pH and solubility | Equilibrium | AP Chemistry | Khan Academy

TL;DR

Changing the pH of a solution can affect the solubility of a slightly soluble salt, increasing it when the salt contains a basic anion.

Transcript

• [Instructor] Changing the pH of a solution can affect the solubility of a slightly soluble salt. For example, if we took some solid lead two fluoride, which is a white solid, and we put it in some distilled water, the solid is going to reach an equilibrium with the ions in solution. Lead two fluoride forms lead two plus ions and fluoride anions i... Read More

Key Insights

• 🧂 Changing the pH of a solution can affect the solubility of a slightly soluble salt if the salt contains a basic anion.
• 😑 Adding H+ ions decreases the concentration of the basic anion, causing the system to shift to the right and increase the solubility of the salt.
• 🧂 Salts with basic anions, such as fluoride, hydroxide, and carbonate, are examples where decreasing the pH increases solubility.
• 💱 However, anions with negligible basicity, like chloride, are not affected by changes in pH.
• 🙂 The Ksp value for a slightly soluble salt remains constant when the pH is decreased, but the molar solubility increases.

Questions & Answers

Q: How does changing the pH of a solution affect the solubility of a slightly soluble salt?

Changing the pH by adding H+ ions can increase the solubility of a slightly soluble salt if the salt contains a basic anion. The decrease in the concentration of the basic anion causes the system to shift to the right and dissolve more of the solid salt.

Q: Why does adding H+ ions decrease the solubility of a slightly soluble salt with a basic anion?

When H+ ions are added to a solution, they react with the basic anion to form a new compound. This reaction decreases the concentration of the basic anion, causing the system to shift to the right and dissolve more of the solid salt.

Q: Are all slightly soluble salts affected by changes in pH?

No, only slightly soluble salts that contain a basic anion are affected by changes in pH. Salts with basic anions, like fluoride and hydroxide, will have increased solubility when the pH is decreased by adding H+ ions.

Q: What happens to the solubility of a slightly soluble salt if the anion is extremely weak?

If the anion is extremely weak, like chloride, changes in pH have no effect on the solubility of the salt. The concentration of the anion remains the same, and the system stays at equilibrium with no change in solubility.

Summary & Key Takeaways

• Adding H+ ions to a saturated solution of lead two fluoride (PbF2) decreases the concentration of fluoride anions, causing the system to shift to the right and increase the solubility of PbF2.

• Increasing the concentration of H+ ions in a solution with a basic anion, such as fluoride or hydroxide, increases the solubility of a slightly soluble salt.

• However, when the anion is extremely weak, like chloride, changes in pH have no effect on the solubility of the salt.