Moles and Molar Calculations Quantitative chemistry deals with calculating quantities of substances involved in chemical reactions. The key concept is the mole...
Quantitative chemistry deals with calculating quantities of substances involved in chemical reactions. The key concept is the mole, which represents 6.022 x 1023 particles (atoms, molecules or ions) of a substance. The relative formula mass (Mr) is the sum of the relative atomic masses of the atoms in a formula unit.
Balanced chemical equations show the whole reactants and products involved in a reaction, and the conservation of mass. The coefficients represent the mole ratio of substances. For example:
2Na + 2HCl → 2NaCl + H2
In this reaction, the mole ratio of Na to HCl to NaCl is 1:1:2.
The mass of a substance in grams can be calculated from its amount in moles using the Mr:
Mass (g) = Moles x Mr
Problem: Calculate the mass of 0.125 mol of carbon dioxide (CO2), given Mr(CO2) = 44.0 g/mol.
Solution:
The limiting reactant determines the maximum amount of product. Concentrations of solutions can be expressed in g/dm3 or mol/dm3. Percentage yield compares the actual and theoretical yield.
For gaseous reactants and products, calculations involve mole ratios and the molar volume of gases (1 mol occupies 24 dm3 at room temperature and pressure).