Quantitative Chemistry

• The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.
• Mr of a compound is the sum of each atoms relative atomic mass.
• The Mr of H2O is 1+1+16=18.
• Ar simply stands for relative atomic mass.
• Errors can be random (unpredictable differences like human error) or systematic (if a measurement is wrong by the same amount each time).
• Zero errors are a type of systematic error where the equipment isn’t zeroed properly.
• Anomalous result is when values don’t follow the pattern.
• Resolution is the smallest change a measuring instrument can detect.
• Uncertainty is when you repeat a measurement you often get slightly different results.
• Uncertainty = Range / 2.
• A mole is a unit of substance (mol) fixed number of particles like a dozen.
• The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
• Avogadro’s constant = 6.02 x 10²³
• Number of moles=mass of an element or compound / Mr or Ar of an element or compound.
• Percentage yield=actual yield / theoretical yield x 100.
• Actual yield is the mass from your experiment.
• Theoretical yield is the expected if 100%.
• Some reactions don’t make 100% of the product because the reaction may be reversible, some product may be lost in containers or wasted, some reactants may react in unexpected ways or not at all, or the reaction may be too slow.
• Atom economy is the percentage of useful products.
• Atom economy=Mr of desired products / Mass of all products x 100.
• How can chemical equations be interpreted in terms of moles?


  Chemical equations show the ratio of moles of reactants and products
• What does the equation Mg + 2HCl → MgCl2 + H2 represent in terms of moles?


  It shows a 1:2:1:1 mole ratio of Mg, HCl, MgCl2, and H2
• In exam questions, how should you use the concept of moles?


  You must apply the moles equation to solve the problem
• What is the limiting reactant in a chemical reaction?


  The limiting reactant is the one that is completely used up
• Why is the limiting reactant important in chemical reactions?


  It limits the amount of products that can be formed
• How do you determine the limiting reactant in exam questions?


  You calculate the moles and see which reactant runs out first
• What is concentration in chemistry?

  Concentration is the amount of solute in a specific volume of solvent
• How is concentration calculated?

  Concentration (g/dm³) = mass of solute (g) / volume of solution (dm³)
• What is the volume equivalent of 1 decimetre cubed (dm³)?


  1 dm³ = 1000 cm³
• What does Avogadro’s Law state about gases?


  Equal amounts of gases occupy the same volume at the same temperature and pressure
• What is the molar gas volume at room temperature and pressure (RTP)?


  The molar gas volume at RTP is 24 dm³
• Volume of gas equation

  Volume of gas = Mass of gass/Mr of gas x 24
• What are the key concepts related to using moles in chemical equations?


  • Chemical equations represent the ratio of moles of reactants and products.
  • Limiting reactant is the one that is completely used up.
  • Concentration is the amount of solute in a specific volume of solvent.
  • Avogadro’s Law states equal amounts of gases occupy the same volume at the same conditions.