📘 CodingMarble Learn

Acids, Bases, Salts and Titration

Inorganic compounds fall into four big classes: oxides, acids, bases and salts. They are linked: a metal makes a basic oxide, that makes a base, and a base plus an acid makes a salt and water. A titration uses this neutralisation to measure an unknown concentration: base of known concentration is added from a burette until all the acid is used up (the equivalence point). Then n(acid) = n(base) by the reaction ratio, so C₁V₁ = C₂V₂ for a 1 : 1 reaction.

🎬 Step-by-step story

  1. Most inorganic compounds belong to four classes: acids, bases, salts and oxides. Each class has a pattern in its formula.
  2. The classes are linked like a family tree. A metal burns to a basic oxide. The oxide plus water gives a base. The base plus an acid gives a salt.
  3. Now we use that last reaction to measure. The flask holds 25 mL of acid. The burette holds a base whose strength we know.
  4. We add the base slowly. The pH climbs only a little at first, because there is still lots of acid left to use up the base.
  5. Near 25 mL the acid is all used up. One more drop and the pH jumps from about 3 to 11. The indicator turns pink. This is the equivalence point.
  6. Your turn. Drag the slider to add base. Find the exact volume where the colour changes, and read the pH.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why is NH₃ a base if it has no OH in its formula?

In the Brønsted–Lowry idea a base is a proton taker. NH₃ grabs H⁺ from water and makes NH₄⁺ + OH⁻.

Why is a metal oxide basic and a non-metal oxide acidic?

Metal oxides give hydroxides with water (CaO → Ca(OH)₂); non-metal oxides give acids (SO₂ → H₂SO₃). Follow the chain in step 2.

Why do we need a burette and not just a measuring cylinder?

A burette reads to 0.05 mL and lets you add single drops, so you can stop exactly at the colour change.

Why does pH barely change for the first 20 mL?

There is still lots of acid. Each mL removes only a small share of the H⁺, and pH is a log scale. Watch the short bars in step 4.

Why does one drop change the colour?

Near equivalence almost no acid is left, so one drop of base turns the solution from slightly acidic to basic and the pH jumps into the indicator's range.

Is the equivalence point always at pH 7?

Only for strong acid + strong base. Weak acid + strong base is above 7; strong acid + weak base is below 7.

The four classes of inorganic compounds

Oxides are compounds of an element with oxygen, like CaO or CO₂. Metal oxides are usually basic; non-metal oxides are usually acidic; some, like Al₂O₃ and ZnO, are amphoteric (they react with both acids and bases).

Acids give H⁺ ions in water (HCl, H₂SO₄, HNO₃). Bases give OH⁻ ions or accept H⁺ (NaOH, Ca(OH)₂, NH₃). A base that dissolves in water is an alkali. Salts are made of a metal (or NH₄⁺) ion and an acid's leftover ion (NaCl, CuSO₄, NH₄NO₃).

Acid–base ideas you should know

Naming acids, bases and salts

The genetic link between the classes

One element can be followed through the classes, like a family line:

Metal: Ca → CaO → Ca(OH)₂ → CaCO₃ (2Ca + O₂ → 2CaO; CaO + H₂O → Ca(OH)₂; Ca(OH)₂ + CO₂ → CaCO₃ + H₂O)

Non-metal: S → SO₂ → H₂SO₃ → Na₂SO₃

The two lines meet in a salt. Salts form in many ways: acid + base, acid + metal, acid + basic oxide, base + acidic oxide, and acid + carbonate.

Introduction to titration

A titration finds the unknown concentration of a solution by reacting it with a solution of known concentration (the standard solution or titrant).

  1. Pipette an exact volume (say 25.0 mL) of the unknown acid into a conical flask. Add 2–3 drops of indicator.
  2. Fill the burette with the standard base. Read the start volume at the bottom of the meniscus, at eye level.
  3. Add base while swirling. Near the end add it drop by drop.
  4. Stop at the first permanent colour change: the end point. Record the final reading. Repeat until two results agree within 0.10 mL (concordant).

Equivalence point = moles of acid and base match exactly by the equation. End point = when the indicator changes. A good indicator makes them almost the same.

Titration curves and choosing an indicator

Plot pH against volume of base added. You get an S-shaped titration curve.

Rule: choose an indicator whose colour-change range sits inside the steep part of the curve.

Titration calculations

Moles = concentration (mol/L) × volume (L). Use the balanced equation's ratio.

For HCl + NaOH (1 : 1): CaVa = CbVb.

For H₂SO₄ + 2NaOH: moles NaOH = 2 × moles H₂SO₄, so 2CaVa = CbVb.

Before equivalence (strong acid): [H⁺] = (leftover moles of acid) ÷ (total volume). After: [OH⁻] = (extra moles of base) ÷ (total volume), then pH = 14 − pOH (at 25 °C).

Key formulas and definitions

Worked examples

1. Name: (a) HNO₂ (b) K₂SO₄ (c) Fe(OH)₂ (d) NaHCO₃.

(a) nitrous acid (less O → -ous). (b) potassium sulfate (sulfuric → sulfate). (c) iron(II) hydroxide. (d) sodium hydrogencarbonate, an acid salt.

2. Write the genetic chain for magnesium ending in magnesium chloride.

2Mg + O₂ → 2MgO; MgO + H₂O → Mg(OH)₂; Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.

3. 25.0 mL of HCl needs 20.0 mL of 0.100 M NaOH. Find [HCl].

n(NaOH) = 0.100 × 0.0200 = 0.00200 mol = n(HCl). C = 0.00200 ÷ 0.0250 = 0.0800 M.

4. 20.0 mL of H₂SO₄ is neutralised by 32.0 mL of 0.250 M NaOH. Find [H₂SO₄].

n(NaOH) = 0.250 × 0.0320 = 0.00800 mol. H₂SO₄ : NaOH = 1 : 2, so n(H₂SO₄) = 0.00400 mol. C = 0.00400 ÷ 0.0200 = 0.200 M.

5. 25.0 mL of 0.100 M HCl. Find the pH after adding 20.0 mL of 0.100 M NaOH.

Acid 0.00250 mol, base 0.00200 mol. Leftover H⁺ = 0.000500 mol in 45.0 mL. [H⁺] = 0.000500 ÷ 0.0450 = 0.0111 M. pH = 1.95.

6. Same flask, after 30.0 mL of NaOH. Find the pH.

Base 0.00300 mol, extra OH⁻ = 0.000500 mol in 55.0 mL. [OH⁻] = 0.00909 M, pOH = 2.04, pH = 11.96. The pH jumped from about 2 to 12 over just 10 mL – the steep part of the curve.

Common mistakes

Practice quiz

1. Which is an amphoteric oxide?
2. The salt of sulfurous acid is called a:
3. For CH₃COOH titrated with NaOH, the best indicator is:
4. At the equivalence point of HCl + NaOH at 25 °C, pH is:
5. Moles of NaOH in 15.0 mL of 0.200 M solution:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is titration in simple words?

Adding a solution of known strength drop by drop to another until they just react completely, so you can work out the unknown strength.

What is the difference between end point and equivalence point?

Equivalence point: moles of acid and base match by the equation. End point: the indicator changes colour. A good indicator makes them nearly equal.

How do you name salts?

Metal name first, then the acid part: -ic acids give -ate salts, -ous acids give -ite salts, and hydro-…-ic acids give -ide salts.

Where this is taught

USA (Common Core, NGSS, AP)Grade 11Chemical Reactions
USA (Common Core, NGSS, AP)Grade 11Thermochemistry
USA (Common Core, NGSS, AP)Grade 11Acids and Bases
Russia11 классTheoretical foundations of chemistry

Learn first

Learn next

Related lessons

All Chemistry lessons