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The following notes were written for the previous IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

# Stoichiometry

### 1.1 Mole concept & Avogadro's constant

#### 1.1.1 Describe the mole concept and apply it to substances.

The mole concept applies to all kinds of particles: atoms, molecules, ions, formula units etc. The amount of substance is measured in units of moles. The approximate value of Avogadro's constant (L), 6.02 x 1023 mol-1, should be known.

#### 1.1.2 Calculate the number of particles and the amount of substance (in moles).

Convert between the amount of substance (in moles) and the number of atoms, molecules or formula units.

1 Mole is equivalent to 6.023 x 1023 (Avogadro's number) units of any substance.

 Mass of 1 mole of atoms (monatomic) Relative atomic mass in grams Symbol used: RAM Mass of 1 mole of a compound Relative molecular mass in grams Symbol used: Mr

 Number of moles = mass mass per mole

### 1.2 Formulae

#### 1.2.2 Distinguish between atomic mass, molecular mass and formula mass.

The term molar mass (in g mol-1) can be used for all of these.

#### 1.2.5 Define the terms empirical formula and molecular formula.

The molecular formula is a multiple of the empirical formula.

#### 1.2.6 Determine the empirical formula and/or the molecular formula of a given compound.

Determine the: empirical formula from the percentage composition or from other suitable experimental data percentage composition from the formula of a compound molecular formula when given both the empirical formula and the molar mass

Chemical formula is the number of each type of atom in the smallest viable unit of the substance. Empirical formula is the simplest possible ratio of elements in a substance.

By definition the molecular formula will be an integral number of empirical formulae (x1, x2 etc)

Examples:

 Compound Sulphuric acid Ethane hydrogen peroxide propane Ethene formula H2SO4 C2H6 H2O2 C3H8 C2H4 empirical formula H2SO4 CH3 HO C3H8 CH2

### 1.3 Chemical Equations

#### 1.3.3 Apply the state symbols (s), (l), (g) and (aq). Encourage the use of state symbols in chemical equations.

The simplest ratio of the number of moles of reactants and products in a chemical reaction. The balancing numbers are known as the coefficients of the reaction. Use of balancing number is essential to fulfill the law of conservation of matter. The overall set of balancing coefficients of a chemical reaction is known as the reaction stoichiometry

State symbols: (s)-Solid , (l)-liquid, (g)-gas, (aq)-aqueous solution... i.e. something dissolved in water. These should be included in all chemical reactions.

### 1.4 Mass relationships in chemical reactions

#### 1.4.1 Calculate stoichiometric quantities and use these to determine experimental and theoretical yields.

Mass is conserved in all chemical reactions. Given a chemical equation and the mass or amount (in moles) of one species, calculate the mass or amount of another species.

#### 1.4.2 Determine the limiting reactant and the reactant in excess when quantities of reacting substances are given. Given a chemical equation and the initial amounts of two or more reactants:

• identify the limiting reactant
• calculate the theoretical yield of a product
• calculate the amount(s) of the reactant(s) in excess remaining after the reaction is complete.

#### 1.4.3 Apply Avogadro's law to calculate reacting volumes of gases.

The ratio of two reacting masses will be always maintained for a specific chemical reaction. These mass ratios can be found from the stoichiometry (balancing numbers) of the equation and the molar masses of the reactants and products

Limiting reagent is a term used to describe the product which will completely react leaving other reactants unused.

The reactant left behind unused at the end of a reaction is said to be in excess

### 1.5 Solutions

#### 1.5.1 Define the terms solute, solvent, solution and concentration (g dm-3 and mol dm-3).

Concentration in mol dm-3 is often represented by square brackets around the substance under consideration, eg [CH3COOH].

#### 1.5.3 Solve solution stoichiometry problems.

Given the quantity of one species in a chemical reaction in solution (in grams, moles or in terms of concentration), determine the quantity of another species.

Solvent - the dissolving medium (usually water)

Solute - the substance that is dissolved.

Solution - the above components when mixed together intimately so that the solid phase becomes indistinguishable from the liquid phase.

Concentration - the amount of solute per volume of solution (units mols per dm3 (litre) or grams per litre)

Concentration = grams or moles/volume (also called Molarity)

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