Chemistry

Basic Concepts in Physical and Inorganic Chemistry

# Formulae, Equations and Amounts of Substance

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Formulae, Equations and Amounts of Substance

**Formulae and Equations**

- Understand the concept of
**chemical formulae**used to represent molecules and ions. These convey the type and ratio of atoms in a compound. - The molecular formula represents the number and type of atoms in a molecule, where the empirical formula conveys the simplest ratio of atoms in a compound.
- Know how to balance
**chemical equations**, which show the number of atoms for each element being conserved in a chemical reaction. - Reactive species in an equation are termed reactants, while the end products are called products.
- Master the concept of
**stoichiometry**, which is the ratio of reactants to products in a balanced equation. - Differentiate between molecular and ionic equations, and be able to write net ionic equations by cancelling out spectator ions.

**Amounts of Substance**

- Familiarize yourself with the term 'mole'. The
**mole**is the unit for amount of substance in chemistry, defined as exactly 6.02 x 10^23 particles. - Understand the concept of
**molar mass**, expressed in g mol^-1, defined as the mass of one mole of a substance. - The molar mass of an element can be found on the Periodic Table and is numerically equal to the relative atomic mass.
- Be able to carry out mole calculations using the equation n = m/M where n is the number of moles, m is the mass, and M is the molar mass.
- Understand how to calculate the number of particles in a sample using the number of moles and Avogadro’s constant.
- Know how to calculate
**molar concentration**in mol dm^-3 by using the formula C = n/V where C is the concentration, n is the number of moles and V is the volume in dm^3. - Familiarize yourself with
**molar volume**at standard temperature and pressure (STP), defined as the volume occupied by one mole of any gas at STP, taken to be 22.4 dm^3 mol^-1.

**Limiting Reagents and Excess**

- Understand the concept of
**limiting reagents**, the substance that is completely consumed in the reaction and determines the maximum amount of product that can be formed. - Identify how to determine the limiting reagent in a chemical reaction from the stoichiometry of the equation and mass of reactants.
- Familiarise yourself with calculations involving
**excess**reagents. The reagent that is not completely used up in a reaction is said to be in excess.

**Percentage Yield and Atom Economy**

- Understand how to calculate
**percentage yield**, a measure of the efficiency of a reaction, calculated with actual yield/theoretical yield x 100%. - Familiarize yourself with the definition and calculation of
**atom economy**, a concept that assesses the efficiency of a reaction in terms of atom utilization. Atom economy is calculated by (mass of desired product/total mass of reactants) x 100%.