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WAEC 2016 Chemistry · Question 48

On evaporation to dryness, 350cm3^3 of saturated solution of salt Z gave 55.5g of salt. What is the solubility of the salt? [Z = 101]

Options

  1. A.1.57 moldm−3dm^{-3}
  2. B.3.14 moldm−3dm^{-3}
  3. C.6.28 moldm−3dm^{-3}
  4. D.12.56 moldm−3dm^{-3}

Answer: A. 1.57 moldm−3dm^{-3}

Explanation written with AI and checked against the answer key

Analyze the question: We need the solubility in mol/dm³ (molar concentration of the saturated solution), given the mass of salt in a known volume.

Concept: Solubility (mol/dm³) = moles of solute ÷ volume of solution in dm³

Moles = mass ÷ molar mass

Solution:

  1. Moles of salt = 55.5 g ÷ 101 g/mol = 0.5495 mol
  1. Volume = 350 cm³ = 0.350 dm³
  1. Solubility = 0.5495 ÷ 0.350 = 1.57 mol/dm³

Check: 1.57 × 0.350 = 0.5495 mol, which matches. Multiplying by 101 gives 55.5 g. ✓

Answer: A. 1.57 mol dm⁻³

Tip: Always convert cm³ to dm³ by dividing by 1000 before calculating concentration. A common mistake is leaving the volume in cm³, which gives a wrong answer such as 0.00157. Also, if you want g/dm³ first: 55.5 × (1000/350) = 158.6 g/dm³, then divide by 101 to get 1.57 mol/dm³.

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