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How to Recognise a Weak Acid

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“The Question Doesn’t Say”

Have you ever looked at an exam question that is asking you about acids, and it doesn’t use the word “weak” or the word “strong”? Maybe it was a question in an assignment, a mock exam or in your own exam practice.

And because it doesn’t tell you, you are left feeling unsure how to go about answering the question.

It is typical that the question won’t be specific about whether the acid is weak or strong. You are probably getting used to A-Level questions being less directive than the questions you have faced at GCSE.

So how should you decide whether your question is about a weak acid or a strong acid?

No need to despair as there will be plenty of clues within the question. We’ll consider what those clues are:

Clue #1: There are Few Strong Acids

The truth is that there are only a small number of strong acids, and you will already be familiar with most of these.

Specifically, these are the mineral acids you will know well:

  • sulfuric acid
  • nitric acid
  • hydrochloric acid
  • hydrobromic acid
  • hydroiodic acid

There are a few more but the list is very small. Almost all acids are weak acids.

So, if the acid in your question is one of these you can go ahead and calculate pH directly from the acid concentration.

Clue #2: The Acid is an Organic Acid

All organic acids are weak acids. If the acid in your question is a weak acid then you need to make any calculations based upon the acid dissociation constant.

But how do you spot an organic acid?

The first way is from the formula. The majority of organic acids include the -COOH group, but any organic compound that has an acidic proton is a weak acid. In other words, any compound described as an acid that includes at least one C-C or C-H bond is a weak acid.

The second way is the name. The -COOH group, or carboxylic acid group, identifies many organic weak acids. The compounds with this group have names ending with “-oic acid”; for example ethanoic acid or benzoic acid. If your compound has this name ending it will be a weak acid. (Beware though: if your compound doesn’t have this type of name it may still be a weak acid).

Clue #3: The Question Indicates Incomplete Dissociation

If the question implies that there is a reversible reaction or an equilibrium related to the dissociation of the acid, this indicates that dissociation is incomplete. In thus case the acid in your question is a weak acid.

Clue #4: The Question Refers to a Ka Value

Mention of a Ka (or pKa) value in the question makes it clear that the question is about a weak acid. (Strong acids don’t have acid dissociation constants).

Clue #5: The Compound is Used in an Acidic Buffer

If the question goes on to imply the use of the acid within an acidic buffer, the acid is definitely a weak acid. The function of an acidic buffer relies on the reversible dissociation of an acid – hence the acid must be a weak acid.

Weak Acid vs Strong Acid

Why does it matter if the acid is weak or strong?

The way we make calculations regarding pH and concentration will depend on the type of acid.

Because a strong acid dissociates quickly it is straightforward to calculate its pH if you know its concentration, and vice versa.

A weak acid only partially dissociates, and it is necessary to know the acid dissociation constant in order to calculate pH or concentration. The methodology of calculations involving weak acids is shown in our previous article.

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