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Worked concentration calculation examples and reversals

Quantitative chemistry • Concentration of solutions

Key concepts

What you'll likely be quizzed about

  • Concentration measures the amount of substance per unit volume of solution.
  • It is commonly expressed as mass concentration in grams per decimetre cubed (g dm−3) and as molar concentration in moles per decimetre cubed (mol dm−3).
  • The symbol for molar concentration is c, and the standard unit is mol dm−3.
  • Including the exact units in the final answer is necessary to indicate whether the concentration refers to mass or the amount of substance.

Flashcards

Test your knowledge with interactive flashcards

Calculate concentration in mol dm−3: 2.92 g NaCl (Mr = 58.5) in 200 cm3.

Click to reveal answer

Moles = 2.92 ÷ 58.5 = 0.05 mol. Volume = 0.200 dm3. Concentration = 0.05 ÷ 0.200 = 0.25 mol dm−3.

Key notes

Important points to keep in mind

Always convert cm3 to dm3 before using concentration formulae.

Track units at every step to prevent factor-of-1000 errors.

Use amount (mol) = mass ÷ Mr when calculating molar concentration from mass.

Rearrange formulae algebraically to solve reverse problems for mass or volume.

Round only the final answer to the appropriate number of significant figures.

Include the correct unit (g dm−3 or mol dm−3) with the numeric answer.

Use consistent units for all quantities before performing arithmetic.

Check calculations by substituting the result back into the original formula where possible.

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