Strong or weak Some common acids and bases Concentrated and dilute pH - combining and measuring acid/base strength and concentration

The acids in the food we eat are dilute weak acids, while the acid in our stomach is a relatively concentrated strong acid. Ammonia solution used in cleaning agents is a dilute weak base, but sodium hydroxide is a strong base.

What is meant by weak and strong and how does this relate to concentrated and dilute?

Strong or weak
Acid or base strength is a measure of the ability of an acid to release hydrogen ions, H⁺_(aq), into solution or of a base to release hydroxide ions, OH^-_(aq), into solution.

	Strong acids completely dissociate, that is 100% of their molecules break up to form hydrogen ions in solution, while weak acids will only dissociate less than 1%. That is, of 100 acid molecules dissolved in a weak acid solution, only one will break up to give a hydrogen ion.
	Similarly strong bases completely dissociate to give hydroxide ions in solution, while a weak base like ammonia will dissociate around 1% only.

Some common acids and bases:

A concentrated acid is one in which many acid molecules are dissolved in a set volume of solution, while a dilute acid will have very few molecules per unit volume. The same applies for bases as acids. By adding water to a concentrated acid or base, the solution is diluted. The more water that is added the more dilute the solution becomes.

pH - combining and measuring acid/base strength and concentration
pH or the "power of hydrogen" is a numerical way of measuring the relative strengths and concentrations of acids and bases.

pH has a numerical scale from 0 to 14 with water in the middle at 7 representing neutral.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

strong acid weak acid neutral weak base strong base

• Acids have a pH less than 7 and bases greater than 7.
• Neutral is neither acid nor base. The only truly neutral substance is pure water.
• Acid or base strength increases the further the pH is away from neutral, 7.

Concentration also increases as pH values move away from 7, but not quite as simply as strength increases. Concentrated strong acids will have a pH of 0 - 2 and their pH will approach 7 as they are diluted with water. Similarly concentrated strong bases will have a pH of 14 - 12 and their pH will approach 7 as they are diluted with water.

Concentrated weak acids will have a pH of around 3 and their pH will approach 7 as they are diluted with water. Similarly a concentrated weak base will have a pH of 0 and their pH will approach 7 as they are diluted with water.

To take into account both strength and concentration, the pH scale is better considered as a series of limits, as shown below.

The pH scale is the best way of measuring the concentration of the hydrogen ions or hydroxide ions present in a solution, because in the end it does not matter if the solution is a weak or strong acid or how dilute it is. It is pH which is important.

A solution with pH 5 will have far fewer hydrogen ions per litre than a solution with pH 2. In fact it will have 1/1000th of the concentration of hydrogen ions as each unit represent a decrease in concentration of hydrogen ions of 1/10th, so a difference of 3 pH units represent a difference of 1/1000th of the concentration.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

concentrated strong acid dilute dilute concentrated strong base concentrated weak acid concentrated weak base neutral

Caution:
All acids and bases are potentially dangerous and concentrated acids and bases, whether weak or strong, pose the most danger. Different acids and bases also have other chemical properties not related to their acid/base characteristics. For example, concentrated ammonia has a very powerful and dangerous smell and concentrated sulphuric acid is a powerful dehydrating agent, capable of ripping skin apart to get water out of it.

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