For these dilute aqueous exercises, pH=−log10[H+], with hydrogen-ion concentration expressed numerically in mol L−1. Reverse the relation using [H+]=10−ᵖᴴ. A decrease of one pH unit means a tenfold increase in hydrogen-ion concentration, not a change of one mole per litre. At 25∘C a neutral aqueous solution has pH 7; acidic values are below 7 and basic values above 7.
For an effectively completely ionised monoprotic strong acid, [H+] approximately equals its concentration when water’s contribution is negligible. This shortcut is unsuitable for a partly ionised weak acid without more information. Dissolved CO2 gives ordinary rain slight acidity; oxides of sulfur and nitrogen can make rain more acidic. Compare supplied pH values by calculating concentration ratios rather than subtracting them as linear concentrations.
Example 1
Find pH for [H+]=2.00×10−3 mol L−1.
pH=−log10(2.00×10−3)=2.699, usually reported as 2.70 here.
Example 2
Compare [H+] at pH 3 and pH 5.
10−510−3=102=100. The pH−3 solution has 100 times the hydrogen-ion concentration.
Watch out:The scale is logarithmic: the ratio is 104, or 10000.
The pH scale and hydrogen ions practice | Year 11 Chemistry | Ascent Arcade