Weak Acid vs. Dilute Acid (Diagram and Explanation)

Описание

In chemistry, the terms "weak" and "dilute" have very different meanings when it comes to acid. When we say an acid is "dilute" we mean that there is not much of the acid in the given volume of water. The opposite of a "dilute acid" would be a "concentrated acid".

Strong acids dissociate completely in water. For example, HCl is a strong acid because it will dissociate completely into H+ and Cl- ions in water. Weak acids only dissociate partially. See the diagram in the video to aid understanding.

It is therefore possible to have a strong acid that is also dilute. This means there the acid dissociates completely into ions but there just isn't much of it in the water.

Likewise it is possible to have a concentrated weak acid. This means there is a lot of the weak acid in water but it only dissociated partially.

There are two ways to determine whether something like HCl is a strong or weak acid. The first it to memorize the seven common strong acids. For general chemistry courses this will be enough to determine if the acid is strong or weak.

The second is to look at the pKa for the acid. If the pKa value is less than zero (some sources say less than one) it is a strong acid. If the pKa is greater than zero it is a weak acid. Values for pKa can be calculated if data is provided or looked up in a table.

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Common Strong Acids
HBr, HCl, HI, HNO3, HClO4, H2SO4, and many lists include HClO3

It is recommended to memorize the list of strong acids.

Common Weak Acids
HF, H3PO4, H2CO3, HNO2

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