Learn: Calculations involving masses

Welcome!Welcome! You've already learned about atomic structure, the periodic table, bonding, and types of substances. Now, let's build on this knowledge with calculations involving masses. This will help you understand how to calculate the amount of substances involved in chemical reactions.

What are calculations involving masses?These calculations involve using the relative atomic mass (the average mass of an atom of an element) and the mole (a unit for the amount of substance). They help determine the quantities of substances in a reaction.

Relative atomic mass (Ar)The relative atomic mass (Ar) is the average mass of all the isotopes of an element compared to 1/12 of the mass of a carbon-12 atom. You can find this number on the periodic table for each element.

What is a mole?A mole is a unit used to measure the amount of substance. One mole contains 6.02 x 1023 particles, which is known as Avogadro's constant. Moles are crucial for calculating how much of a substance is involved in a chemical reaction.

Using moles in calculationsYou can calculate the number of moles using the formula: moles = mass ÷ relative formula mass. This formula helps you find out how much of a substance you have if you know its mass and relative formula mass.

What is the relative formula mass?Relative formula mass (Mr) is the sum of the relative atomic masses of all the atoms in a compound. For example, the Mr of H2O is calculated as: 2 × Ar of H + Ar of O = 2 × 1 + 16 = 18.

Conservation of massThe law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction. This means the mass of the reactants equals the mass of the products. It is important to ensure chemical equations are balanced.

Using ratios to calculate massesIn chemical reactions, the mole ratio from the balanced equation helps you calculate the mass of reactants and products. For example, if the reaction is 2H2 + O2 → 2H2O, the ratio of H2 to H2O is 1:1, meaning 1 mole of hydrogen produces 1 mole of water.

Review Time!Great work! You've learned all about calculations involving masses, including relative atomic mass, relative formula mass, the mole, and conservation of mass. Let's test your understanding with a few questions!