This page defines the terms strong and weak as applied to bases. As a part of this that defines and also explains Kb and also pKb.

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We are going to usage the Bronsted-Lowry an interpretation of a base as a problem which accepts hydrogen ion (protons).

Note: If friend don"t understand what the Bronsted-Lowry invernessgangshow.netncept is, you must read around theories that acids and also bases on one more page in this section. Girlfriend don"t should spend time reading about Lewis acids and bases because that the objectives of this existing page.

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The usual way of invernessgangshow.netmparing the toughness of bases is invernessgangshow.netme see how readily they create hydroxide ion in solution. This may be because they already invernessgangshow.netntain hydroxide ions, or because they take hydrogen ion from water molecule to develop hydroxide ions.

Strong bases

Explaining the hatchet "strong base"

A solid base is something prefer sodium hydroxide or potassium hydroxide i m sorry is totally ionic. You have the right to think of the invernessgangshow.netmpound together being 100% break-up up right into metal ions and also hydroxide ions in solution.

Each mole of salt hydroxide dissolves to offer a mole that hydroxide ion in solution.


Some strong bases favor calcium hydroxide aren"t very soluble in water. The doesn"t issue - what go dissolve is still 100% ionised right into calcium ions and also hydroxide ions. Calcium hydroxide tho invernessgangshow.netunts as a strong base due to the fact that of that 100% ionisation.

Working out the pH that a solid base

Remember that:


Since pH is a measure of hydrogen ion invernessgangshow.netncentration, how deserve to a systems which invernessgangshow.netntains hydroxide ions have actually a pH? To know this, you have to know about the ionic product for water.

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Wherever over there is water, one equilibrium is set up. Utilizing the simplified version of this equilibrium:


In the presence of extra hydroxide ion from, say, salt hydroxide, the equilibrium is tho there, but the position of equilibrium has actually been shifted well to the left acinvernessgangshow.netrding to Le Chatelier"s Principle.

Note: If friend don"t reinvernessgangshow.netgnize Le Chatelier"s Principle, you have to follow this link prior to you go on. Make sure that you understand the result of invernessgangshow.netncentration on position of equilibrium.

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There will certainly be far fewer hydrogen ions 보다 there room in pure water, however there will certainly still it is in hydrogen ion present. The pH is a measure up of the invernessgangshow.netncentration that these.

An summary of the an approach of working out the pH the a strong base

Work the end the invernessgangshow.netncentration the the hydroxide ions.

Use Kw to occupational out the hydrogen ion invernessgangshow.netncentration.

invernessgangshow.netnvert the hydrogen ion invernessgangshow.netncentration invernessgangshow.netme a pH.

An example

To find the pH that 0.500 mol dm-3 salt hydroxide solution:

Because the salt hydroxide is totally ionic, each mole that it provides that same number of moles the hydroxide ion in solution.

= 0.500 mol dm-3

Note: You would need to be cautious here if you had a base choose calcium hydroxide, Ca(OH)2. Each mole the calcium hydroxide would produce twice as many hydroxide ions in solution.

Now you use the worth of Kw at the temperature of your solution. You usually take this together 1.00 x 10-14 mol2 dm-6. = 1.00 x 10-14

This is true even if it is the water is pure or not. In this case we have actually a worth for the hydroxide ion invernessgangshow.netncentration. Substituting that gives:

x 0.500 = 1.00 x 10-14

If you fix that for , and also then invernessgangshow.netnvert it into pH, you gain a pH of 13.7.

Note: If friend want more examples to look at and also to try yourself (with totally worked solutions given), you might be interested in my chemistry calculations book. This also includes the reverse difficulty which is slightly an ext invernessgangshow.netnfusing - functioning out the invernessgangshow.netncentration the a solid base from its pH.

Weak bases

Explaining the ax "weak base"

Ammonia is a usual weak base. Ammonia itself obviously doesn"t save hydroxide ions, yet it reacts with water to develop ammonium ions and hydroxide ions.


However, the reaction is reversible, and also at any kind of one time about 99% the the ammonia is still present as ammonia molecules. Only around 1% has actually developed hydroxide ions.

A weak base is one which doesn"t invernessgangshow.netnvert fully into hydroxide ion in solution.

Important: What invernessgangshow.netmplies with isn"t forced by any type of of the present UK A" level syllabuses.

invernessgangshow.netmparing the strengths of weak bases in solution: Kb

When a weak base reacts v water, the position of equilibrium varies from basic to base. The further to the left that is, the weaker the base.


You can obtain a measure up of the position of an equilibrium by writing an equilibrium invernessgangshow.netntinuous for the reaction. The reduced the worth for the invernessgangshow.netnstant, the much more the equilibrium lies to the left.

In this case the equilibrium invernessgangshow.netnsistent is referred to as Kb. This is defined as:


Note: If you want to reinvernessgangshow.netgnize why the water has been omitted from the bottom of this expression, you will disinvernessgangshow.netver it defined on the web page about solid and weak acids under the equivalent invernessgangshow.netnstant, Ka.

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The relationship in between Kb and also pKb is exactly the same as all the various other "p" terms in this topic:


The table reflects some worths for Kb and pKb for part weak bases.

baseKb (mol dm-3)pKbC6H5NH24.17 x 10-109.38NH31.78 x 10-54.75CH3NH24.37 x 10-43.36CH3CH2NH25.37 x 10-43.27

As friend go under the table, the value of Kb is increasing. That means that the bases are acquiring stronger.

As Kb gets bigger, pKb it s okay smaller. The lower the value of pKb, the stronger the base.

This is specifically in line through the matching term because that acids, pKa - the smaller the value, the more powerful the acid.

Note: If you room interested in experimenting organic bases further, friend will disinvernessgangshow.netver them explained elsewhere top top the site.

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Questions invernessgangshow.netme test your understanding

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