# pH

Solving pH pOH H3O and OH.wmv

Tip: This isn't the place to ask a question because the teacher can't reply.

## Key Questions

• pH is a measure of hydrogen ion concentration. The pH scale was originated by Sorensen. He set 0 pH as the hydrogen ion concentration of a 1.0 molar solution of a strong monoprotic acid (like HCl). 14 on the pH scale uses its standard as the hydrogen ion concentration in a 1.0 molar concentration of a strong base (like NaOH). pH is actually a negative logarithmic scale. To determine pH, take the negative logarithm of the hydrogen ion concentration.

Decreasing the pH increases the solubility of sparingly soluble bases and basic salts.

#### Explanation:

Increasing the pH has the opposite effect.

Many sparingly soluble compounds have solubilities that depend on pH.

It all involves the application of Le ChÃ¢telier's Principle.

Here are two common examples.

Bases

${\text{Zn(OH)}}_{2}$ is a sparingly soluble base.

$\text{Zn(OH)"_2"(s)" â‡Œ "Zn"^"2+""(aq)" + "2OH"^"-""(aq)}$

If you increase the pH by adding $\text{OH"^"-}$ ions, Le ChÃ¢telier's Principle says that the position of equilibrium will move to the left.

The solubility of the ${\text{Zn(OH)}}_{2}$ decreases.

If you decrease the pH by adding ${\text{H"_3"O}}^{+}$ ions, the added ${\text{H"_3"O}}^{+}$ ions will react with the $\text{OH"^"-}$ ions and form water.

$\text{OH"^"-}$ ions are removed from solution.

According to Le ChÃ¢telier's Principle, the position of equilibrium moves to the right to replace the $\text{OH"^"-}$ ions.

The solubility of the ${\text{Zn(OH)}}_{2}$ increases.

Salts of Strong Bases and Weak Acids

${\text{CaCO}}_{3}$ is the sparingly soluble salt of the strong base ${\text{Ca(OH)}}_{2}$ and the weak acid ${\text{H"_2"CO}}_{3}$.

$\text{CaCO"_3"(s)" â‡Œ "Ca"^"2+""(aq)" + "CO"_3^"2-""(aq)}$

$\text{CO"_3^"2-}$ is a base.

$\text{CO"_3^"2-""(aq)" + "H"_2"O(l)" â‡Œ "HCO"_3^"-""(aq)" + "OH"^"-""(aq)}$

The overall equilibrium is

$\text{CaCO"_3"(s)" + "H"_2"O(l)" â‡Œ "Ca"^"2+""(aq)" + "HCO"_3^"-""(aq)" + "OH"^"-""(aq)}$

If you increase the pH by adding $\text{OH"^"-}$ ions, Le ChÃ¢telier's Principle says that the position of equilibrium will move to the left.

The solubility of the ${\text{CaCO}}_{3}$ decreases.

If you decrease the pH by adding ${\text{H"_3"O}}^{+}$ ions, the added ${\text{H"_3"O}}^{+}$ ions will react with the $\text{OH"^"-}$ ions and form water.

$\text{OH"^"-}$ ions are removed from solution.

According to Le ChÃ¢telier's Principle, the position of equilibrium moves to the right to replace the $\text{OH"^"-}$ ions.

The solubility of the ${\text{CaCO}}_{3}$ increases.

Here's a video that discusses the effect of pH on solubility.

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