E. Chemical Systems and Equilibrium · Acid-Base and Solubility Equilibria

Acid-Base Properties of Salt Solutions

E3.7 E3.6 E2.4 E2.1About 70 minutes

Learning goals

I can predict whether a salt solution is acidic, basic or neutral from the ions it contains.
I can write equations for the reactions of ions with water (salt hydrolysis).
I can calculate the pH of a solution of a salt that contains a weak acid or a weak base ion.
I can compare Ka and Kb to predict the pH of salts in which both ions react, including amphiprotic anions.

Words to know

salt
An ionic compound made of a cation other than H⁺ and an anion other than OH⁻ (or O²⁻), often formed in a neutralization.

hydrolysis (of an ion)
The reaction of an ion with water that produces H₃O⁺ or OH⁻ and changes the pH.

spectator ion (for pH)
An ion that does not react with water, such as Na⁺, K⁺, Cl⁻ or NO₃⁻. It has no effect on pH.

hydrated metal ion
A metal ion surrounded by bonded water molecules, such as [Al(H₂O)₆]³⁺. Small, highly charged ones are weak acids.

Why isn't every salt neutral?

In Grade 11, "acid + base → salt + water" made salts sound neutral. Dissolve some salts in water, though, and the pH is anything but 7. Baking soda solution is basic. Ammonium chloride, used in some fertilizers, makes an acidic solution. Alum, sold in Canadian grocery stores for pickling, is acidic too. Brønsted-Lowry theory explains all of these: some ions are weak acids or weak bases themselves, and they react with water.

Look at the ions one at a time

A salt dissolves into its ions. Ask two questions:

1. Does the anion accept protons from water?

Conjugate bases of strong acids (Cl⁻, Br⁻, I⁻, NO₃⁻, ClO₄⁻) are negligibly weak bases: no effect.
Conjugate bases of weak acids (F⁻, CH₃COO⁻, CN⁻, CO₃²⁻, PO₄³⁻, ClO⁻) are weak bases: they make the solution basic. For example: F⁻(aq) + H₂O(l) ⇌ HF(aq) + OH⁻(aq).

2. Does the cation donate protons to water?

Cations of strong bases (Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺): no effect.
Conjugate acids of weak bases (NH₄⁺, CH₃NH₃⁺) are weak acids: they make the solution acidic. For example: NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq).
Small, highly charged metal ions (Al³⁺, Fe³⁺, Cr³⁺) are surrounded by water molecules in solution. The positive charge pulls electrons away from the O–H bonds of those water molecules, so one of them can give up a proton:

[Al(H₂O)₆]³⁺(aq) + H₂O(l) ⇌ [Al(H₂O)₅(OH)]²⁺(aq) + H₃O⁺(aq)

That is why alum, KAl(SO₄)₂·12H₂O, and solutions of AlCl₃ or FeCl₃ are acidic.

Classify a salt by asking where each ion comes from. Ions from strong acids and strong bases are spectators; ions from weak acids and weak bases react with water.

When both ions react

In ammonium fluoride, NH₄F, the cation is a weak acid and the anion is a weak base. Compare their constants:

Ka(NH₄⁺) = 5.6 × 10⁻¹⁰
Kb(F⁻) = Kw ÷ Ka(HF) = 1.0 × 10⁻¹⁴ ÷ 6.6 × 10⁻⁴ = 1.5 × 10⁻¹¹

Ka > Kb, so the acid reaction wins and the solution is acidic. For NH₄CN, Kb(CN⁻) = 1.6 × 10⁻⁵ is larger than Ka(NH₄⁺), so it is basic. For NH₄CH₃COO, Ka(NH₄⁺) = 5.6 × 10⁻¹⁰ and Kb(CH₃COO⁻) = 5.6 × 10⁻¹⁰ are equal, so the solution is about neutral.

Amphiprotic anions

Ions such as HCO₃⁻, H₂PO₄⁻, HPO₄²⁻ and HSO₄⁻ can act as an acid or a base. Compare the two possibilities:

HCO₃⁻ (in baking soda): as an acid, Ka = Ka2 of H₂CO₃ = 4.7 × 10⁻¹¹. As a base, Kb = Kw ÷ Ka1 = 1.0 × 10⁻¹⁴ ÷ 4.5 × 10⁻⁷ = 2.2 × 10⁻⁸. Kb > Ka, so NaHCO₃ solution is basic.
H₂PO₄⁻: Ka = 6.3 × 10⁻⁸; Kb = 1.0 × 10⁻¹⁴ ÷ 7.1 × 10⁻³ = 1.4 × 10⁻¹². Ka > Kb, so NaH₂PO₄ solution is acidic.
HSO₄⁻: Ka = 1.0 × 10⁻² and its conjugate acid, H₂SO₄, is strong (so it is a negligible base). NaHSO₄ solution is acidic (it is used to lower the pH of swimming pools).

Worked example

Example 1: Classify and explain

Predict whether each 0.10 mol/L solution is acidic, basic or neutral, and write the hydrolysis equation if there is one: (a) KNO₃ (b) CH₃COONa (c) NH₄NO₃ (d) FeCl₃.

(a) K⁺ comes from KOH (strong base); NO₃⁻ comes from HNO₃ (strong acid). Neither reacts with water: neutral.
(b) Na⁺ is a spectator. CH₃COO⁻ is the conjugate base of a weak acid: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻: basic.
(c) NO₃⁻ is a spectator. NH₄⁺ is a weak acid: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺: acidic.
(d) Cl⁻ is a spectator. Fe³⁺ is small and highly charged: [Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺: acidic.

(a) neutral (b) basic (c) acidic (d) acidic.

Worked example

Example 2: pH of an acidic salt

Find the pH of 0.10 mol/L NH₄Cl. Ka(NH₄⁺) = 5.6 × 10⁻¹⁰.

Cl⁻ is a spectator. NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺, with C = 0.10 mol/L.
x = √(5.6 × 10⁻¹⁰ × 0.10) = 7.5 × 10⁻⁶ mol/L = [H₃O⁺]. (Tiny ionization, so the approximation is fine.)
pH = −log(7.5 × 10⁻⁶) = 5.13.

pH = 5.13.

Worked example

Example 3: pH of a basic salt

Find the pH of 0.10 mol/L sodium acetate, CH₃COONa. Ka(CH₃COOH) = 1.8 × 10⁻⁵.

Na⁺ is a spectator. CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻.
Kb = Kw ÷ Ka = 1.0 × 10⁻¹⁴ ÷ 1.8 × 10⁻⁵ = 5.6 × 10⁻¹⁰.
x = √(5.6 × 10⁻¹⁰ × 0.10) = 7.5 × 10⁻⁶ mol/L = [OH⁻].
pOH = 5.13; pH = 14.00 − 5.13 = 8.87.

pH = 8.87. This is the kind of solution present at the equivalence point when acetic acid is titrated with NaOH.

Calculated pH of five 0.10 mol/L salt solutions at 25 °C. The farther an ion's conjugate partner is from being strong, the larger its effect on pH.

Just for fun

Baking soda walks into a café and orders a vinegar. The server says, "Are you sure? Last time you got all fizzy." Baking soda replies, "Relax, I'm basically harmless."

Hands-on activity

Test kitchen salts with red cabbage indicator

You need: red cabbage indicator, 5 clear glasses, a teaspoon, warm tap water, and these salts if you have them: table salt (NaCl), baking soda (NaHCO₃), Epsom salt (MgSO₄·7H₂O, from a pharmacy), pickling alum (KAl(SO₄)₂·12H₂O, from the spice or canning aisle), and cream of tartar (potassium hydrogen tartrate, from the baking aisle).

Grown-up note: an adult should supervise. These are food or pharmacy products, but do not taste the solutions. Wash hands afterwards.

Dissolve about ½ teaspoon of each salt in ¼ cup of warm water in its own glass. (Cream of tartar dissolves only partly; that's fine.)
Add 2 tablespoons of cabbage indicator to each glass, and compare with a glass of indicator in plain water.
Record each colour and classify each solution as acidic, neutral or basic.
For each salt, name the ion responsible (if any) and write its reaction with water.

What to look for: table salt stays purple (neutral: Na⁺ and Cl⁻ are spectators). Baking soda turns blue or blue-green (basic: HCO₃⁻ has Kb > Ka). Alum turns pink to reddish (acidic: hydrated Al³⁺ donates protons). Cream of tartar turns pink (acidic: the hydrogen tartrate ion is an amphiprotic ion whose Ka is larger than its Kb). Epsom salt stays close to purple (Mg²⁺ and SO₄²⁻ have only small effects).

Your turn

Is a solution of KBr acidic, basic or neutral? Explain.

Write the hydrolysis equation for the ion that makes Na₂CO₃ solution basic.

Write the hydrolysis equation that makes NH₄NO₃ solution acidic.

Find the pH of 0.20 mol/L NH₄NO₃.

Find the pH of 0.10 mol/L NaF.

Predict whether NH₄CN solution is acidic or basic. Show the comparison.

More practice: warm-up

Name two anions that are spectators (no effect on pH).

Name two cations that are spectators.

Is the acetate ion an acid or a base in water?

Is the ammonium ion an acid or a base in water?

A salt made from a strong acid and a weak base gives what kind of solution?

More practice: on your own

Find the pH of 0.25 mol/L sodium methanoate (sodium formate), HCOONa. Ka(HCOOH) = 1.8 × 10⁻⁴.

Explain why a solution of AlCl₃ is acidic even though Al³⁺ has no hydrogen atoms in its formula.

Is NaH₂PO₄ solution acidic or basic? Compare Ka and Kb of H₂PO₄⁻.

Rank 0.10 mol/L solutions of NaCl, NH₄Cl, NaCN and CH₃COONa from lowest to highest pH.

A 0.10 mol/L solution of the salt NaA has pH 9.50. Find Kb of A⁻ and Ka of HA.

Challenge

For an amphiprotic salt such as NaHCO₃, a good estimate is pH ≈ ½(pKa1 + pKa2). Estimate the pH of baking soda solution, and check it against your cabbage result.

What mass of NH₄Cl (M = 53.50 g/mol) must be dissolved in 500.0 mL of water to make a solution with pH 5.00?

Ammonium acetate solution is about neutral. Explain why, and explain why ammonium fluoride is acidic.

Check your understanding

Quiz

Which salt forms a basic solution?

Which salt forms an acidic solution?

What is the pH of 0.10 mol/L CH₃COONa? (Ka of CH₃COOH = 1.8 × 10⁻⁵)

For HCO₃⁻, Ka = 4.7 × 10⁻¹¹ and Kb = 2.2 × 10⁻⁸. A solution of NaHCO₃ is therefore

Which ion has no effect on the pH of water?

In NH₄F, Ka(NH₄⁺) = 5.6 × 10⁻¹⁰ and Kb(F⁻) = 1.5 × 10⁻¹¹. The solution is

For parents and teachers

Big idea (E3.7, E3.6): a salt solution's pH depends on its ions. Ions from strong acids and strong bases are spectators; conjugate bases of weak acids make solutions basic; conjugate acids of weak bases (and small, highly charged metal ions) make them acidic. The pH is calculated with Ka or Kb exactly as for any weak acid or base.

How to help: Name a salt and ask your teen to split it into ions and say where each came from ("Na⁺ from NaOH, a strong base"). The cabbage activity is a striking way to see that salts are not all neutral; supervise, and keep all products away from food prep surfaces.

Common difficulties:

Thinking every product of neutralization is neutral.
Using Ka of the wrong acid for an anion's Kb (for CO₃²⁻ use HCO₃⁻; for HCO₃⁻ as a base use H₂CO₃).
Forgetting that metal ions like Al³⁺ and Fe³⁺ are acidic.
Calling an amphiprotic salt acidic just because it contains an H; Ka must be compared with Kb.