Chapter 18 (Aqueous ionic equilibrium)

Buffer

A solution that resists changes in pH when small amounts of acid or base are added.

Components of a buffer

Weak acid + conjugate base OR weak base + conjugate acid.

Best buffer pH range

pH ≈ pKa ± 1.

Henderson–Hasselbalch equation

pH = pKa + log([A⁻]/[HA]).

When to use Henderson–Hasselbalch

When both weak acid and conjugate base are present (buffer present).

What happens when strong acid is added to a buffer

H⁺ reacts with A⁻ → HA; pH decreases slightly.

What happens when strong base is added to a buffer

OH⁻ reacts with HA → A⁻; pH increases slightly.

Buffer capacity

How much acid/base a buffer can neutralize before pH changes significantly.

Strong acid + buffer reaction equation

H⁺ + A⁻ → HA.

Strong base + buffer reaction equation

OH⁻ + HA → A⁻ + H₂O.

How to choose a buffer

Choose a weak acid with pKa close to desired pH.

Titration

A method to determine concentration by reacting unknown solution with known solution.

Equivalence point

Point where moles of acid = moles of base.

Strong acid–strong base equivalence point

pH = 7.

Weak acid–strong base equivalence point

pH > 7 due to basic conjugate base formed.

Weak base–strong acid equivalence point

pH < 7 due to acidic conjugate acid formed.

Half-equivalence point

The point where [HA] = [A⁻]; pH = pKa.

Indicator

A substance that changes color at specific pH ranges; used to detect equivalence point.

Solubility

The amount of solute that dissolves in a given amount of solvent at a given temperature.

Molar solubility

The number of moles of solute that dissolve per liter of solution.

Solubility product constant (Ksp)

Equilibrium constant for the dissolution of an ionic solid.

Expression for Ksp

Product of ion concentrations raised to their coefficients; solids not included.

Common ion effect

Decreased solubility when a common ion is added.

Ion product (Qsp)

Calculated like Ksp but using initial concentrations.

If Qsp < Ksp

Solution is unsaturated; more solid dissolves.

If Qsp = Ksp

Solution is at equilibrium; saturated.

If Qsp > Ksp

Precipitation occurs; solution is supersaturated.

Factors that increase solubility of ionic compounds

Adding acid to dissolve basic salts (like carbonates).

Ksp vs solubility

A higher Ksp usually means higher solubility.

Effect of pH on solubility

Salts with basic anions dissolve better in acidic solutions.

Ksp calculation for 1:1 salts (AB)

If AB → A⁺ + B⁻, then Ksp = s².

Ksp calculation for 1:2 salts (AB₂)

If AB₂ → A⁺ + 2B⁻, then Ksp = 4s³.

Ksp calculation for 2:3 salts (A₂B₃)

If A₂B₃ → 2A³⁺ + 3B²⁻, then Ksp = 108s⁵.

Selective precipitation

Using differences in Ksp values to precipitate one ion before another.