Stoichiometry & Chemical Calculations Review

These flashcards cover fundamental definitions, calculation methods, and sample problems for stoichiometry, limiting reactants, yields, and formula determination.

What is a mole?

The amount of substance that contains 6.022 × 10²³ particles (Avogadro's number).

What is Avogadro's number?

6.022 × 10²³ particles per mole.

What is the relationship between mass, moles, and molar mass?

Moles = Mass (g) ÷ Molar Mass (g/mol) OR Mass (g) = Moles × Molar Mass (g/mol).

What is molar mass?

The mass of one mole of a substance in g/mol, calculated from the relative atomic or molecular masses.

What is stoichiometry?

The calculation of reactants and products in chemical reactions using mole ratios from a balanced equation.

What are the steps to calculate the mass of a product in a reaction?

1) Write the balanced equation. 2) Find moles of the known substance. 3) Use the mole ratio to find moles of product. 4) Convert moles of product to mass (Mass = Moles × Mr).

What is the general method for reacting-mass calculations?

1) Calculate moles of the known substance. 2) Use the balanced equation to obtain moles of the unknown. 3) Convert moles of the unknown to mass using moles × Mr.

What is a limiting reactant?

The reactant that is completely used up first in a reaction and therefore limits the amount of product formed.

How do you identify the limiting reactant?

1) Calculate moles of each reactant. 2) Use the balanced equation to compare their mole ratios. 3) The reactant that would form fewer moles of product is the limiting reactant.

How do you calculate theoretical yield?

Start with moles of the limiting reactant, apply the mole ratio to obtain moles of product, then convert to mass (Mass = Moles × Mr).

What is percentage yield and how is it calculated?

Percentage Yield = (Actual Yield ÷ Theoretical Yield) × 100%.

What is an empirical formula?

The simplest whole-number ratio of atoms present in a compound.

How do you calculate an empirical formula from masses or percentages?

1) Divide each element’s mass by its Ar. 2) Divide each result by the smallest value obtained. 3) Convert to whole-number ratios as needed.

What is a molecular formula?

The actual number of atoms of each element present in a molecule.

How do you determine a molecular formula from an empirical formula?

Molecular Formula = Empirical Formula × (Molecular Mr ÷ Empirical Mr).

Example: What mass of CO₂ is formed when 24.0 g of CH₄ reacts with excess O₂?

Balanced equation: CH₄ + 2O₂ → CO₂ + 2H₂O. Moles of CH₄ = 24.0 g ÷ 16.0 g/mol = 1.5 mol. Mole ratio CH₄:CO₂ = 1:1, so moles CO₂ = 1.5 mol. Mass CO₂ = 1.5 mol × 44.0 g/mol = 66.0 g.

Example: Find the empirical formula of a compound containing 7.20 g C and 1.20 g H.

Moles C = 7.20 g ÷ 12.0 g/mol = 0.60 mol. Moles H = 1.20 g ÷ 1.0 g/mol = 1.20 mol. Divide by smallest: C = 1, H = 2. Empirical formula = CH₂.