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Acids and Bases
Strong and Weak Acids/Alkalis
A proton produced as we dissolve an acid in water is hydrated. Hydrated hydrogen ions are
represented as H+ (aq)
An alkali is a base which dissolves in water, producing OH- ions.
Because acids act as a source of protons, we call them proton donors. The hydroxide ions
from an alkali combine readily with protons to form water. We call them proton acceptors.
A strong acid or alkali is one that is 100% ionised in water. A weak acid is only partly ionised
The pH scale measures the concentration of hydrogen ions in a solution. A strong acid will
have a lower pH as it is fully ionised.
Acidity can also be distinguished by observing the rate of reaction when a reactive metal is
added to an acid.
Titration is used to measure exactly how much alkali is needed to react completely with a
known amount of acid (or vice versa).
The end point is the point at which the acid and the alkali have reacted completely. This is
shown using an indicator.
A neutral solution only forms when exactly the right amounts of a strong acid react with a
If a strong acid reacts completely with a weak alkali, the solution formed at the end point is
If a strong alkali reacts completely with a weak acid, the solution formed at the end point is
A suitable indicator should be chosen to show the end point of an acid-base reaction:
o Strong acid + strong alkali = any indicator is suitable
o Weak acid + strong alkali = use phenolphthalein
o Strong acid + weak alkali = use methyl orange
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The concentration of a solute in a solution is given in units as moles per cubic decimetre
To calculate the concentration of a solution:
o Calculate the mass (in grams) of solute in 1cm3 of solution
o Calculate the mass (in grams) of solute in 1000cm3 of solution
o Convert the mass(in grams) to moles
To calculate the mass of solute in certain volume of solution:
o Calculate the mass (in grams) of the solute there is in 1dm3 of solution