# How do you get pH and pOH given pka or pkb?

**Answer:**

If you are given pKa use the Henderson-Hasselbalch equation to find pH:

pH = pKa + log(conc. of base / conc. of acid)

To convert pKb to pKa or vice versa use the following equation:

pKa x pKb = 1 x 10^-14

And use this equation to convert between pH and pOH:

pH + pOH = 14

Given pKa or pKb you can get Ka and Kb not pH or pOH.

For example pKa (HF)= 3.17

To get Ka = 10^- 3.17 =0.000671

If you want to get pH you have to know initial concentration HF.

We suppose = 0.1 M

HF <> H+ + F-

let x= moles/l HF that dissolve at equilibrium

Concentration HF = 0.1 - x

Concentration H+ = Concentration F- = x

Ka = 6.71 10 ^-4 = (x)(x) /0.1-x

x = 0.00819 M

pH = - log 0.00819 = 2.09

The answer of sureshma is valid ONLY for buffer solutions

who cares

It is very easy with the following formula.

log Ka = log [H+] + log [A--]/[HA]

Derivation:

HA --------> H+ + A—

(Weak Acid) (proton) (conjugate Base)

Ka (acid dissociation constant) = [H+]* [A--]/[HA]

log Ka = log [H+] + log [A--]/[HA]

--log [H+] = --log Ka + log [A--]/[HA]

pH = pKa + log [conjugate base]/[acid]

Henderson Hasselbach equation

pH = -log[H+] and pKa = --log Ka

When [A-]= [HA]

pH = pKa + log [HA]/[HA] or pH = pKa + log 1

So pH = pKa

if the concentration of the conjugate base and your weak acid are equal, than your pKa is you pH and visversa

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