GCSEChemistryAQARevision NotesQuantitative Chemistry

Quantitative Chemistry (AQA GCSE Chemistry)

Revision Note

Quantitative Chemistry 

What is quantitative Chemistry?

Quantitative chemistry is an area of chemistry that allows chemists to calculate known quantities of materials. For example, how much product can be made from a known starting material or how much of a given component is present in a sample.

For example, chemists can use quantitative methods to:

  • Determine the formulae of compounds 
  • Determine the chemical equations for reactions
    • Chemical equations are a way of representing chemical reactions and are one way that chemists can communicate ideas about possible reactions
  • Calculate the amount / yield of product that can be made from a known starting material
  • Determine the purity of the products of a chemical reaction
  • Monitor the progress or yield of chemical reactions.

Chemical reactions can be classified in various ways. Identifying different types of chemical reactions enables chemists to understand how chemicals react together, establish patterns and predict the behaviour of other chemicals. Some of the key quantitative chemistry topics include:

  • Relative atomic and relative formula mass
  • Conservation of mass 
  • Chemical equations
  • Changes in mass
  • Chemical measurements 
  • The mole, including concentrations in mol / dm3 and gas volumes
  • Reacting mass calculations
  • Concentration of solutions
  • Percentage yield and percentage atom economy of chemical reactions
  • Use of amount of substance in relation to volumes of gases

Relative atomic and relative formula mass

Relative atomic mass, Ar, is the average mass of one atom of an element compared to one-twelfth of the mass of a carbon-12 atom. This value can be found as the mass number on the periodic table.

The relative formula mass or relative molecular mass, Mr, of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

For example, CaCO3 contains one calcium atom, one carbon atom and three oxygen atoms.

The relative atomic mass of each element is:

  • Calcium, Ca = 40
  • Carbon, C = 12
  • Oxygen, O = 16

Therefore, the Mr of calcium carbonate is 40 + 12 + (16 x 3) = 100. 

Conservation of mass 

The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.

For example, calcium carbonate decomposes to form calcium oxide and carbon dioxide:

CaCO3 → CaO + CO2 

  • The Mr of calcium carbonate is 100.
  • The Mr of calcium oxide is 40 + 16 = 56.
  • The Mr of carbon dioxide is 12 + (16 x 2) = 44.

These Mr values confirm that the Law of Conservation of Mass is followed:

CaCO3 → CaO + CO2 

100 → 56 + 44

Note: If you tried to monitor the decomposition of calcium carbonate, you would actually see that the mass of the reaction decreases. This is because carbon dioxide is a gas and will escape into the atmosphere / surroundings.

Chemical equations

Word equations are a qualitative way to show what is happening in a chemical reaction. 

For example, magnesium reacts with hydrochloric acid (HCl) to form magnesium chloride (MgCl2) and hydrogen (H2):

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Balanced chemical / symbol equations give quantitative information about the same reaction.

The unbalanced equation is:

Mg + HCl → MgCl2 + H2 

The left-hand side of the equation has one hydrogen atom, while the right-hand side has two hydrogen atoms. The same applies to the chlorine atoms.

If we double the amount of hydrochloric acid, this will balance the equation:

Mg + 2HCl → MgCl2 + H2 

This shows that one magnesium atom reacts with two hydrochloric acid molecules to form one magnesium chloride molecule and one hydrogen molecule

All of the atoms at the start of the chemical reaction are still present at the end, they have just moved around - this confirms that chemical reactions follow the Law of Conservation of Mass.

Chemical measurements

Any chemical reaction that involves measurements is subject to systematic errors and / or random errors.

Systematic errors will affect the results by moving the value of the reading away from its true value. 

  • For example, forgetting to press the zero / tare button on an electronic balance will result in the mass reading being higher than the true value. 
  • Systematic errors always act in the same direction, i.e. too high OR too low. 

Random errors are also errors that affect the results by moving the value of the reading away from its true value. 

  • For example, a thermometer may show a reading of 23.2 oC but this may accidentally be misread as 23.1 or 23.3 oC. 
  • Random errors act in both directions, i.e. too high AND too low.

Uncertainty is a specific type of random error.

  • Uncertainty is often seen on pieces of equipment such as thermometers, burettes and balances where there is an accepted error
  • For example, a thermometer may have an uncertainty of ± 0.5 oC
     
  • More information: “Chemical Measurements”

The mole (Higher tier only)

The mole is a standard measurement of any chemical substance. One mole of any substance contains 6.02 x 1023 particles:

  • One mole of silver = 6.02 x 1023 atoms of silver
  • One mole of carbon dioxide = 6.02 x 1023 molecules of carbon dioxide
  • One mole of hydrogen ions = 6.02 x 1023 hydrogen ions

The mole is linked to:

  1. Relative atomic mass and relative formula / molecular mass by:
    • Moles = mass / (Ar or Mr)
      More information: “The Mole”
  2. Concentration and volume by:
  3. Volume and molar volume, specifically at 20 oC and 1 atmosphere pressure, by:

Reacting mass calculations (Higher tier only)

Balanced chemical equations can be used to determine the number of moles or masses of the reactants and products involved in a chemical reaction.

An exam question will give you enough information to be able to determine the number of moles of at least one chemical. A ratio can then be taken from the balanced chemical equation and applied to calculate the required answer.

Percentage yield and percentage atom economy of chemical reactions

Yield describes the amount of desired product that is obtained from a chemical reaction. The percentage yield of any reaction can be calculated by:

Percentage yield = fraction numerator actual space yield over denominator theoretical space yield end fractionx 100

The actual yield is typically given in the question and the theoretical yield can be calculated using a reacting mass calculation (HT) or may be given (FT)

The industrial aim of any company is to have a reaction with as high a percentage yield as possible to maximise profits and minimise costs and waste

Atom economy is a measure of the efficiency of a chemical reaction in producing a desired product. The percentage atom economy can be calculated by:

Percentage atom economy = fraction numerator total space M subscript r space of space desired space product over denominator total space straight M subscript straight r space of space all space products end fraction x 100

In industry, companies will try to use chemical reactions with high atom economies for sustainable development.

This is a quick summary of some key concepts in quantitative Chemistry - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!