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GCSE Chemistry Revision
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GCSE Chemistry revision
Using concentrations of solutions in mol/dm3
Using concentrations of solutions in mol/dm3 (chemistry only) (HT only)
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Key knowledge
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The Key Formula
- The number of moles of an aqueous substance is calculated using: moles =concentration × volume
- Concentration is measured in moles per decimetre cubed (mol dm³) and volume must always be in decimetres cubed (dm³).
- The formula can be rearranged to find concentration: c = (n / V) or volume: V=(n / c), where n = moles, c = concentration, and V = volume.
Converting Units: cm³ to dm³
- Volume is often given in centimetres cubed (cm³), which must be converted to dm³ by dividing by 1000 before using the formula.
- For example, 800 cm³ becomes (800 / 1000) = 0.8 dm³ and 30 cm³ becomes 0.03 dm³.
Calculating Moles from Concentration and Volume
- To find moles, multiply the volume in dm³ by the concentration in mol dm³: n = c ×V
- For example, 0.8 dm³ of sodium hydroxide at 0.2 mol dm³ contains 0.8 × 0.2 = 0.16 moles.
Calculating Concentration from Moles and Volume
- To find concentration, divide the number of moles by the volume in dm³: c = (n / V)
- For example, 0.6 moles of hydrochloric acid in 1.5 dm³ gives a concentration of (0.6 / 1.5) = 0.4 mol dm³.
Step 1: Find Moles of the Known Substance
- In titration-style calculations, always start by identifying the known substance and calculating its number of moles using n = c × V
- For example, 0.03 dm³ of potassium hydroxide at 0.5 mol dm³ gives 0.5 × 0.03 = 0.015 moles of KOH.
Step 2: Use the Molar Ratio
- The molar ratio is found from the balanced chemical equation by looking at the coefficients (numbers) in front of each substance.
- If no number appears before a substance in the equation, it is assumed to have a coefficient of 1.
- For the reaction of potassium hydroxide with sulfuric acid, the ratio is 2:1, meaning 2 moles of KOH react with 1 mole of H₂SO₄.
- Using the ratio, 0.015 moles of KOH reacts with (0.015 / 2) = 0.0075 moles of sulfuric acid.
Step 3: Calculate the Unknown Concentration
- Once the moles of the unknown substance are found, divide by its volume (in dm³) to find its concentration: c = (n / V)
- For example, 0.0075 moles of sulfuric acid in 0.025 dm³ gives (0.0075 / 0.025) = 0.3 mol dm³.
Summary of the Three-Step Method
- Step 1: Calculate the moles of the known substance using n = c × V, ensuring volume is in dm³.
- Step 2: Use the molar ratio from the balanced equation to find the moles of the unknown substance.
- Step 3: Calculate the concentration of the unknown substance using c = (n / V).