quantitative chemistry

• Conservation of mass


  No atoms are lost or made during a chemical reaction so the mass of the products = mass of the reactants
• Balanced chemical equation


  The numbers of atoms of each element involved are exactly the same on both sides of the equation
• Relative formula mass (Mr)


  Sum of the relative atomic masses of the atoms in the numbers shown in the formula
• Relative formula mass


  • For HCl: Mr= 1 + 35.5 = 36.5
• In a balanced chemical equation, the sum of Mr of reactants in quantities shown = sum of Mr of products in quantities shown
• If a reaction appears to involve a change in mass


  Check to see if this is due to a reactant or a product as a gas and its mass has not been taken into account
• When a metal reacts with oxygen


  • Mass of metal oxide product > mass of metal
• Whenever a measurement is made there is always some uncertainty about the result obtained
• Be prepared to make estimations of uncertainty
• Mole


  Chemical amount measured in the unit mole, symbol mol
• Mass of one mole of a substance in grams


  Numerically equal to its relative formula mass
• Mass of one mole

  • Iron (Mr 56) is 56g
  • Nitrogen gas, N2 (Mr 28) is 28g
• One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance
• Converting between moles and grams

  Use triangle or equation: mass = moles x molar mass
• Converting moles to grams

  • 42g of carbon = 3.5 moles
• Amounts of substances in equations

  Masses of reactants & products can be calculated from balanced symbol equations
• Avogadro constant


  Number of atoms, molecules or ions in a mole of a given substance: 6.02 x 10^23 per mole
• Chemical equations


  Can be interpreted in terms of moles
• E.g. Mg + 2HCl -> MgCl2 + H2


  • Shows that 1 mol Mg reacts with 2 mol HCl to produce 1 mol MgCl2 and 1 mol H2
• The total moles of one element must be the same on both sides of the equation
• Using moles to balance equations

  1. Convert the masses in grams to amounts in moles

  2. Convert the numbers of moles to simple whole number ratios, then you know how many moles you have of one element/compound compared to another
• Limiting reactant


  The reactant that is used up / not in excess (since it limits the amount of products)
• If a limiting reactant is used, the amount of product produced is restricted to the amount of the excess reactant that reacts with the limiting one</b>
• Concentration of a solution

  Can be measured in mass per given volume of solution e.g. grams per dm3 (g/dm3)
• To calculate mass of solute in a given volume of a known concentration


  Use mass = conc x vol i.e. g = g/dm3 x dm3 (think about the units!)
• A smaller volume or larger mass of solute


  Gives a higher concentration
• A larger volume or smaller mass of solute


  Gives a lower concentration
• Percentage yield


  Amount of product produced x 100 / Maximum amount of product possible
• It is not always possible to obtain the calculated amount of a product for 3 reasons:
• Reasons why it is not possible to obtain the calculated amount of product

  • 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
• Yield


  Amount of product obtained
• Atom economy ● A measure of the amount of starting materials that end up as useful products ● Important for sustainable development and for economic reasons to use reactions with high atom economy = (Mr of desired product from reaction / sum of Mr of all reactants) x 100 ● Possible reasons why a particular reaction pathway is chosen/not chosen: atom economy, yield, rate, equilibrium position and usefulness of by-products
• To calculate the theoretical mass of a product from a given mass of reactant and the balancing equation for the reaction

  1. Calculate mol. of reactant by using mol. = mass / molar mass

  2. Use balancing numbers to find mol. of product

  3. Calculate theoretical mass of a product by then using mass = mol. x molar mass
• Concentration of a solution

  Measured in mol. per given volume of solution e.g. mol. per dm 3 (mol./dm 3)
• Finding moles from mass and volume

  1. Moles = mass / molar mass

  2. Concentration = moles / volume
• Calculating concentration of unknown solution

  1. Work out moles of known solution by concentration x volume

  2. Work out moles of unknown solution by mole ratio from equation

  3. Calculate unknown concentration by moles / volume
• Equal amounts (in mol.) of gases occupy the same volume under the same conditions of temperature and pressure (e.g. RTP) ● Volume of 1 mol. of any gas at RTP (room temperature and pressure: 20 degrees C and 1 atmosphere pressure) is 24 dm 3 ● This sets up the equation: Volume of gas (dm 3 ) at RTP = Moles x 24 ● using this equation, if the reaction is at RTP, you can calculate moles of a gas produced and then x24 to get volume produced (e.g. if you produce 5 moles of hydrogen, you produce 24 x 5 = 120 dm 3 )