Revision notes · Quantitative chemistry
Chemical measurements, conservation of mass and the
Conservation of mass and balanced chemical equations4.3.1.1
Definition: The law of conservation of mass states that no atoms are made or destroyed during a chemical reaction, so the total mass of the products equals the total mass of the reactants.
Mass is always conserved in a chemical reaction because the total number and type of atoms remain unchanged.
- •The sum of the relative formula masses of reactants equals the sum of the relative formula masses of products.
- •Chemical equations must be balanced to show that the same number of atoms of each element exists on both sides.
- •No atoms are created or destroyed during any chemical change.
- •If mass appears to change in an unsealed container, a gas has either entered or escaped.
⚠️ Common mistake: Thinking that mass is lost or destroyed when a gas is produced during a chemical reaction.
🧠 Remember: Mass in = Mass out (atoms are rearranged, never created or destroyed).
Relative formula mass4.3.1.2
Definition: The relative formula mass (Mᵣ) of a compound is the sum of the relative atomic masses (Aᵣ) of all the atoms in its chemical formula.
Relative formula mass allows you to calculate the total mass of a formula unit relative to carbon-12.
- •The relative atomic mass (Aᵣ) of an element is found on the Periodic Table as the larger number.
- •To calculate Mᵣ, multiply the Aᵣ of each element by the number of its atoms in the formula and add them together.
- •In a balanced equation, the sum of the Mᵣ of all reactants equals the sum of the Mᵣ of all products.
- •The relative formula mass does not have any units.
⚠️ Common mistake: Multiplying the relative formula mass by the big balancing number in front of a formula when calculating Mᵣ.
🧠 Remember: Mᵣ is the sum of all Aᵣ values in the formula.
Mass changes when a reactant or product is a gas4.3.1.3
Definition: A non-enclosed system is a reaction container where gases can freely enter or leave, which can cause observed mass to change.
Reactions involving gases can show an apparent change in mass if conducted in an unsealed container.
- •If a product is a gas, mass appears to decrease as the gas escapes into the surrounding air.
- •If a reactant is a gas from the air (such as oxygen), mass appears to increase as gas atoms bond to form a solid.
- •When heating magnesium in air, the measured mass increases because oxygen atoms from the air join the solid metal.
- •When reacting thermal decomposition of calcium carbonate, the mass decreases because carbon dioxide gas escapes.
⚠️ Common mistake: Claiming that mass was lost because atoms disappeared, rather than explaining that a gas escaped into the atmosphere.
🧠 Remember: Gas escapes = mass drops; Gas added = mass pops up!
Chemical measurements4.3.1.4
Definition: The uncertainty is the interval within which the true value of a measured quantity is expected to lie.
Every scientific measurement has a degree of uncertainty caused by apparatus limitations and human error.
- •Whenever a measurement is taken, there is always some uncertainty in the result.
- •The uncertainty can be estimated as ± half the smallest scale division of a measuring instrument.
- •For a set of repeated measurements, uncertainty can be calculated as Range ÷ 2.
- •Calculating the mean of repeated results improves reliability, provided anomalous results are ignored.
⚠️ Common mistake: Including anomalous results (outliers) when calculating the mean or range of repeated measurements.
🧠 Remember: Uncertainty = Range ÷ 2 (always ignore anomalies when calculating the mean!).
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