Stoichiometry and Empirical Formulas

Stoichiometry

• Stoichiometry is the study of the relationships between reactants and products in a chemical reaction.
• Chemists use stoichiometry to determine the amounts of reactants needed to produce a certain amount of product.

Calculating Percentage Composition

• To find the percentage composition of an element in a compound, calculate the relative molecular mass of the compound.
• Divide the atomic mass of the element by the relative molecular mass of the compound, then multiply by 100%.

Empirical Formula

• Molecular formula: Shows the actual number and type of atoms in a molecule.
• Empirical formula: Shows the simplest whole number ratio of atoms in a compound.
• Molecular formula = empirical formula x n (where n is a whole number)

Calculating Empirical Formula from Masses of Reactants and Products

• An experiment was carried out to find the empirical (simplest) formula of a copper oxide.
• 5.4 g of the oxide gave 4.8 g of copper.
• Find the mass of oxygen in the oxide (5.4 g – 4.8 g = 0.6 g).
• Calculate the number of moles of copper and oxygen.
• Divide the number of moles of each element by the smallest number of moles to get the simplest whole number ratio. This will be the subscripts of the elements in the empirical formula.

Calculating Masses of Reactants or Products from Balanced Chemical Equations

• The balanced equation for the reaction between magnesium and oxygen to form magnesium oxide is : 2Mg + O2 → 2MgO
• If 9 g of magnesium is burned in excess oxygen, the mass of magnesium oxide formed can be calculated using the molar masses and the stoichiometric ratios from the balanced equation.

Calculating Volumes of Gases

• The balanced equation for the reaction between carbon and oxygen to produce carbon dioxide is: C + O2 → CO2
• If 30 g of carbon is burned in excess oxygen, the volume of carbon dioxide produced at standard temperature and pressure (STP) can be calculated by using the molar volume of a gas, which is 22.4 L/mol.