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| The pH of an aqueous solution is a measure of its hydrogen
ion concentration; it depends on temperature because the degree of dissociation of an acid
does, i.e. Ka changes. Before you go on, write down the definition of
pH. It is: pH = - lg{[H+]/mol dm-3} Note that the concentration of the hydrogen ion has to be divided by its unit to get a number. You can only take logarithms of numbers, not of physical quantities. A neutral solution can be at pH 8. How? By being cold. If you look up the values of Kw at 0oC, 25oC and 100oC you will be able to calculate the pH of water at these three temperatures; they are roughly 8, 7 and 6. All of the solutions are, however, neutral. The definition of this is pH-independent: a neutral solution is one where [H+] = [OH-] . This is at pH 7 only at 25oC. |
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Dr Rod Beavon 17 Dean's Yard London SW1P 3PB
e-mail: rod.beavon@westminster.org.uk