Chemistry IGCSE Chapter 4 – Stoichiometry

Symbols of Elements

·         A letter or two letters which stand for one atom of the element

·         Element symbols are sometimes taken from latin

Compound Formulas

·         For every compound, there is a formula containing the symbols of the elements present and the ratio in which they are present.

·         The compound Carbon Dioxide consists of molecules, each molecule having one atom of carbon and 2 atoms of oxygen. (CO₂) à [Where 2 multiplies the O infront of it].

·         Therefore three molecules of carbon dioxide is 3CO­₂

Determining the formula of a compound/solution:

• Name of compound
• Symbols for elements and radicals in compound
• Write down their valencies
• Adjust the number of atoms of each element or radical until the valencies are the same on both the left and right side

·         When dealing with diagram, each outer shell represents one atom of a certain element also provided. Therefore, write down the element symbol and the total number of atoms that are visible.

·         Compounds can have simpler, reduced formula

E.g. Na₃Cl₃ can be reduced, the three cancelled, to give NaCl

·         When the charges of ions are presented, you must make negative charge balance positive charge by increasing the atoms of the element(s) present accordingly. E.g. Mg²⁺ and Cl^- , Cl atoms are increased by one to give MgCl₂.

Chemical Equations

·         In reactions, reactants are converted into products

·         In word equations, the name of the elements used are shown with + sign indicating ‘and’ and an arrow indicating “yields”.

·         Chemical equations give the symbols, formulas and state symbols of reactions.

·         Note in equations, the two sides, the atoms on the left hand side and the atoms on the right hand side are the same in kind and in number.

Calcium carbonate à calcium oxide + carbon dioxide

CaCO₃ (s) à Ca0 (s) + CO₂ (g)

·         Balancing chemical equations: make sure the no. of atoms of each element on the LHS equals the no of atoms of each element on the RHS.

Relative Atomic Mass

·         Carbon was chosen as standard atom for comparison

·         Masses of atoms can be measured using the mass spectrometer

·         RAM or A_r = the average mass of an atom of the element divided by 1/12^th of the mass of one atom of C-12 isotope.

·         Not always a whole #

RAM = (% Abundance of Isotope 1 x Mass of Isotope 1) + ((% Abundance of Isotope 2 x mass iso 2)

100                                                                                                                                      100

Example: ²⁴Mg = 80%,   ²⁵Mg = 9%, ²⁶Mg = 11%

RAM of Mg = (80  x 24) +  ( 9  x  25)  +   ( 11  x 26)

                    100              100               100          

2.25 + 19.2 + 2.86

A_r (Mg) = 24.31

Relative Molecular Mass

·         the sum of the relative atomic masses

·         it is used in the calculation of mass of compounds/ionic compounds.

·         The mass of an element or compound in the form it exists in nature.

·         M_r or RMM

RMM (O₂) = 16 + 16 = 32

Note: M_r and A_­r have no units

% Composition

Calculated by: M_r of element/compound   x 100

                       M_r of entire compound

Purity

·         A substance is 100% pure when it consists of only 1 material

·         A substance is 98% pure when 2% is impurities

The Mole/Avogadro’s Constant

·         The mole is amount of substance that contains as many particles as there are in exactly 12 grams of Carbon 12 isotope.

·         12g of C-12 isotope contains 6.023 x 10²³ particles (ions, atoms, molecules, electrons)

Avogadro’s Constant

·         the number of particles (molecules, ions, atoms) found in one mole of any substance and has the value of 6.023 x 10²³ particles.

Example: 1 mole of H atoms = 6.023 x 10²³

1 mole of H₂ atoms = 6.023 x 10²³

1 mole of H atoms in H₂ molecule = 2(6.023 x 10²³)

·         Mole = mass provided

          Molar mass of substance

·         The molar mass of a substance is the mass found in 1 mole of that substance

·         Molar mass, M = in units g/mol

Molar Volume

·         Space occupied by 1 mole of a gas (of any substance)

·         Measured in dm³

·         R.t.p = Conditions of Room Temperature and Pressure

·         At r.t.p the volume of 1 mole of any gas is 24dm³.

Note: 1000cm³ = 1 dm³

·         For problems involving the above properties and values, use simple proportion to solve it. Find equation for reaction, take balanced equation and then use mole ratios to find answer.

The Empirical Formula

·         The simplest whole # ratio of the number of atoms of each element in a compound

·         The ratio is the same as the moles in the compound

·         Mass composition or % composition of each element should be available

·         The aim of the empirical formula is to deduce the atoms present per element

·         Most importantly, the mole ratio (moles per element) needs to be determined

Using information about the % composition or the mass composition these are the steps 2 follow:

 Molecular Formula

·         The formula of a compound showing the actual # of atoms of each element in a compound

·         The molecular formula of a compound may differ from Empirical formula since the RMM of the compound is different

·         When the RMM of the compound is stated, it is possible to calculate the molecular formula, using the empirical formula.

Molecular Formula = (Empirical Formula)_x

Where x represent subscript of compound, that will be multiplied to obtain actual atoms.

Molecular Mass of CH₂Br = 188

188 = (12 + 2 +80) _x

188  = x

 94

x = 2, THERFORCE, Molecular Formula = [CH₂Br]₂, C₂H₄Br

*Molecular formula is especially required with organic substances

NOTE: CONCENTRATIONS OF FORMULA (QUANTITATIVE ANALYSIS AND TITRATION) HAS NOT BEEN MADE INTO NOTES