Revision notes · Quantitative chemistry

Use of amount of substance in relation to masses of pure

Moles4.3.2.1

Definition: A mole is the unit for the amount of substance, containing 6.02 × 10²³ particles.

Chemical amounts are measured in moles.

•One mole of any substance contains 6.02 × 10²³ atoms, molecules, or ions, known as Avogadro's constant.
•The mass of one mole of a substance in grams is equal to its relative formula mass (Mᵣ) or relative atomic mass (Aᵣ).
•Calculate mass, moles, or Mᵣ using the formula: mass = moles × Mᵣ.
•Mass must always be in grams (g) when calculating moles.

⚠️ Common mistake: Believing one mole of different substances has the same mass, rather than the same number of particles.

🧠 Remember: Mass = Moles × Mᵣ

Definition: The stoichiometry of a reaction is the ratio of the amounts of reactants and products shown in a balanced chemical equation.

Balanced symbol equations tell us the exact mole ratios of reacting substances.

•The big numbers in front of formulas in a balanced equation show the relative moles of each substance reacting.
•You can calculate reacting masses by converting a known mass to moles using moles = mass ÷ Mᵣ.
•Use the molar ratio from the equation to find the moles of the desired product or reactant.
•Convert the calculated moles back into mass using mass = moles × Mᵣ.

⚠️ Common mistake: Multiplying the Mᵣ of a substance by its balancing coefficient when calculating mass from moles.

🧠 Remember: Mass to moles, ratio, moles to mass

Reacting masses can be used to determine the balanced symbol equation for a chemical reaction.

•Convert the mass of each reactant and product into moles by dividing mass by its Mᵣ or Aᵣ.
•Divide all calculated mole values by the smallest number of moles obtained.
•Convert the resulting numbers into simple whole-number ratios.
•Place these whole numbers in front of the chemical formulas to balance the equation.

⚠️ Common mistake: Forgetting to divide all calculated mole values by the smallest mole value.

🧠 Remember: Moles, divide by smallest, whole numbers

Definition: A limiting reactant is the reactant that is completely consumed in a chemical reaction and limits the amount of product formed.

The amount of product formed depends on the reactant that is fully used up first.

•The reactant that is completely used up is the limiting reactant.
•Any reactant that is left over after the reaction stops is in excess.
•The mass of product produced is directly proportional to the amount of the limiting reactant.
•To identify it, calculate the moles of each reactant and compare them using the equation's molar ratio.

⚠️ Common mistake: Assuming the reactant with the smaller initial mass is always the limiting reactant.

🧠 Remember: The limiting reactant determines the maximum product formed.

Definition: Concentration is a measure of the amount of solute dissolved in a given volume of solution.

Solution concentration describes how much solute is present in a specific volume of liquid.

•Concentration can be measured in g/dm³ using the formula: concentration = mass ÷ volume.
•Volumes must be converted from cm³ to dm³ by dividing by 1000.
•Increasing the mass of solute in a fixed volume increases the concentration.
•Increasing the volume of solvent while keeping solute mass constant decreases the concentration.

⚠️ Common mistake: Forgetting to convert volume from cm³ to dm³ before calculating concentration.

🧠 Remember: Divide cm³ by 1000 to get dm³

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