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1.4.4Additional lessons

Molar Concentration — IB Chemistry SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 14 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Curriculum Equation Key Moles & Stoichiometry FormulasFast Exam Solution
n = m / M_r | N = n × N_A (6.022 × 10²³) | M = n / V | V_gas = n × 22.4 L (STP)
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Part 1 of 5Concentration

1.4.4 Molar Concentration

Concentration states how much solute is dissolved in a given volume of solution.

Molar concentration
  • The amount of solute in moles per cubic decimetre of solution, written mol dm⁻³ or M.
Mass concentration
  • The mass of solute per unit volume, in g dm⁻³, which becomes mol dm⁻³ after dividing by molar mass.
Equation
  • c = n ÷ V, with n in mol and V in dm³,
    • so n = c × V and V = n ÷ c.
      What the symbols mean: c = concentration (mol dm−3) · n = amount (mol) · V = volume (dm3; divide cm3 by 1000)
Volume units
  • To convert from cm³ to dm³ divide by 1000, because 1 dm³ = 1000 cm³.

Part 2 of 5Making and Diluting

Concentrations change when more solvent is added.

1
To make a standard solution, weigh the solute and dissolve it in less than the final volume of solvent in a beaker.
2
Transfer to a volumetric flask, rinse the beaker into it, and fill to the mark with solvent before mixing.
3
To dilute, take a measured volume of the concentrated solution and add solvent up to the new volume.
4
The moles of solute do not change on dilution, so c₁V₁ = c₂V₂.

Part 3 of 5Checks

Checks
Solution volume
  • The volume in c = n ÷ V is the volume of the final solution, not of the solvent alone.
Square brackets
  • The concentration of a species is written with square brackets, so [NaCl] means its concentration in mol dm⁻³.
Ion concentration
  • In 0.10 mol dm⁻³ MgCl₂ the concentration of Cl⁻ is 0.20 mol dm⁻³, because each formula unit gives two chloride ions.
Same units
  • In c₁V₁ = c₂V₂ the two volumes must use the same unit, but they need not be in dm³.

Part 4 of 5Worked Example

Question

  • 5.85 g of sodium chloride (M = 58.44 g mol⁻¹) is dissolved in water to make 250 cm³ of solution.
  • Calculate the concentration in mol dm⁻³.

  1. n = 5.85 ÷ 58.44 = 0.1001 mol.
  2. V = 250 ÷ 1000 = 0.250 dm³.
  3. c = 0.1001 ÷ 0.250 = 0.4004 mol dm⁻³.

Answer 0.400 mol dm⁻³

Part 5 of 5Exam Checklist & Tips

1.4.4 – Key Points for the Exam

  • Concentration The amount of solute per unit volume of solution.
  • Standard solution A solution with an accurately known concentration.
  • Dilution Lowering the concentration of a solution by adding more solvent.
  • Volumetric flask Glassware with a single mark, used to make solutions of accurate volume.

Common Mistakes

  • Using the volume in cm³ without converting it to dm³.
  • Using the volume of the solvent added instead of the volume of the final solution.
  • Forgetting to multiply by the number of ions when asked for ion concentration.
  • Believing the moles of solute change when a solution is diluted.

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