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### Course: Chemistry library>Unit 11

Lesson 2: Acid-base equilibria

# Weak base equilibrium

Quick overview of Kb and pKb. Examples of calculating the pH of a weak base solution.

## Want to join the conversation?

• By finding the ph of ammonia, shouldn't we technically be solving for the pOH since ammonia is a base. It makes no sense to me that the pH is a larger number than the pOH. I would expect the pH to be 2.52 and pOH to be 11.48
• Keep in mind that a pH >7 is classified as a base and a pH < 7 is considered acidic .
We know that ammonia is a base so we expect the pH to be above 7.
Remember that pH + pOH = 14
This can be rearranged to pH = 14 - pOH
If the pOH was larger than the pH then the solution would have to be acidic.
• I don't understand why we assume that .003 is for the OH. Couldn't it be for the NH4+ also since they are both set to x?!
• You are correct, `x` represents both the [OH¯] and [NH₄⁺].

Why do you think that is a problem?

Remember the goal was to work out the pH, so [OH¯] matters while [NH₄⁺] does not ...
• Why do we say that the concentration of pure liquids and solids is one? Is it molarity? If it is, are we saying that there is always one mole of the solid/liquid in every litre of the solid/liquid?
• It is the activity of the liquids and solids that equals 1.
The concentration of water is 55.5 mol/L. The typical concentration of a solute in a reaction
is 0.1 mol/L to 1 mol/L.
Even if the water is a reactant in the reaction, its concentration stays almost constant.
We can consider that its constant is incorporated into the equilibrium constants K_a, K_b. and K_w.
• how come we can assume that x<< 0.500 or x<< 1 in the two examples discussed above?
• let us consider the case of NH3. x in the above example is the concentration of NH4+ and OH- formed ( also called degree of ionization), and we know that NH3 is a weak base. Since Weak bases ( or acids) only disassociate by a very small amount, the conc. of NH4+ and OH- ( which is x) is very small. Hence we ignore it to simplify our calculations, and assume that 0.5 - x is almost the same as 0.5. Hope this helped :)
(1 vote)
• I've also seen a version of finding the pH of a base by dividing the number you get from the ICE table calculation by 1.01x106-14. It got me a pH of 11.47 so I was wondering if it was the same thing or if it was just lucky for this calculation?
(1 vote)
• Assuming you mean x = [OH-] when you say what we get from the ICE table.

1.0 x 10^(-14) is Kw, or the autoionization constant of water at 25°C. We can relate the hydroxide concentration to the hydronium concentration by the autoionization of water reaction.

H2O(l) + H2O(l) ⇌ H3O^(+)(aq) + OH^(-)(aq) so Kw = [H3O^(+)][ OH^(-)] = 1.0 x 10^(-14). So if we know [OH-] then we can find [H3O^(+)] and therefore get pH because pH = -log([H3O^(+)]).

This basically the same method used in the video. The Kw equation from before can be written as pKw = pH + pOH by taking the –log of both sides. And pKw is 14 at 25°C. So if we know [OH-] then we can find pOH since pOH = -log([OH-]). And then we can use the pKw equation to find pH.

So in both methods you use the autoionization of water to relate hydroxide and hydronium concentrations.

Hope that helps.
• At 0.05, why does the water donate proton and ammonia don't?Why ammonia can't act as acid? How to figure out which one donates proton and which one not?
• That is because NH2 (it's apparently written as H2N- and called azanide) is a better base than water. So for that reason NH3 will have to be the Bronsted-Lowry Acid here. You'd usually know which way it goes by practicing and seeing questions after all the range of compounds you will be given will be limited and you'd normally instantly be knowing the answer.
(1 vote)
• for example at (as with previous videos), why is the B-L definition of a base used when the Lewis definition is more inclusive?
• The B-L definition touches more on bases having one less proton (H+) that acids, relating more on the Arrhenius definition.
(1 vote)
• why did you mention concentration as I and not C?
(1 vote)
• I stands for initial concentration
C stands for change in concentration
E stands for equilibrium concentration