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1Everything Science www.everythingscience.co.za8. Quantitative aspects of chemical changeChemistryGrade 11
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2Everything Science www.everythingscience.co.zaReactions and gases●The volume of one mole of gas at STP is 22,4 dm3● For any number of moles of gas at STP we can use to find the volume.●The volume relationship for two gases in a reaction is given by:where VAis the volume of gas A, VBis the gas B, a is the stoichiometric coefficient of gas Aand b is the stoichiometric coefficient of gas B.Vg=22,4 ngV A=abVB
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3Everything Science www.everythingscience.co.zaSolutionsThe concentration of a solution can be calculated using:where C is the concentration (in mol·dm3), n is the number of moles of solute dissolved in thesolution and V is the volume of the solution (in dm3).A standard solution is a solution in which the concentration is known to a high degree ofprecision. For the purposes of calculations, a standard solution can be thought of as one inwhich the concentration is a set value.A titration is a technique for determining the concentration of an unknown solution. We cancalculate the unknown concentration using:where CAand CBare the concentrations of A and B (in mol·dm3), VAand VBare the volumes ofA and B and a and b are the stoichiometric coefficients of A and B.C=nVCA V Aa=CB VBb
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4Everything Science www.everythingscience.co.zaStoichiometryThe following diagram is useful for stoichiometric calculations
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5Everything Science www.everythingscience.co.zaStoichiometry calculations1. Limiting reagentsA limiting reagent is a reagent that is completely used up in a chemical reaction.An excess reagent is a reagent that is not completely used up in a chemical reaction.This is determined by calculating the amount of product obtained from each reactant and thenseeing which reactant produces the least amount of product.2. Percent yieldPercent yield is calculated using:where the actual yield is the amount of product that is produced when you carry out thereaction and the theoretical yield is the amount of product that you calculate for the reactionusing stoichiometric methods.yield=actual yieldtheoretical yield×100
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6Everything Science www.everythingscience.co.zaStoichiometry calculations contd...3. Empirical and molecular formulaeThe empirical formula is the simplest formula of a compound.The molecular formula is the full formula of a compound.4. Percent purityPercent purity is calculated using:purity=mass of compoundmass of sample×100
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7Everything Science www.everythingscience.co.zaFor more practice see:www.everythingscience.co.zaShortcode: ESBPG
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