Chemguide: Support for CIE A level Chemistry

Learning outcome 25: Equilibria

25.1: Acids and bases

Learning outcomes 25.1.5 and 25.1.6

These statements are about how buffer solutions work, and how you do simple calculations with them.

Before you go on, you should find and read the statements in your copy of the syllabus.

Read the first half of the Chemguide page buffer solutions which talks about how buffer solutions work.

When you are happy with that, read the second half of the same page where the calculations are introduced, and then find a source of similar questions that you can practise.

Extra for statement 25.1.5(d)

This is about the uses and importance of buffer solutions, including keeping the blood at a constant pH between 7.35 and 7.45.

There are several things which buffer the pH of the blood, one of which is the following equilibrium involving hydrogencarbonate ions.

The carbon dioxide in the blood comes from, for example, the break-down of carbohydrates in the body.

If the hydrogen ion concentration in the blood increases (if the pH falls), then this equilibrium will move to the left to remove the extra hydrogen ions.

If the hydrogen ion concentration in the blood decreases (if the pH rises), then this equilibrium will move to the right to replace the missing hydrogen ions.

This is just a simple application of Le Chatelier's Principle.

The syllabus talks about "uses" of buffers - which implies more than one use. You need to be able to quote another use just in case you are asked.

The simplest to understand is their use in the lab to check the readings on a pH meter. For accurate work, you would make up a buffer solution with a pH of, say, 4.0, and then adjust your pH meter so that it was reading exactly that value.

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© Jim Clark 2020