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Chemistry
Stoichiometry
The Mole Ratio Roadmap
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Stoichiometry is the chemistry skill of using a balanced equation to relate amounts of reactants and products. It matters because chemical formulas tell us not only what substances react, but also the exact particle ratios in which they combine. The mole is the bridge between the microscopic world of atoms and molecules and the measurable world of grams, liters, and particles. A clear roadmap helps you move from a given quantity to the quantity you need to find without guessing.
Key Facts
- Moles from mass: n = m / M, where n is moles, m is mass in grams, and M is molar mass in g/mol.
- Mass from moles: m = nM.
- Particles and moles: particles = n × 6.022 × 10^23.
- For gases at STP: V = n × 22.4 L, and n = V / 22.4 L.
- Mole ratio comes from coefficients in the balanced equation, such as 2 mol H2 / 1 mol O2 in 2H2 + O2 -> 2H2O.
- Stoichiometry roadmap: given quantity -> moles given -> mole ratio -> moles unknown -> final quantity.
Vocabulary
- Stoichiometry
- Stoichiometry is the calculation of amounts of reactants and products using a balanced chemical equation.
- Mole
- A mole is an amount of substance containing 6.022 × 10^23 particles.
- Mole ratio
- A mole ratio is a conversion factor made from the coefficients of a balanced chemical equation.
- Molar mass
- Molar mass is the mass of one mole of a substance, usually measured in grams per mole.
- Limiting reactant
- The limiting reactant is the reactant that runs out first and determines the maximum amount of product formed.
Common Mistakes to Avoid
- Using an unbalanced equation, which gives the wrong mole ratios and makes every later calculation incorrect.
- Applying the mole ratio to grams directly, which is wrong because coefficients compare moles, not masses.
- Flipping the mole ratio incorrectly, which changes the conversion direction and produces an answer for the wrong substance.
- Rounding too early, which can cause noticeable error in the final answer, especially in multi-step stoichiometry problems.
Practice Questions
- 1 For the reaction 2H2 + O2 -> 2H2O, how many moles of H2O form from 3.50 mol of O2?
- 2 For the reaction N2 + 3H2 -> 2NH3, how many grams of NH3 can form from 10.0 g of H2 if N2 is in excess? Use H = 1.01 g/mol and N = 14.01 g/mol.
- 3 A student converts grams of a reactant directly to grams of a product using the coefficients from the balanced equation. Explain why this method is incorrect and describe the correct roadmap.