Core Concept

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It allows chemists to predict how much of each substance is needed or produced in a reaction.

Key Concepts

1. Mole Ratio:

   • The coefficients in a balanced chemical equation represent the ratio of moles of each substance involved in the reaction.

   • For example, in the reaction $\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}2$, the mole ratio is 2:1:2, meaning 2 moles of $\text{H}_2$​ react with 1 mole of $\text{O}_2$​ to produce 2 moles of $\text{H}_2\text{O}$.

2. Balanced Chemical Equation:

   • A balanced equation is essential for stoichiometry calculations because it reflects the conservation of mass, with the same number of atoms of each element on both sides.

   • Example: In $2\text{Al} + 3\text{Cl}_2 \rightarrow 2\text{AlCl}_3$​, the equation is balanced, so stoichiometry calculations can be made accurately.

3. Molar Mass:

   • The molar mass (g/mol) of a substance, found on the periodic table, allows conversion between grams and moles.

   • Example: The molar mass of $\text{H}_2\text{O}$ is 18.02 g/mol (2 H atoms at 1.01 g/mol each + 1 O atom at 16.00 g/mol).

4. Conversions in Stoichiometry:

   • Stoichiometry problems often involve converting between grams, moles, and molecules using molar mass and Avogadro's number ($6.022 \times 10^{23}$ particles/mol).

Basic Stoichiometry Steps

1. Write and Balance the Chemical Equation:

   • Ensure the equation is balanced with the correct coefficients to use in mole ratios.

2. Convert Known Quantities to Moles:

   • If you start with grams of a substance, convert to moles using the molar mass.

3. Use Mole Ratios to Find Unknown Quantities:

   • Use the mole ratio from the balanced equation to relate moles of the given substance to moles of the unknown substance.

4. Convert Moles Back to Desired Units:

   • If needed, convert moles back to grams or other units using molar mass.