)pKb + (1/2)p(Concentration)

pH + pOH = 14
pH = 14 - pOH ...violaaa