topic 3 - quantitative chemistry

• The relative atomic mass of an element is the relative mass of its atoms compared to the mass of a carbon-12 atom
• The relative formula mass of a substance made up of molecules is the sum of the relative atomic masses of the atoms in the substance
• The law of conservation of mass states that the total mass of the products is equal to the total mass of the reactants - no atoms are lost or made in a chemical reaction, instead the atoms join together in different ways to form products
• In a reaction the sum of the relative formula masses of the reactants is equal to the sum of the relative formula masses of the products
• In closed systems, total mass of reactants = total mass of products but in open systems, gases could escape so total mass of the reactants doesn’t always appear to equal the total mass of the products
• When a measurement is made in chemistry there is always some uncertainty in the result obtained
• There are two ways of estimating uncertainty:

  • considering the resolution of measuring instruments
  • from the range of a set of repeat measurements
• The resolution of a measuring instrument is the smallest change in a quantity that gives a change in the reading that can be seen
• The uncertainty of a measuring instrument is estimated as (+/-) half the smallest scale division. For a thermometer with a mark every 1 degree celsius, the uncertainty is +- 0.5 degrees celsius
• A solution forms when a solute dissolves in a solvent
• The concentration of a solution is the amount of solute in a given volume of the solution
• Concentration in $g/dm^3$= mass of solute in g / volume in $dm^3$
• $1 dm^3 =$$1000 cm^3$ $cm^3 —> dm^3 =$$/1000$
• The concentration of a solution can be changed by:

  • dissolving more solute in a given volume of solution
  • allowing some solvent to evaporate
• Chemical amounts are measured in moles
• The mass of one mole of a substance is equal to the relative atomic mass of its formula in grams if the substance is an element
• The mass of one mole of a substance is equal to the relative formula mass of its formula in grams if the substance is a compound
• The number of atoms, molecules or ions in one mole of a substance is called the Avogadro constant. It is equal to 6.02 x 10^23
• Number of particles = avogadro constant x amount of substance in moles
• Mass = relative formula mass x moles
• A balanced reaction shows the amounts in moles of reactants that react and the amounts of products that are made
• A reaction finishes when one of the reactants is all used up. The other reactant has nothing to react with, so is left over.

  The reactant that is used up is the limiting reactant

  The reactant that is left over is in excess
• The concentration of a solution can be calculated using:

  • the amount of dissolved solute in moles
  • the volume of solution in cubic decimetres
• To convert from mol/dm^3 to g/dm^3, multiply by the relative formula mass. To go back, divide by the relative formula mass.
• The amount of moles of a solute in a given volume of solution can be calculated if the concentration is known.

  amount of solute = concentration x volume
• No atoms are gained or lost in reactions, however some atoms in the reactants may not end up in the desired product. Instead they form other products and are regarded as by-products
• The atom economy of a reaction is a measure of the amount of starting materials that are converted into useful products
• atom economy = (total Mr of the desired product/ total Mr of all reactants) x 100
• The theoretical yield of a reaction is the maximum mass of product that can be made in a chemical reaction. It can be calculated from:

  • the balanced equation
  • the mass and relative formula mass of the limiting reactant
  • the relative formula mass of the product
• There are several reasons why it is not always possible to obtain the calculated amount of a given product:

  • the reaction not going 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 different ways to the expected reaction
• Percentage yield = mass of product made/theoretical mass of product x 100
• Usually there is more than one way to make a particular substance. A reaction pathway describes the sequence of reactions needed to produce a desired product
• The reaction pathway chosen for a product depends on factors like:

  • Percentage yield
  • Atom economy
  • Rate of reaction
  • Equilibrium position
• Avogadro’s law states that when the temperature and pressure stay the same, equal volumes of different gases contain an equal number of molecules. This means that equal amounts in moles of gases occupy the same volume under the same temperatures and pressures
• The molar volume is the volume occupied by one mole of any gas at room temperature and pressure. It is equal to 24dm^3
• Volume = amount in moles x molar volume