Strong or weak acid

The idea

The strength of an acid or a base depends on the number of H+ or OH– ions it produces; at the same concentration, acids giving more H+ ions are strong acids and those giving fewer are weak acids.

Same amount, same water

Hydrochloric acid and acetic acid are made with the same concentration: one molar. Each has the same number of acid particles.

Will they give the same number of H⁺ ions?

• No: one gives many more than the other
• Yes: the same number
• Neither gives any H⁺

Count the H⁺ ions

Add water to each acid. Then count the H⁺ ions.

More H⁺ means a stronger acid

The strength of an acid or a base depends on the number of H⁺ or OH⁻ ions it produces. At the same concentration, an acid that gives more H⁺ ions is a strong acid; one that gives fewer is a weak acid.

Strong does not mean concentrated. Strength is about how many H⁺ ions the acid gives out.

• Acid | H⁺ at the same concentration | Kind
• Hydrochloric acid, HCl | many | strong
• Acetic acid, CH₃COOH | few | weak

Notes

The strength of an acid or a base depends on the number of H⁺ or OH⁻ ions it produces; at the same concentration, acids giving more H⁺ ions are strong acids and those giving fewer are weak acids.

Check yourself

Two acids have the same concentration. Acid P gives more H⁺ ions than acid Q. Which is stronger?

Which of these is a weak acid?

Strength of a base depends on the number of …

Same concentration: HCl gives 8 H⁺ and acetic acid gives 1 H⁺. Which has the lower pH?

• P — correct. Yes! More H⁺ ions means a stronger acid.
• Q. Q gives fewer H⁺ ions, so it is the weaker acid.
• They are equally strong. They give different numbers of H⁺ ions, so they differ in strength.
• Hydrochloric acid, HCl. HCl gives many H⁺ ions: a strong acid.
• Acetic acid, CH₃COOH — correct. Yes! It gives only a few H⁺ ions.
• Both are weak. Hydrochloric acid is a strong acid.
• H⁺ ions it produces. H⁺ comes from acids. Bases give OH⁻.
• Metals in it. It depends on the OH⁻ ions it gives out.
• OH⁻ ions it produces — correct. Yes! A strong base gives more OH⁻.
• HCl — correct. Yes! More H⁺ means a lower pH.
• Acetic acid. Acetic acid has fewer H⁺, so its pH is higher.
• Both the same. Different H⁺ amounts give different pH.