Chemguide: Support for CIE A level Chemistry
Learning outcome 7.2(b)
This statement is an introduction to strong and weak acids and bases.
Before you go on, you should find and read the statement in your copy of the syllabus.
You need to start by reading bits of the page about strong and weak acids.
This page covers work that you will need in the second half of the course as well as the first. You should ignore all the numerical parts of the page.
Read down as far as the first paragraph under the heading "Strong acids and pH". For now, all you need to know is that strong acids of the sort of concentrations that we normally use in the lab have pHs of around 0 - 1.
For example, 1 mol dm-3 hydrochloric acid has a pH of 0. (More concentrated solutions of hydrochloric acid will have slightly negative pHs.)
Ignore the sections headed "Defining pH" and "Working out the pH of a strong acid", and go on to the section on weak acids.
In the weak acid section, just read the introductory bit about what a weak acid is. Weak acids of the sort of concentrations we use in the lab, will typically have pHs in the 2 - 6 range.
For example, 1 mol dm-3 ethanoic acid has a pH of 2.4.
Now do much the same thing on the page about strong and weak bases.
In the strong base section, just read the introductory bit about what a strong base is, and then jump down to the similar introductory bit about weak bases. Ignore everything else on the page.
Typically, a strong base like sodium hydroxide solution will have a pH of 14 for a 1 mol dm-3 solution. Ammonia solution, a weak base, has a pH of 11.6 for a solution of the same concentration.
This next bit isn't mentioned on either of the Chemguide pages, but is mentioned in CIE's teacher support material.
The electrical conductivity of a solution depends on the number and nature of the ions present. That means that the conductivity of a strong acid or base will be higher than a weak one of the same concentration.
A 1 mol dm-3 solution of hydrochloric acid, for example, contains a far greater concentration of ions than a solution of ethanoic acid of the same concentration. Its conductivity will also be far greater.
© Jim Clark 2011 (last modified April 2014)