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These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

Acids and bases (sl)

9.3 The pH scale

pH vales ranges from 0 to 14 (7 being the neutral value of pure water at 20c and 1 atm).

Lower pH value are acidic, higher values are basic.

pH can be measured with a pH meter, or with pH paper (paper containing a mixture of indicators to cause a continuous color change).

pH is a measure of the dissociation of an acid or base, and also of the concentration of that acid / base (actually its related to the concentration of H3O+ ions).

If we have two solutions with their pH values, the lower one will be more acidic and the higher one will be more basic (though they could both still be basic/acidic with respect to water -- pH 7).

Relationship between pH and acid concentration

A change of 1 in the pH scale represents a 10 times change in the acidity or basicity of the solution (because it's a log scale).

pH = - log [H+]

9.4 Buffer solutions

These are solutions that resist changes in pH when small amounts of acid or base are added

There are two types of buffer.

1. Weak acid and the salt of the same weak acid, (for example a solution containing ethanoic acid and sodium ethanoate). This gives a buffer solution with a pH less than 7
2. Weak base and the salt of a the same weak base (for example ammonia and ammonium chloride solution). This gives a buffer with a pH greater than 7

The first (acidic) buffer works in the following way.

If an acid is added it combines its free hydrogen ions with the ions from the salt of the weak acid making molecular weak acid that cannot affect the pH.

If a base is added the OH- ions from the base react with the H+ ions that are present from the weak acid dissociation. Having been removed from the solution this stimulates the weak acid to produce more H+ ions (Le Chatelier's Principle) and the original pH is re-established.

9.5 Acid - Base Titrations

A titration is where small quantities of one component is added a little at a time to a solution of the other component in the presence of an indicator until the indicator registers the neutral point.

A graphical plot of the volume of one component added against the pH, gives distinctive curves depending on the strength of the acid and base. Examples...