CDS Chemistry · Formula sheet

Mole Concept and Stoichiometry formulas

5 formulas and 13 common traps for CDS Chemistry Mole Concept and Stoichiometry, grouped by subtopic.

The Mole, Equivalent Weight and Concentration

Moles, molar mass and Avogadro's number

Moles and particles

n = \frac{m}{M} \qquad N = n \times N_A, \quad N_A = 6.022 \times 10^{23}

$n$ number of moles
$m$ mass in grams
$M$ molar mass in g/mol
$N$ number of particles

Equivalent weight

E = \frac{M}{n\text{-factor}}

$E$ equivalent weight
$M$ molar mass

Molarity and molality

M = \frac{n_{\text{solute}}}{V_{\text{solution}}\,(\text{L})} \qquad m = \frac{n_{\text{solute}}}{m_{\text{solvent}}\,(\text{kg})}

Common traps

Count atoms, not molecules, when asked for atoms

After finding the moles of a compound, multiply by the number of that atom in the formula. Glucose has 12 hydrogen atoms per molecule, so hydrogen atoms = 12 × the molecules.

Equal in number is not equal in mass

Ethyne, C₂H₂, has as many carbon atoms as hydrogen atoms, but the carbon's mass is 24 and the hydrogen's 2. Multiply each count by its atomic mass before comparing masses.

Helium is monatomic

Helium exists as single atoms, so its molar mass is 4 g/mol, not 8. Moles of helium = mass ÷ 4.

A mole is a number, not a mass

One mole of anything has 6.022 × 10²³ particles, but its mass depends on the substance: 1 mol of H₂ is 2 g, 1 mol of O₂ is 32 g.

Divide by the n-factor, not by the number of atoms

A base with two OH⁻ per formula, such as Ca(OH)₂, has acidity 2, so its equivalent weight is half its molar mass, not the molar mass itself.

Basicity counts only replaceable hydrogens

Acetic acid, CH₃COOH, has four hydrogens but only the one in –COOH is given up as H⁺, so its basicity is 1.

Molarity uses the solution, molality the solvent

Molarity divides by the volume of the solution in litres; molality divides by the mass of the solvent in kilograms. Mixing them up gives the wrong number.

Molality does not change with temperature

Heating a solution expands its volume, so its molarity falls slightly. Molality uses mass, which does not change, so it stays the same.

Balancing Equations and Reacting Amounts

Learn this subtopic in the notes

Balancing a chemical equation

Conservation of atoms

\sum \text{atoms of each element (reactants)} = \sum \text{atoms of that element (products)}

Mole ratios and the limiting reactant

Mole ratio from coefficients

aA + bB \rightarrow cC: \qquad n_C = \frac{c}{a}\, n_A

Common traps

Re-check the first element after fixing the second

In Fe + Cl₂ → FeCl₃, fixing chlorine to 3Cl₂ → 2FeCl₃ puts 2 Fe on the right, so the iron must go back to 2Fe: 2Fe + 3Cl₂ → 2FeCl₃.

Never change a subscript

Changing H₂O to H₂O₂ to balance oxygen makes a different substance (hydrogen peroxide). Only the numbers in front of formulas may change.

The reactant in excess does not set the yield

In CH₄ + ½O₂ → CO + 2H₂ with 3 mol CH₄ and 1 mol O₂, the methane would need 1.5 mol O₂, so oxygen is limiting: 1 mol O₂ reacts with 2 mol CH₄ and gives 2 mol CO.

Mass is conserved, moles need not be

In 2H₂ + O₂ → 2H₂O, three moles of gas give two moles of water, yet the mass on both sides is the same (36 g). Conservation applies to mass and atoms, not to the number of moles.

Per gram means divide by mass

To find which reaction gives the most product per gram, divide the moles of product by the reactant's molar mass. A larger coefficient does not mean more per gram if the reactant is heavy.

Mole Concept and Stoichiometry formulas

The Mole, Equivalent Weight and Concentration

Moles, molar mass and Avogadro's number

Common traps

Balancing Equations and Reacting Amounts

Balancing a chemical equation

Mole ratios and the limiting reactant

Common traps

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