Ph calculations

A brief overview of all the calculations needed when calculating pH in different conditions (i.e different strengths of acids and bases)

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  • Created by: melvin
  • Created on: 17-12-11 17:22

Ph calculations - Strong acids

ph is the measure of the concentration of h+ ions in the solution. The "p" is actually representative of the negative log (-log) of H-3O+ concentration.

In a strong acid, the ions dissociate 100%. this means that all the molecules of the acid split up into their conjugate bases and acids. for example, in HCl, no molecules stay as HCl in water. the all split into H+ and Cl- ions.

With this assumption, you can work out the pH of a solution of a strong acid simply by doing the negative log of the concentration of HCl.

question: What is the pH of a solution of HCl if the concentration is found to be 0.1mol-1dm-3?

Answer: -log 0.1 = 1

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pH calculations - Weak Acids

2 assumptions are made when calculating the concentration of weak acids, those being:

the dissociation of the weak acid is so small it is neglible, so the concentration of the weak acid stays the same.

the little that does dissociate is the same concentration of the h+ ions that it contributes to the solution. So the concentration of the salt ion is = the H+ ion concentration (no H+ donated to solution from water)

To work out the pH, you use the formula Ka = [H+][A-]/[HA]. Ka is the acid dissociation constant. A is the other part of the acid. for example, in ethanoic acid (CH3COOH) the A would be CH3COO-.

It is also important to note that [] signifies "the concentration of".

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pH calculations - Weak acids question

Question: What is the pH of a solution of CH3COOH with a concentration of 0.5M when Ka is 0.00012?

Answer: Ka = [H+][A-]/[HA]

0.00012 = [H+][A-]/0.5

Nb: look back to the assumptions. We see that H+ = A- so that means [H+][A-] could be expressed as [H+][H+]=[H+]^2.

0.00012=[H+]^2/0.5 Multiply both sides by 0.5


Sqrt(0.00006 = [H+]

0.00775 = [H+]

-log[H+] = 2.11 = pH

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pH calculations - Bases

On occasion, you may be asked to work out the pH of a basic solution e.g. KOH or NaOH. to do this, you use the negative log of the OH- ion concentration and take this away from 14.

Question: What is the pH of a solution of 0.1M KOH?

Answer: 14 - (-log0.1) = pH = 14 -1 = 13

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pH calculations - Buffer solutions

A buffer solution is a solution with an excess of an acid and its corresponding salt i.e. CH3COOH + CH3COO-. by having both of these in a solution, they can counteract an addition of a small amount of acid or base. this is because if a base is added, then the acid can donate protons to it, and there wont be a significant change in pH. Likewise, addition of an acid the salt ion will accept the new protons.

Because we have changed the concentrations of the A- ions, the assumption deduced before doesn't hold true and therefore, you cannot work out the pH of a buffer in the same way as you would a weak acid.

the new equation to solve is [H+] = Ka x [acid]/[salt]

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