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ChemistryGCSE, IGCSE & A Level

What is ph?

pH is a logarithmic measure of hydrogen ion concentration: the lower the pH, the more acidic the solution.

pH explained

Because the scale is logarithmic, each unit represents a tenfold change — pH 3 is ten times more acidic than pH 4 and a hundred times more than pH 5.
At A Level you calculate pH from concentration for strong acids directly, and for weak acids through the acid dissociation constant KaK_{a}.

Key formula

pH=−log⁡10[H+]\text{pH} = -\log_{10}[\mathrm{H^{+}}]

Worked example

0.01 mol dm−30.01\,\text{mol dm}^{-3} HCl has [H+]=0.01[\mathrm{H^{+}}] = 0.01, so pH =2= 2.

Examiner tip

Give pH to two decimal places at A Level. Rounding to a whole number throws away a mark on almost every calculation.

pH: common questions

What makes an acid strong or weak?
Strong acids dissociate completely in water; weak acids only partially, so a weak acid of the same concentration has a higher pH.
Can pH be negative?
Yes, for very concentrated strong acids where the hydrogen ion concentration exceeds 1 mol dm⁻³.

Related terms

Titration

A titration is a quantitative technique that finds an unknown concentration by reacting it with a solution of known concentration until the end point.

Concentration

Concentration is the amount of solute dissolved in a given volume of solution, usually in moles per cubic decimetre.

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