1. Balanced Chemical Equation:
* The first step is to have a balanced chemical equation for the reaction. This equation tells you the exact number of moles of each reactant and product involved in the reaction. For example:
* 2H₂ + O₂ → 2H₂O
* This equation tells us that 2 moles of hydrogen gas (H₂) react with 1 mole of oxygen gas (O₂) to produce 2 moles of water (H₂O).
2. Mole Ratios:
* The coefficients in the balanced equation represent the mole ratios of reactants and products. In the example above, the mole ratio of H₂ to H₂O is 2:2, which simplifies to 1:1. This means that for every 1 mole of H₂ consumed, 1 mole of H₂O is produced.
3. Converting Mass to Moles:
* We often work with masses of reactants rather than moles. To use stoichiometry, you need to convert the mass of the reactant to moles using its molar mass.
* Molar mass is the mass of one mole of a substance. You can find it on the periodic table.
4. Calculating Moles of Product:
* Using the mole ratio from the balanced equation, you can calculate the number of moles of product that will be formed from the given number of moles of reactant.
* For example, if you have 3 moles of H₂, you will produce 3 moles of H₂O because the mole ratio is 1:1.
5. Converting Moles to Mass:
* Finally, you can convert the moles of product back to grams using its molar mass.
Example:
Let's say you want to calculate the mass of water produced from 10 grams of hydrogen gas.
1. Balanced Equation: 2H₂ + O₂ → 2H₂O
2. Moles of H₂:
* Molar mass of H₂ = 2 g/mol
* Moles of H₂ = 10 g / 2 g/mol = 5 moles
3. Moles of H₂O:
* Mole ratio of H₂ to H₂O is 1:1
* Moles of H₂O = 5 moles (H₂) x (1 mole H₂O / 1 mole H₂) = 5 moles
4. Mass of H₂O:
* Molar mass of H₂O = 18 g/mol
* Mass of H₂O = 5 moles x 18 g/mol = 90 g
Therefore, 10 grams of hydrogen gas will produce 90 grams of water.
Key Points:
* Stoichiometry is based on the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.
* The balanced chemical equation is essential for stoichiometric calculations.
* Stoichiometry allows us to predict the amount of product that can be formed from a given amount of reactant, which is essential for chemical synthesis and analysis.
