Chemical Calculations

Relative Masses and Moles

- the masses of atoms are compared by measuring them relative to atoms of carbon-12

- can work out the relative formula mass of a compound by adding up the relative atomic masses of elements in it, in the ratio shown by its formula

- one mole of any substance is its relative formula mass, in grams

    number of moles = mass / Ar or mass / Mr

- avogadro constant is 6.02 x 10²³

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Equations and Calculations

- balanced symbol equations tell you the number of moles of substances involved in a chemical reaction

- can use balanced symbol equations to calculate the masses of reactants and products in a chemcial reaction

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From Masses to Balanced Equations

- can deduce balanced symbol equations from the masses (and hence the ratio of the numbers of moles) of substances involved in a chemical reaction

- the reactant that gets used up first in a reaction is called the limiting reactant, this is the reactant that is not in excess

- therefore, the amounts of product formed in a chemical reaction are determined by the limiting reactant

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Yield of a Chemical Reaction

- the yield of a chemical reaction describes how much product is made

- the percentage yield of a chemical reaction tells how much product is made compared with the maximum amount that could be made (100%)

- factors affecting the yield of a chemical reaction include product being left behind in the apparatus, reversible reactions not going to completion

- some reactants may produce unexpected reactions, and losses in separating the products from the reaction mixture

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Atom Economy

- it is important to maximise atom economy in industrial processes to conserve the Earth's resources and minimise pollution

- the atom economy of a reaction uses its balanced equation to compare the relative formula mass of the desired prouct with the sum of the relative formula masses of the reactants, it is usually expressed as a percentage

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Expressing Concentration

- concentration (g/dm³) = amount of solute (g) / volume of solution (dm³)

- to calculate the mass of solute in a certain volume of solution of known concentration:

    1. calculate the mass (in grams) of the solute there is 1dm³ (1000cm³) of solution

    2. calcualte the mass (in grams) of solute in 1cm³ of solution 

    3. calculate the mass (in grams) of solute there is in the given volume of the solution

- a more concentrated solution has more solute in the same volume of solution than a less concentrated solution

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Titrations

- titration is used to measure accurately what volumes of acid and alkali react together completely, the point at which a reaction bewteen an acid and an alkali is complete is called the end point of the reaction

- use an acid/ base indicator to show the end point of the reaction between an acid and an alkali

- to calculate the concentration of a solution in mol/dm³, given the mass of the solute in a certain volume:

    1. calculate the mass (in grams) of solute in 1cm³ of solution

    2. calculate the mass (in grams) of solute in 1000cm³ of solution

    3 convert the mass (in grams) to moles

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Titration Calculations

- can use titration to find the unknown concentration of a solution

- need to know the accurate concentration of one solution, then once the end point is established, then the balanced equation gives you the number of moles in a certain volume of solution

- this value is multiplied up to give the concentration in moles per decimetre cubed (which can be converted to grams per decimetre cubed if necessary)

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Volume of Gases

- a certain volume of gas always contains the same number of gas molecules under the same conditions

- the volume of 1 mole of any gas at room temperature and pressure is 24dm³ (24000cm³)

- can use the molar gas volume and balanced symbol equations to calculate volumes of gaseous reactants or products

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