Acid- Base: pH/pOH Calculations

Strong Acids
If it is not a stong acid, it is a weak acid.
Know the strong acids (monoprotic and diprotice)
Strong acids ionize virtually 100%

HCl(aq)  + H2O => H3O+(aq)  +  Cl-(aq)

pH = -log[H3O+]; 10^-pH = [H3O+];


Question: Determine the pH of a 0.150M HCl solution
(since strong acid ionizes 100%, [HCl] = [H3O+]=0.150M

pH = -log(0.150M) = 0.824
Notice: 3 sgfigs in concentration 0.150M; 3 decimal places
 in pH value 0.824

Determine the concentration of a monoprotic acid solution if the pH
of the solution is 4.35
10^-4.35 =4.5x10^-5  M
Notice: 2 decimal places n pH value, 2 sigfigs in concentration answer

Only the decimal places of pH values are significant.


In a similar way that pH/[H3O+] connect, so can
pOH and [OH-]

pOH = -log[OH-] and 10^-pOH = [OH-]

Connecting pH and pOH?

@25C:   pH + pOH = 14.00

Question: Determine the pH of a 0.250M NaOH solution
Strong bases dissociate virtually 100%
NaOH is a strong base, dissociates 100%
NaOH(aq) => Na+(aq)  +   OH-(aq)
So, [NaOH] = [OH-] = 0.250M

When working with a base, one cannot determine pH directly from the given base concentration . Determine pOH, then determine pH

pOH = -log(0.250M) = 0.602
pH = 14.00-0.602 = 13.398

there is another way to carry out this calculation
- saving that for another time

A few basic jokes to end this post: What happened when the lion ate the comedian? Ans: It felt funny. Where do sheep go to get a haricut? Ans: To the Baa Baa shop. They are simple, basic jokes. Get it? This post dealt with bases, so it ended with some basic jokes!