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Welcome to GCSE Edexcel Science revision.

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Unit S C 14: Quantitative analysis.

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(Higher tier) Concentration in moles per cubic decimetre equals amount in moles divided by volume in cubic decimetres.

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Convert cubic centimetres to cubic decimetres by dividing by 1000 before using concentration equals amount divided by volume;

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rearrange to amount equals concentration times volume or volume equals amount divided by concentration.

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(Higher tier) Concentration in grams per cubic decimetre equals concentration in moles per cubic decimetre times relative formula mass.

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Divide a mass concentration by the relative formula mass to obtain molar concentration.

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A titration finds the volume of one solution needed to react exactly with a measured volume of another.

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Use a balanced equation to establish the mole ratio, which is not always 1 to 1.

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For the core practical, use a pipette and filler to transfer a fixed volume of sodium hydroxide of unknown concentration into a conical flask.

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Add a few drops of a suitable indicator; hydrochloric acid of known concentration goes in the burette.

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Read the meniscus at eye level and swirl the flask near the endpoint.

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Rinse the burette with the solution it will contain, and rinse the pipette with the solution it will measure.

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Rinse the conical flask with distilled water; extra water there changes dilution but not the moles of alkali already pipetted.

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Fill the burette tip and remove air bubbles.

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Remove the filling funnel before measuring.

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Read the bottom of a colourless meniscus at eye level and record initial and final readings; titre equals final reading minus initial reading.

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Place the flask on a white tile, swirl as acid is added and add acid dropwise near the endpoint.

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The endpoint is the indicator’s lasting colour change; methyl orange or phenolphthalein can be suitable, but universal indicator gives too gradual a change.

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Do a rough titration, then repeat accurately to obtain close, concordant titres using the tolerance specified by the teacher or question.

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Calculate a mean from the concordant accurate results, excluding the rough trial.

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(Higher tier) Find moles of the known solution with concentration times volume,

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use the equation ratio to find moles of the unknown,

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then divide by the unknown solution’s volume in cubic decimetres.

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Carry units through each step and round only the final answer.

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Wear eye protection and use a pipette filler, never mouth pipetting.

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Handle acid and alkali according to the risk assessment; wash spills as instructed.

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Theoretical yield is the maximum product predicted from the balanced equation and limiting reactant.

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Actual yield is what is collected experimentally; percentage yield equals actual yield divided by theoretical yield times 100.

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Yield can be reduced by incomplete reaction, unwanted side reactions and losses during separation or transfer.

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A measured yield above 100 percent suggests wet or impure product or measurement and calculation error, not creation of extra atoms.

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Atom economy is the percentage of the reaction’s product mass that is the desired product.

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Calculate it as: total M R of desired products divided by total M R of all products times 100.

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Multiply each M R by its number in the balanced equation before adding.

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Because mass is conserved, the denominator can also be the total relative formula mass of the reactants with their coefficients.

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Atom economy describes the reaction equation, while yield describes practical success; they are different quantities.

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Include balanced-equation coefficients in atom-economy totals.

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An addition reaction with a single product can have 100 percent atom economy even if its experimental yield is low.

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A reaction giving several products can have high yield but low atom economy for the chosen product.

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(Higher tier) When choosing how to make a product,

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compare atom economy,

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yield,

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rate,

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equilibrium position,

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energy use,

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raw-material costs and the usefulness or hazards of the other products (by-products).

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Lower atom economy may be acceptable if those other products can also be used.

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(Higher tier) At room temperature and pressure, one mole of gas occupies about 24 cubic decimetres for GCSE calculations when specified.

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Use V equals n times molar volume with matching volume units; detailed gas-reaction calculations continue in S C 15.

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That completes Quantitative analysis.

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Revisit the notes and test yourself on the revision website.