Question

The pH of a solution describes its acidity or alkalinity: Describe how pH and H3O+ concentration are related and explain why diluting an acid raises the pH, but diluting a base lowers the pH.

Answer 1

Answer on Question #76522, Chemistry / General Chemistry

The pH of a solution describes its acidity or alkalinity: Describe how pH and $\mathrm{H}_3\mathrm{O}^+$ concentration are related and explain why diluting an acid raises the pH, but diluting a base lowers the pH.

Answer

pH shows the concentrations of $\mathsf{H}^+$ ions in the solutions. As $\mathsf{H}^+$ ion is a single proton, it is very small and positive particle with high charge density, that's why it founds a molecule to join. It joins usually to a molecule of water (because there are a lot of them in the solution).

$$\mathrm{H}_2\mathrm{O} + \mathrm{H}^+ \rightarrow \mathrm{H}_3\mathrm{O}^+$$

In other words in water solution $\mathsf{H}^+$ ions exist in the form of $\mathrm{H}_3\mathrm{O}^+$. So their concentrations are equal.

pH is **negative** logarithm of the concentration of $\mathsf{H}^+$.

$$\mathrm{pH} = -\lg[\mathrm{H}^+].$$

That means that if the concentration is 0.1 M or $10^{-1}$ M, pH is 1. If the concentration is 0.01 M or $10^{-2}$ M, pH is 2. Less concentration of acid means higher pH. If acidic solution is very diluted like $10^{-6}$ M, pH is 6.

In case of bases there is another unit pOH that shows the concentration of $\mathrm{OH}^-$ ions in the same way like pH

$$\mathrm{pOH} + \mathrm{pH} = 14, \text{ or } \mathrm{pH} = 14 - \mathrm{pOH}$$

In this case diluting the base we increase pOH and increase pH at the same time.

If the concentration of base is $10^{-1}$ M, pOH is 1, so pH is 13. If the base is more diluted like $10^{-4}$ M, pOH is 4, and pH is 10. The lower concentration of base means the lower magnitude of pH only because it is **negative** logarithm.