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Acids, bases and salts

Chemistry · WAEC and JAMB · SS2 and SS3

Practical chemistry in written form. Expect a titration calculation and at least one question on preparing a named salt.

What you need to know

  • Acids produce hydrogen ions in solution; bases accept them. A soluble base is an alkali.
  • Strong acids ionise completely (HCl, H2SO4, HNO3); weak acids only partially (CH3COOH, H2CO3).
  • Neutralisation: acid + base -> salt + water.
  • Acid + carbonate -> salt + water + carbon dioxide. The gas turns limewater milky.
  • Acid + reactive metal -> salt + hydrogen. The gas burns with a pop.
  • Soluble salts of sodium, potassium and ammonium are prepared by titration; insoluble salts by precipitation.
  • Indicators: litmus is red in acid and blue in alkali; methyl orange is red to yellow; phenolphthalein is colourless to pink.
  • pH below 7 is acidic, 7 is neutral, above 7 is alkaline.

Key terms

Basicity of an acid
The number of replaceable hydrogen ions per molecule. HCl is monobasic, H2SO4 dibasic.
Deliquescence
Absorbing moisture from the air until the substance dissolves in it.
Efflorescence
Losing water of crystallisation to the air, so the crystals turn powdery.

Formulae

  • CaVa / CbVb = na / nb
  • pH = -log[H+]

Worked example

25.0 cm^3 of NaOH needed 20.0 cm^3 of 0.1 mol/dm^3 HCl for neutralisation. Find the concentration of the NaOH.

  1. HCl + NaOH -> NaCl + H2O, so na : nb = 1 : 1
  2. CaVa / CbVb = na / nb
  3. (0.1 x 20.0) / (Cb x 25.0) = 1/1
  4. 2.0 = 25.0 Cb

The mistake to avoid

Phenolphthalein is colourless in acid and pink in alkali — not the other way round. Getting the end point backwards invalidates the whole titration answer.

In the exam

In titration questions state the mole ratio from the balanced equation before substituting. For H2SO4 against NaOH the ratio is 1 : 2, not 1 : 1.