Y1 · III · #31Stoichiometry – chemical calculations

Calculations using n = N/NA, n = m/M, n = V/Vm (II)

Recap and Where We're Going

In the last lesson, you met three key formulas for finding the amount of substance, n (measured in moles):

n = N / N_A — using the number of particles and Avogadro's constant (N_A = 6.02 × 10²³ mol⁻¹)
n = m / M — using mass and molar mass
n = V / V_m — using gas volume and molar volume (22.7 dm³ mol⁻¹ at STP, or 24.8 dm³ mol⁻¹ at SATP, depending on which conditions your syllabus data booklet specifies)

Today we combine these formulas with balanced chemical equations to solve real stoichiometry problems — converting between mass, particles, and gas volumes across a reaction (Talbot, p.418).

The General Strategy

Whatever combination of quantities you're given, the method is always the same three-step chain:

Convert the given quantity (mass, particle number, or gas volume) into moles using the correct formula.
Use the mole ratio from the balanced equation to find moles of the substance you want.
Convert those moles into whatever quantity is asked for (mass, particles, or volume).

This mirrors the mass–mass method described by Talbot (p.424), just extended to particles and gases too.

Worked Example 1 – Mass to Gas Volume

Question: What volume of oxygen gas (at STP, V_m = 22.7 dm³ mol

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Source excerpts

From Chemistry for the IB Diploma 3e · Talbot

p.424relevance 14.3

R2: How much, how fast and how far? 412 Stoichiometry problems There are four basic types of stoichiometry problem involving the mole concept: ■ mass–mass ■ mass–gas volume ■ gas volume–gas volume ■ concentration. ■Mass–mass stoichiometry problems In a mass–mass stoichiometry problem, you will use a given mass of a reactant or product to determine an unknown mass of reactant or…

p.525relevance 12.4

HL ONLY R2.3 How far? The extent of chemical change 513 Going further Calculations using the quadratic formula For the esterification reaction: CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l) calculate the amount of ethyl ethanoate that formed at equilibrium when 1.0 mole of ethanol reacted with 2.0 moles of ethanoic acid at 373 K. T he value of K is 4.0 at this temperature. Le…

p.12relevance 11.8

Chemistry for the IB Diploma Programme x n Tool 3: Mathematics Skill Description Applying general mathematics • Use basic arithmetic and algebraic calculations to solve problems. • Carry out calculations involving decimals, fractions, percentages, ratios, r eciprocals and exponents. • Carry out calculations involving logarithmic functions. • Carry out calculations involving exp…

p.418relevance 11.4

R2: How much, how fast and how far? 406 How much? The amount of chemical changeR2.1 • How are chemical equations used to calculate reacting ratios? Guiding question SYLLABUS CONTENT By the end of this chapter, you should understand that:  chemical equations show the ratio of reactants and products in a reaction  the mole ratio of an equation can be used to determine:  the ma…