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5% Approximation Rule

Definition

The 5% approximation rule is a guideline used in chemistry to simplify calculations involving weak acids and bases. It states that if the ionization of a weak acid or base is less than 5%, then the concentration of the un-ionized species can be approximated as equal to the initial concentration.

Analogy

Imagine you're at a concert with 10,000 people and only 500 people (or less) decide to leave early. The venue would still look pretty full, right? That's because those who left represent less than 5% of the total crowd. Similarly, when less than 5% of a weak acid or base ionizes, we can say that its concentration hasn't changed significantly - just like how our concert venue still looked pretty full!

Related terms

Weak Acid: A weak acid is one that does not completely dissociate into its ions in solution. This means it doesn't release all its hydrogen ions into the solution.

Ionization: Ionization refers to any process by which electrically neutral atoms or molecules are converted to electrically charged atoms or molecules (ions).

Equilibrium Constant (Ka): Ka is an equilibrium constant specific for reactions involving weak acids in aqueous solutions. It provides information about the degree of ionization for a particular acid.