Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Stoichiometry and Stoichiometric Calculations
Stoichiometry and Stoichiometric Calculations
Stoichiometry and Stoichiometric Calculations
The word stoichiometry is derived from two Greek words — stoicheion (meaning element) and metron (meaning measure). Stoichiometry deals with the calculation of masses (sometimes volumes also) of the reactants and the products involved in a chemical reaction. Before understanding how to calculate the amounts of reactants required or the products produced in a chemical reaction, let us study what information is available from the balanced chemical equation of a given reaction.
What a Balanced Equation Tells Us
Let us consider the combustion of methane. A balanced equation for this reaction is:
Here, methane and dioxygen are called reactants, and carbon dioxide and water are called products. Note that all the reactants and the products are gases in the above reaction, indicated by the letter (g) in brackets next to each formula; in the case of solids and liquids, (s) and (l) are written respectively.
The coefficient 2 for O₂ and H₂O is called the stoichiometric coefficient; similarly the coefficient for CH₄ and CO₂ is one in each case. The stoichiometric coefficients represent the number of molecules (and moles as well) taking part in the reaction or formed in the reaction. Thus, according to the above chemical reaction:
- One mole of CH₄(g) reacts with two moles of O₂(g) to give one mole of CO₂(g) and two moles of H₂O(g)
- One molecule of CH₄(g) reacts with 2 molecules of O₂(g) to give one molecule of CO₂(g) and 2 molecules of H₂O(g)
- 22.7 L of CH₄(g) reacts with 45.4 L of O₂(g) to give 22.7 L of CO₂(g) and 45.4 L of H₂O(g)
- 16 g of CH₄(g) reacts with 2 × 32 g of O₂(g) to give 44 g of CO₂(g) and 2 × 18 g of H₂O(g)
From these relationships, the given data can be interconverted as follows:
The 22.7 L figure is the molar volume of an ideal gas at STP in NCERT's convention (0 °C, 1 bar). Older references using 1 atm quote 22.4 L — the difference is purely the pressure convention.
The General Strategy for Mass–Mass Calculations
- Write the balanced chemical equation.
- Convert the given mass of the substance to moles (divide by its molar mass).
- Use the mole ratio from the balanced equation to find the moles of the desired substance.
- Convert those moles back to mass (multiply by the molar mass).
The chapter's Problems 1.3 and 1.4 (combustion of methane — mass of water produced from 16 g of CH₄, and moles of CH₄ needed for 22 g of CO₂) apply exactly this strategy; each is worked in full on its own question page. …