What Is a Salt?

A salt is an ionic compound formed when an acid and a base react. That already makes it a little different from everything else in this chapter. The acids we have been studying (an H followed by some anion) are molecular compounds made entirely of non-metals. The strong bases — the Group 1 and Group 2 metal hydroxides — are technically ionic compounds and could be called salts, but we treat them as their own category. This lesson covers all the remaining salts: the vast middle ground of ionic compounds you encounter in solution.

3 Classes of Salts

When you dissolve a salt in water, one of three things happens to the pH:

• Acidic salts drive the pH below 7.
• Basic salts drive the pH above 7.
• Neutral salts leave the pH right around 7.

Your job is to look at any ionic compound and correctly predict which category it falls into. The good news: you can do this almost entirely from memory — as long as you have your strong acids and strong bases memorized cold.

Cations and Anions: The Lewis Acid/Base Framework

The strategy for classifying salts is to evaluate the cation and the anion separately, then combine the results. The Lewis definition of acids and bases gives us an elegant way to think about which ion will do what in water.

Cations Are Lewis Acids

A Lewis acid is an electron-pair acceptor. Which ions are more likely to accept electrons? Positively charged ones. Cations are therefore naturally predisposed to behave as Lewis acids in water — they pull electron density toward themselves, which can disrupt water molecules and release H⁺. This is precisely why metal cations were introduced as Lewis acids earlier in the chapter: species like boron compounds with empty orbitals and metal ions with high positive charge are classic electron-pair acceptors.

Anions Are Lewis Bases

A Lewis base is an electron-pair donor. Negatively charged anions have extra electron density and are primed to donate it. Many anions also qualify as Bronsted-Lowry bases, accepting a proton from water and releasing hydroxide in the process. This process — an ion reacting with water to alter pH — is called salt hydrolysis.

The Key Insight: Remember the Exceptions

The default assumption is: cations are acidic, anions are basic. The exceptions to this rule are small and predictable — and knowing them is the entire skill. There are only a handful of cations that are not acidic, and only a handful of anions that are not basic. Once you know the exceptions, everything else follows automatically.

6 Negligible Anions: Conjugate Bases of Strong Acids

There is a fundamental rule in acid-base chemistry: the stronger the acid, the weaker its conjugate base. Strong acids sit at the extreme end of this scale — their conjugate bases are so incredibly weak that they cannot measurably act as bases in water. We call these negligible bases.

The 6 Negligible Conjugate Bases

Six of the seven strong acids produce negligible conjugate bases:

• Cl⁻ (from HCl)
• Br⁻ (from HBr)
• I⁻ (from HI)
• NO₃⁻ (from HNO₃)
• ClO₄⁻ (from HClO₄)
• ClO₃⁻ (from HClO₃)

These anions have zero ability to act as bases in water. They do not affect pH. When you see one of them in a salt, cross it off — it contributes nothing to the acid-base behavior of the solution.

The HSO₄^- Exception

H₂SO₄ is a special case because it is the only polyprotic strong acid. Only the first proton dissociates completely; the second dissociates only partially. Once H₂SO₄ donates its first proton, it becomes HSO₄⁻ — and that species is a weak acid, not a negligible base.

Anion Classification Summary

Negligible Cations: Groups 1, 2, and Low-Charge Metals

On the cation side, the parallel logic applies: most cations are acidic, but a specific set are negligible — they have no meaningful effect on pH.

Which Cations Are Negligible?

The negligible cations are:

• Group 1 metal ions: Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺
• Group 2 metal ions: Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺
• Transition metal ions with only a +1 charge (e.g., Ag⁺)

Any cation outside this list — most transition metal ions, post-transition metals, and polyatomic cations like ammonium (NH₄⁺) — is an acidic cation that will lower the pH of the solution.

2 Lewis Acid Strength Trends for Metal Ions

For the non-negligible metal cations, two factors determine how strong a Lewis acid they are:

Cation Classification Summary

Combined Quick-Reference: Negligible Ions at a Glance

4 Rules for Classifying Salts

With the negligible lists established, classifying any salt reduces to a simple two-step check: evaluate the cation, evaluate the anion, then combine. The four possible outcomes are:

The fourth case — both cation and anion active — is rare on exams and requires comparing the K_a of the cationic acid with the K_b of the anionic base. Whichever is larger wins: a higher K_a makes the salt acidic, a higher K_b makes it basic. Without table values, this case cannot be determined by inspection alone. One special sub-case: if the anion is hydroxide, the salt is simply basic regardless of the cation, because hydroxide is a strong base.

Worked Examples: 5 Ionic Compounds

Let us work through five ionic compounds and classify each one. The method has two steps: check the cation against the negligible list, then check the anion.

Step 1: Evaluate the Cation

Ask: is this cation Group 1, Group 2, or a +1 transition metal? If yes, it is negligible. If no, it is an acidic cation.

• Na⁺ — Group 1. Negligible.
• Ba²⁺ — Group 2. Negligible.
• Mg²⁺ — Group 2. Negligible.
• Al³⁺ — Not Group 1 or 2; charge is +3. Acidic cation.
• NH₄⁺ — Not a metal at all. Acidic cation.

Step 2: Evaluate the Anion

Ask: is this anion the conjugate base of one of the six (non-sulfuric) strong acids? If yes, it is negligible. If no, it is a basic anion.

An alternative approach: add H⁺ to the anion and see what acid you get. If that acid is a strong acid, cross the anion off — it is negligible. Similarly, add OH^- to the cation; if you get a strong base (e.g., NaOH), cross the cation off.

• Cl^- — conjugate base of HCl (strong acid). Negligible.
• F^- — conjugate base of HF (weak acid). Basic anion.
• ClO₄^- — conjugate base of HClO₄ (strong acid). Negligible.
• OH^- — a strong base in its own right. Basic anion.
• CN^- — conjugate base of HCN (weak acid). Basic anion.

Results for All 5 Examples

Calculating the pH of Salt Solutions

Once you have classified a salt, calculating the pH of its solution is a direct extension of the weak acid and weak base pH calculations you already know. There are only four types of pH calculations in this chapter: strong acid, strong base, weak acid, and weak base. Salt solutions reduce to one of the weak acid or weak base cases.

Example 1: pH of 0.1 M Sodium Fluoride (Basic Salt)

Na⁺ is negligible; F^- is the active ion. Fluoride undergoes salt hydrolysis — but not as a strong base (strong bases are Group 1 and Group 2 metal hydroxides). Therefore, F^- is a weak base, and we use the weak-base shortcut:

[OH^-] = √(K_b × C)

Finding K_b for F^- Using K_a × K_b = K_w

Ionic species rarely appear in equilibrium tables directly. What you will find tabulated is K_a for the corresponding neutral acid, HF: K_a = 6.8 × 10⁻⁴. Use the conjugate pair relationship:

K_a × K_b = K_w   ⇒   K_b = K_w / K_a

Example 2: pH of 0.1 M Ammonium Chloride (Acidic Salt)

Cl^- is negligible; NH₄⁺ is the active ion. Ammonium chloride is an acidic salt — NH₄⁺ is not a Group 1 or Group 2 metal, and it is not even a metal. It is a weak acid that donates a proton to water, producing H⁺ and ammonia (NH₃). Use the weak-acid shortcut:

[H⁺] = √(K_a × C)

Finding K_a for NH₄⁺ Using K_a × K_b = K_w

Again, the ion itself will not appear in tables, but its conjugate base, ammonia (NH₃), is a neutral species with a well-known K_b = 1.76 × 10⁻⁵. Apply the same relationship:

K_a = K_w / K_b

Why Salt pH Calculations Are the Trickiest

Salt pH problems are consistently the most challenging pH calculations on general chemistry exams. The trap is deceptively simple: you are given a salt like NaF and a K_a value for HF. Many students immediately multiply K_a by the concentration and take the square root — and get the wrong answer.

The mistake is conflating two different solutions. The K_a shortcut [H⁺] = √(Ka × C) applies when C is the concentration of the weak acid itself — in this case, HF. But you do not have an HF solution. You have a sodium fluoride solution. The active species is F^-, a weak base, not a weak acid.

The pattern is always the same: one ion is negligible, the other is active, and the active ion is either a weak acid or a weak base. Identify which, write the shortcut, and figure out how to get the K you need. Systematic process beats intuition here every time.

Looking Ahead

At this point in the chapter, there are four types of pH calculations: strong acid, strong base, weak acid, and weak base. Salt solutions fold neatly into the weak acid and weak base categories — the challenge is recognizing which one you have and executing the method cleanly.

The difficulty will increase in the next chapter, where buffers and titrations introduce additional layers of pH calculation. Those topics build directly on everything covered here. The more automatic your salt-classification and pH-calculation process becomes now, the better equipped you will be when the water gets murkier.