C3- Quantitative Chemistry

• What is the law of conservation of mass?

  No atoms are lost or made during a chemical reaction. The mass of products equals the mass of the reactants
• How is conservation of mass shown in balanced equations?

  The number of atoms of each element is the same on both the reactant and product side of the equation
• Why is conservation of mass not always observed?

  Gases can escape there is an open system
• What is an enclosed system?

  Mass will be observed to be conserved as no mass can enter or leave
• What is an non-enclosed/open system?

  Gases can enter and leave the system which makes it look like the mass has changed when it hasn't
• What is relative formula mass (RFM)?

  The sum of the relative atomic masses of the atoms in the numbers shown in its formula

  In balanced chemical equation:

  Sum of RFM of reactants = sum of RFM of products
• How can you calculate percentage by mass?

  % by mass= total RAM of desired element / RFM of compound x 100
• How can you estimate uncertainty from a set of repeated measurements?

  1. Work out mean
  2. Work out the range of results
  3. Uncertainty is ± half the range
  4. The estimate of uncertainty is the mean ± half range
• How do you estimate uncertainty from measuring instruments using resolution?

  Analogue measuring instruments- ± half of smallest division on scale

  Digital measuring instruments- ± Half of smallest division on scale
• What is resolution?

  The smallest division in the apparatus
• What is mass of 1 mole of a substance equal to?

  its relative formula mass
• What is avogadro's constant?

  6.02 x 10^23 (number of particles in one mole of a substance)
• How can we calculate moles?

  moles= mass / Mr or Ar
• How can we balance equations using moles and masses?

  1. Work the RFM of all substances in the equation
  2. Calculate number of moles of each substance
  3. Work out the sumplest whole number ratio by divideing by the smallest
  4. Use the number to balance equation
• How can we use balanced equations to work out the mass of one of the reactants or products?

  Need the mass of one of the substances

  Work out the number of moles in the substance of the given mass

  Use molar ratio to work out the moles of the substance that the question asks the mass for

  Use mass = Mr x moles to work out the mass
• What does balanced equations give?

  The ratio of moles in a reaction
• What is the limiting reactant?

  The reactant that runs out before the others

  The amount of limiting reactant affects the maximum mass of the products produced
• If one reactant is limiting what is the other?

  The remaining reactant is in excess
• What is the equation for concentration of a solution?

  concentration (g/dm^3) = mass (g) / volume (dm^3)

  concentration (mol/dm^3) = moles / volume (dm^3)
• How to convert from centimetres cubed to decimetres cubed?

  divide by 1000
• How to convert from g/dm^3 to mol/dm^3?

  x Mr
• How can we calculate the concentration of a solution?

  If we know the concentration of 1 solution and the volume of both solutions
• How is percentage yield calculated?

  % yield = mass of product actually made / max theoretical mass of product x 100
• Why do most reactions not have 100% percentage yield?

  • The reaction may be reversible so it doesn't go to completion
  • Some of the products may be lost when it is separated from the mixture
  • Some reactant may react in unexpected ways
• What is atom economy?

  The measure of the amount of starting materials that end up as useful products
• What is the equation of atom economy?

  atom economy = RFM of desired product / sum of RFM of all reactants x 100
• Why is high atom economy important?

  • Sustainable development
  • If we have a high atom economy we have less waste and fewer natural resources are needed
  • Economic reasons
  • The more unwanted useful products made, the more money it costs to dispose of and you may need more reactants
• IF REACTION HAS ONLY 1 PRODUCT THE ATOM ECONOMY IS 100%
• What is the volume of any gas at room temperature and pressure?

  1 mole of any gas occupies 24dm^3 at room temp (20 degrees) and pressure (1 atm)
• What is the formula for volume of gases?

  Volume of gases = moles x 24