Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations revision notes

Conservation of Mass in Chemical Reactions

The law of conservation of mass states that no atoms are lost or made during a chemical reaction. This means:

• The total mass of the reactants equals the total mass of the products.
• Atoms are simply rearranged to form new substances.

Balanced Symbol Equations

Chemical reactions are represented using balanced symbol equations, where:

• Formulae show the exact number and type of atoms in each substance.
• Coefficients (numbers before formulae) indicate how many molecules or moles are involved.
• Subscripts (numbers within formulae) show the number of atoms of each element in a molecule.

Example:

*Unbalanced equation:* H₂ + O₂ → H₂O *Balanced equation:* 2H₂ + O₂ → 2H₂O

Here, the coefficients ensure equal numbers of H and O atoms on both sides.

Key Steps to Balance Equations

1. Write the unbalanced equation with correct formulae.
2. Count atoms of each element on both sides.
3. Adjust coefficients to balance atoms (never change subscripts!).
4. Check all elements are balanced.

Common Pitfalls

• Confusing coefficients (multipliers) with subscripts (fixed in formulae).
• Forgetting to balance diatomic elements like O₂ or H₂.
• Changing subscripts, which alters the substance's identity.

Relative Formula Mass (Mr)

The relative formula mass is the sum of the relative atomic masses (Ar) of all atoms in a formula:

• Example: For H₂O, Mr = (2 × Ar of H) + (1 × Ar of O) = (2 × 1) + 16 = 18.
• Used to calculate percentage by mass of an element in a compound.

Percentage by mass formula:

\[ \text{Percentage by mass} = \left( \frac{\text{Total Ar of element}}{\text{Mr of compound}} \right) × 100 \]

Mass Changes in Reactions

Reactions may appear to change mass if:

• A gas is produced and escapes (e.g., CO₂ from thermal decomposition).
• A gas is absorbed (e.g., O₂ reacting with a metal to form an oxide).

Example: Heating magnesium in air increases mass as it forms magnesium oxide (MgO) by absorbing oxygen.

Measurement Uncertainty

All measurements have some uncertainty due to:

• Equipment limitations (e.g., balance precision).
• Human error in reading instruments.
• Environmental factors (e.g., temperature fluctuations).

Estimating uncertainty:

• Calculate the range (difference between highest and lowest values).
• Use the mean as the most reliable value.

Exam Tips

1. Always check atom counts when balancing equations.
2. Remember diatomic elements (e.g., O₂, N₂) in equations.
3. Use Mr calculations to verify conservation of mass.
4. Explain mass changes with reference to gases entering/leaving the system.
5. State uncertainty as a range (e.g., ±0.5 g) when reporting measurements.

Common Mistakes

1. Altering subscripts instead of coefficients when balancing.
2. Forgetting to multiply all atoms in a formula by the coefficient.
3. Ignoring measurement uncertainty in calculations.
4. Misinterpreting mass changes as violating conservation of mass.
5. Confusing Mr with Ar in percentage-by-mass calculations.

Unit 4.3 quantitative chemistry focus

Use Conservation of Mass and Balanced Chemical Equations to connect formula selection, substitution, numerical calculation, final answer, and units. Keep each explanation tied to the exact AQA GCSE Chemistry 8462 subtopic instead of using a broad statement that could fit any calculation page.

Calculation method

For calculation questions, start by naming the formula. Substitute the values with units, carry out the calculation clearly, then give the final answer with the expected unit. For ratio, empirical formula, and molecular formula questions, show how the relationship or simplest whole-number ratio was obtained.

Common boundaries to keep clear

Do not confuse relative atomic mass with relative formula mass, molecules with moles, mass with amount of substance, concentration in g/dm3 with concentration in mol/dm3, percentage yield with atom economy, or coefficients with subscripts.

Practice method

After reading each section, cover the worked example and attempt the formula, substitution, calculation, answer, and unit from memory. Then compare your working with the model method and correct any unit conversion or significant-figure mistakes.

Extra revision note support: write one sentence naming the quantitative idea, one sentence choosing the formula or ratio, one sentence substituting values with units, and one sentence checking that the final answer matches the question wording.

Extra revision note support: write one sentence naming the quantitative idea, one sentence choosing the formula or ratio, one sentence substituting values with units, and one sentence checking that the final answer matches the question wording.