Hydrolysis of salt

The hydrolysis of salt tells you whether a salt is acidic or basic. But it does not tell you how acidic or basic a salt is. Is there a way to determine this?

My guess is that taking of the weak base part of a salt( presuppose that it is acidic) and see how many hydronium ion it could produce. So the more hydronium ions, the more acidic this salt is?

• Why not just use pKa or Ka values for the ion of the salt with acid-base properties? – Dissenter Jun 11 '14 at 0:21
• Why for pKa you take the negative log of the ionization constant for acid? What the log represents? Why do this. I know the definition of log, just asking for the reason to take the negative log. – most venerable sir Jun 11 '14 at 0:43
• The negative log returns a positive pH value (generally), since concentrations of hydronium and hydroxide ion are typically very low, so they are expressed using negative exponents - i.e. $1.0 \times 10^-6$. The negative is mainly there for convention - it's a bit easier dealing with positive values in general. – Dissenter Jun 11 '14 at 1:02
• Alright, then is my guess correct or not? – most venerable sir Jun 11 '14 at 1:11
• Why would you take the base part of the salt and see how much hydronium ion it creates? Why not hydroxide ion? – Dissenter Jun 11 '14 at 1:16

Steps for determining acidity or basicity of a salt by hydrolysis:

1. Write the neutralization reaction in reverse (salt and water makes base and acid).
2. Break up the salt into its ions.
3. To write the base, add an $\ce{OH-}$ to the cation (the positive ion) for each + charge.
4. To write the acid, Add $\ce{H+}$ to the anion (the negative ion) for each negative charge.
5. If you for a strong base, the salt is basic. $\ce{(NaOH, KOH, LiOH, Mg(OH)_2, ...)}$

If you form a strong acid it is acidic. $\ce{(HCl, HNO_3, HClO_4, HBr,...)}$

If you form both a strong acid and a strong base it is neutral.

EXAMPLE:

$\ce{NH_4Cl}$

$\ce{NH4Cl + H_2O <=> NH4OH + HCl}$

Since $\ce{HCl}$ is a strong acid it breaks up and yields $\ce{H+}$, the salt is acidic.

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