GCSE C2 Chapter 3

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  • Created by: emma998
  • Created on: 08-03-14 16:13

The mass of atoms

  • Protons and neutrons both have a relative mass of 1
  • Electrons have a relative mass of 0.0005
  • Protons have a charge of +1 and electrons have a charge of -1
  • Mass number = number of protons + neutrons
  • Atomic number = number of protons (which is also equal to number of electrons)
  • Atoms of the same element all have the same atomic number
  • Isotopes are elements with the same number of protons but a different number of nuetrons
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Masses of atoms and moles

  • Because atoms are much too small to weigh in grams, we use relative atomic masses (Ar)
  • With all elements there is a main isotope and all other isotopes are compared to it. In the periodic table an average value for the mass number is calculated from all the isotopes of the element. However when rounded to a whole number the mass number is often the same as the main isotope.
  • The relative atomic mass of an element in grams is known as 1 mole of atoms of that element
  • Relative formula mass is the relative mass in grams of a substance and is found by adding up all the relative atomic masses of elements in a comound
  • Relative formula mass of a substance is known as 1 mole of the substance
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Percentages and formulae

Working out percentages of elements in a compound substance:

  • Calculate the relative formula mass of substance
  • Find relative atomic mass of element you are finding the percentage of
  • Divide (Ar) of element by relative formula mass of substance then multiply by 100

Working out the formula of a compound from its percentage composition:

  • Find the relative atomic masses of each element
  • Do one element at a time:
    • divide its (Ar) by the percentage of it in the substance which is given
  • Compare results of each element to find the smallest result calculated
  • Divide each result by the smallest result to find the simplist ratio
  • If the any of the answers are not whole numbers then multiply all the answers by the smallest number possible which will make them all whole
  • Put the final answers with each element to give the formula
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Equations and calculations

Calculating masses from chemical equations:

Calculate the mass of calcium oxide that can be made from 10g of calcium carbonate in the reaction: CaCO3 = CaO + CO2

  • Calculate relative formula mass(Mr) of reactant in question (Mr of CaCO3)
  • Calculate relative formula mass of product in question (Mr of CaO)
  • Write "(Mr of reactant) of (reactant) makes (Mr of substance)" of (product)
  • Find out how much one relative gram of reactants can make (divide the Mr of the products by the Mr of the reactants)
  • Multiply the answer by the amount in the question (10g) to calculate the mass of product which can be produced by the amount of reactants given in the question
  • If you have to give result in moles:
    • Multiply the amount in grams by the Mr of the product
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Yield of chemical reactions

Percentage yield = (total amount produced / maximum possible amount) X 100 A student made 4.4g of CaO from 4.0g of Ca. Calculate percentage yield

  • Calculate Mr of reactant
  • Calculate Mr of product
  • Write (Mr of reactant) should make (Mr of product)
  • Divide Mr of reactant by Mr of product to find out how much one gram of reactant should make
  • Multiply this by the amount of reactant specified in question to find the maximum amount possible of product
  • Divide the amount produced by the maximum possible
  • Multiply result by 100

It is usually impossibe to get 100% yield when you complete chemical reations because:

  • Other reactions may occur
  • The reaction may not go to completion
  • Some product may be lost when seperated or collected from apparatus

Maximising %yield helps to save limited resources and reduces waste. Sustainability

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Reversible reactions

  • Reversible reactions are reactions in which the products can react to form the reactants again
  • They go in both directions
  • Ammonium Chloride = Ammonia + hydrogen chloride
  • When heated the ammonium chloride decomposes to form ammonia and hydrogen chloride which forms ammonium again once cooled
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Analysing substances

  • Substances can be added to food to imrove its qualities
  • These substances can be natural or artificial (synthetic chemicals)
  • Foods can be checked to ensure that only legal additives have been added to food
  • This can be done using paper chromotography or mass spectrometry
  • Paper chromotography:
    • A spot of the artificial colour can be placed at the top of a piece of paper
    • A solvent is allowed to move through the paper
    • The spot of colour will move through the paper, the distance dependent upon the solubility of the food colouring
    • The result can then be compared to those of known additives to see if the colouring is a safe and permitted one to use in foods
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Instrumental analysis

  • Modern instrumental methods analysis rapid, accurate + sensitive using small samples
  • Samples for analysis are often substances which are mixtures which must be sperated to identify the different compounds

Gas chromotography linked to a mass spectrometer can be used to do this


  • The mixture of gases are carried by a gas through a long collomn packed with solid particles
  • The different compounds travel at different speeds through the collumn
  • As each gas comes out of the collumn the retention time is recorded
  • The retention times of compounds can compared to retention times of known compounds
  • This can be linked directly to a mass spectrometer which can provide more details about the compounds to quickly identify them

Relative molecular masses

  • A mass spectrometer can also be used to give relative molecular masses
  • For an individual compound the peak with largest mass corrosponds to an ion with just one electrons removed. This is called molecular ion peak (furthest to right)
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