Topic 3 Calculations

1.26 Calculate relative formula masses (Mr) from relative atomic masses (Ar)

• Ar (relative atomic mass) = the weighted average of the different isotopes of the same element.

• Mr (relative formula mass) of a substance = sum of the Ar(relative atomic masses) of the atoms of the elements in the numbers shown in the formula

• e.g. Mr of CaBr₂ = 40 × 1 + 80 × 2 = 200g/mol

• Mr is measured in grams per mol

1.27 know that mole(mpl) is the unit for the amount of a substance

• Chemical amounts are measured in moles.

• The mass of 1 mole of a substance in grams is numerically equal to its relative formula mass.

• For example, the Ar of Iron is 56, so one mole of iron weighs 56g.

• Careful: 1mol of bromine ATOM (Br) = 1 × 80 = 80g

• 1 mol of bromine MOLECULE (Br₂) = 2 × 80 = 160 g

• A mole of any element contains the same number of particles (6.022 × 10²³), regardless if it’s atom, ion or molecule.

• (Element (atomic form) → atoms

  • Example: 1 mole of oxygen atoms = 6.022 × 10²³ atoms of O

  • Example: 1 mole of iron atoms = 6.022 × 10²³ atoms of Fe

• Ionic compound → ions

  • Example: 1 mole of NaCl = 6.022 × 10²³ formula units, containing 6.022 × 10²³ Na⁺ ions and 6.022 × 10²³ Cl⁻ ions

• Molecular substance → molecules

  • Example: 1 mole of H₂O = 6.022 × 10²³ water molecules

  • Example: 1 mole of O₂ = 6.022 × 10²³ oxygen molecules)

1.28 Conversion between moles amnd grams

mol = Mass/Mr

Mass = mol x Mr

e.g. how many moles are there in 42 g of carbon?

mol = mass/Mr

= 42/12

= 3.5 mol

1.29 calculate reacting masses using experimental data and chemical equations

Key : A balanced equation tells you the RATIO of moles of reagent and product

1) Write out a table : Formula of substances, mass, mr, moles ratio

2) Fill in known values

3) Calculate one number of mole by dividing Mass/Mr

3) Use ratio to find the other number of mole

4) Calculate the mass of this substance using mass = mol x Mr

1.30 Percentage yield calculation

percentage yield = amount of product produced/ maximum amount of product possible (which may need to be calculated using reacting mass calculation)

theoretical yield = the mxaimum amount of product possible

some common reasons why yield is not 100%: substance is not pure, not cooled for long enough, some precipitate remained in the filtrate,….

1.31 determine the formulae of salts containing water of crystallisation experimentally

1) Heat the hydrated substance (e.g. magnesium sulfate)

2) Allow it to cool when no more steam seems to be produced

3) Reweigh and repeat heating until 3 consecutive masses are produced to ensure that all the water is removed.

4) Calculate mass of water by subtracting end mass from mass of hydrated substance

5) Calculate empirical formula by calculating ratio of the moles using reacting mass calculation method (e.g. if ratio is 1:7, empirical formula = MgSO₄.7H₂O

Main sources of error in water of crystallisation practical

• heated for too long causes decomposition of MgSO₄, so further mass is lost /mass is lower than expected

• heated for not long enough, so water vapour/steam condensed in the mouth of the boiling tube

1.32 Empirical formula and molecular formula

Empirical formula = the simplest ratio of the atoms of each different element in a compound

Molecular formula = the actual number of atoms of each different element in a molecule of a compount

• Any molecular compound can have both formulae but compounds that have giant structures (ionic or giant covalent) only have empirical formula because they do not consist of concrete molecules in their structure.

Calculate empirical and molecular formulae from experimental data

Data given will be mass of element/compound or percentage mass (is the same as the mass per 100g so its not a problem :)

Method:

1. Work out mass of each element in the sample

2. calculate no. of moles (mol = mass/Mr)

3. calculate ratio by dividing both num of moles by the smallest no. of moles

4. Simplify the ratio by converting to whole numbers

Empirical formula from Molecular : simplify actual number of atoms into a ratio

Molecular from Empirical:

• Work out the multiple by doing multiple = Mr(relative formula mass/ efm (empirical formula mass)

• Molecular formula = Multiple x empirical formula

• e.g. multiple = 4, empirical formula = CH_2, molecular formula = 4 x CH₂ = C₄H₈

1.36 Practical; know how to determine the formula of a metal oxide by combustion (e.g. magnesium oxide) ….

Combustion:

1. Heat crucible containing magnesium ribbon with the lid on the top.

2. Every minute, lift the lid slightly to allow oxygen from the air enter the crucible in order to successfully oxidise(so that magnesium can react with oxygen)

3. Magnesium should glow red and burn with a bright white flame. Finally it should turn to a white solid.

4. Heat until 3 consecutive constant masses to ensure all the magnesium has reacted with oxygen to form magnesium oxide.

Sources of error in combustion

1. Crucible lid was not completely closed during heating so some of the magnesium oxide escaped from the crucible. As a result the mass is lower than expected.

2. Not heated for long enough. Not all the magnesium has reacted with oxygen to form magnesium oxide, which leads to lower mass than expected.

or by reduction (e.g. copper oxide)

• Add copper oxide to reduction tube. Make sure that it's spread over the bottom of the tube.

• Turn on hydrogen/methane supply and wait 2 minutes before igniting excess gas leaving the end of the reaction tube.

• This is to ensure that all the air containing oxygen is flushed out of the reaction tube, preventing hydrogen/methane mixing with oxygen while heating to avoid an explosive reaction.

• Heat the reduction tube until copper turns completely red/blown. This is because oxygen has reacted with the gas passing over the copper oxide, removing the oxygen. Pure copper is red/brown.

• The mass of the solid would increase over time as copper reacts with oxygen from the air to form copper oxide again.