Chemistry Topic 3

• What is the law of conservation of mass?

  That no atoms are lost or made during a chemical reaction, so the mass of the products equals the mass of the reactants
• What is the relative formula mass?

  The sum of the relative atomic masses of the atoms in a compound
• How do gasses sometimes make it appear that mass has been lost during a chemical reaction?


  If a gas is formed this will escape, and therefore the mass will decrease
• How do gasses sometimes make it appear that mass has been gained during a chemical reaction?

  If a metal reacts with oxygen, the metal oxide is heavier than the initial metal (by the same mass as the oxygen added)
• What is a mole?

  Just a number - equal to 6.023 x 10^23
• How is the relative formula mass related to one mole of a substance?


  One mole of a substance will weigh the same as the relative formula mass in grams
• How can the relative formula mass be used to calculate how many moles of a substance there is?

  Mass = moles x relative formula mass
• What is a limiting reactant?


  The reactant that runs out first
• What happens when the limiting reactant runs out?

  The reaction stops
• What does an 'excess' of reactant mean?


  When the reaction stops there is still some of that reactant left
• What is the concentration of a solution?

  The mass of something dissolved in a given volume (e.g. grams per dm3)
• How do you convert from cm3 into dm3

  divide by 1000
• What is a dm3?

  1000cm3
• How is the concentration, volume and number of moles related?

  moles = concentration x volume
• Why might you not get 100% yield of a product?


  The reaction might not go to completion as its reversible OR some product might be lost when separated from reaction mixture OR some of the reactants might react in different ways to expected
• What is the percentage yield?

  The measurement of the actual yield compared to the maximum theoretical yield
• What does 'yield' mean?

  The amount of product obtained
• What is the formula for percentage yield?


  (Actual Yield / Maximum theoretical yield) x 100
• What is the atom economy?


  A measure of the amount of reactants that end up as useful products
• What is the formula for percentage atom economy?

  (Relative mass of desired product / Sum of relative mass of all products) x 100
• How might you increase atom economy?

  Find a use for the waste product
• What is the volume of one mole of gas at room temp and pressure?

  24dm3
• How do you calculate the number of moles of gas from its volume?


  moles = volume / 24