Chemistry C5

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  • Reacting masses can be calculated from a given set of amounts by simple ratio
  • The realtive atomic mass of an element is the average mass of an atom of the element compared to one-twelth of the mass of an atom of carbon-12
  • The empirical formula of C₆H₁₂ is CH₂
  • The emprical formula of C₄H₁₀ is C₂H₅
  • An emprical formula is the simplest whole number ratio of each type of atom in a compound
  • The percentage mass of HNO₃ (Mɾ is 1+14+48 = 63) is H = 1.6%, N = 22.2%, O = 76.2%
  • The number of moles in a chemical = mass/molar mass
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  • The more concentrated a solution, the more crowded the solute particles
  • The GDA on food packaging is the guideline daily amount, often expressed as a percentage
  • A simple pH curve shows a sudden change in pH for a very small volume change in acid or alkali at the end point
  • When using a titration it is important to get several consistant titre readings for reliability
  • For solutions, the amount of moles = concentration x volume
  • A single indicator such as litmus, gives a sharp colour change at the neutral point and not a range of colours like a mixed indicator
  • The concentration of an acid can be calculated from a titration with alkali if the volumes are known or found and the concentration of the alkali is known
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Gas volumes can be: measured after the gas has been:

  • Collected using a gas syringe, collected over water in an upturned burette/measuring cylinder
  • Calculated from the loss on the mass of reactants
  • The reactant that is not on excess and is all used up at the end of the reaction is called the limiting reactant
  • In a reversible reaction the rate of the forward reaction equals the rate of the backward reaction
  • A change in temperature, pressure or concentration of a reactant or product may change the position of the equilibrium
  • The amount of product formed varies with the amount of limiting reactant used
  • The total volume of a gas can be deduced from a graph of a reaction
  • The molar gas volume is 24dm₃ at room temperature and pressure (rtp)
  • The contact process uses a catalyst of V₂O₅, a temperature of around 450⁰c and an atmospheric pressure
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  • Acids ionise in water to produce H+ ions
  • Strong acids ionise completely but weak acids do not ionise fully and form an equilibrium mixture
  • There are three stages in preparing an insoluble compound by precipitation : 1) mixing the reactants, 2) filtering to get the residue, 3) washing and drying the residue
  • Acid strength is a measure of the degree of ionisation of the acid
  • Acid concentration is the number of moles of acid in 1dm₃
  • The pH of a weak acid is much higher than the pH of a strong acid
  • The more H+ ions present, the lower the pH
  • In a precipitation reaction, ions must collide with other ions to form a precipitate
  • Barium chloride is used to test for sulphates by precipitation
  • Ionic equations with state symbols (aq/s) represent precipitation reactions
  • The spectator ions are the ions that remain in solution
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