Quantitave Chemistry

• Law of conservation of mass

  No atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants
• Balanced equation of magnesium reacting with hydrochloric acid

  Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
• Relative atomic mass (RAM)


  Average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to 12C
• Relative formula mass (RFM)


  Sum of RAM's of all atoms in the formula
• Avogadro's constant

  The number of atoms, molecules or ions in a mole of a given substance. The value of the constant is 6.02 x 10^23.
• Limiting reactant

  In a chemical reaction involving two reactants, the reactant that is completely used up is called the limiting reactant because it limits the amount of products
• Calculating the mass of oxygen produced from 40.8 g of hydrogen peroxide

  1. Write the balanced equation

  2. Calculate the number of moles in 40.8 g

  3. Use the ratio in the balanced equation to calculate the moles of oxygen

  4. Calculate the mass of oxygen
• Concentration (g per dm3)

  Mass (g)/Volume (dm3)
• Concentration (mol per dm3)


  Number of moles/Volume (dm3)
• Calculating the concentration of potassium hydroxide solution

  1. Calculate the moles of HNO3 used

  2. Calculate the moles of KOH

  3. Calculate the concentration of KOH
• Molar volume of a gas at room temperature and pressure

  1 mole of a gas occupies 24 dm3
• Titration

  A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration
• Conducting a titration

  1. Rinse the pipette and measure out the known volume

  2. Add an indicator

  3. Rinse the burette and gradually add the solution of known concentration

  4. Record the volume added when the indicator changes colour

  5. Obtain concordant volume results

  6. Perform suitable calculations
• Reasons why it is not always possible to obtain the theoretical amount of product in a chemical reaction

  • The reaction may not go to completion because it is reversible
  • Some of the product may be lost when it is separated from the reaction mixture
  • Some of the reactants may react in ways different to the expected reaction (side reactions may occur)
• Percentage yield of a product in a chemical reaction

  Actual mass of a product / Maximum theoretical mass of product x 100%
• Calculating the percentage yield of ammonia

  1. Write the balanced equation

  2. Calculate the theoretical amount of ammonia

  3. Calculate the percentage yield
• Atom economy

  A measure of the amount of starting materials that end up as useful products. It is a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants.
• Reaction II (CuO + 2HCl → CuCl2 + H2O) has a better atom economy than Reaction I (CuCO3 + 2HCl → CuCl2 + H2O + CO2)