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Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations
This topic builds the foundation for quantitative chemistry by connecting balanced symbol equations, mass conservation, formula masses and experimental measurement uncertainty.
28
Objectives
140
Flashcards
140
Questions
90 min
Study time
AQAGCSEChemistryQuantitative chemistry
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Syllabus checklist
What you need to know
28 objective pages available
Conservation of mass and balanced chemical equations8 objectives
- State that the law of conservation of mass means no atoms are lost or made during a chemical reaction.
- Explain that the mass of the products equals the mass of the reactants in a chemical reaction.
- Represent chemical reactions using balanced symbol equations.
- Balance symbol equations by making the number of atoms of each element the same on both sides.
- Interpret multipliers in normal script before formulae in chemical equations.
- Interpret subscripts within formulae when counting atoms in balanced chemical equations.
- Distinguish equation multipliers from formula subscripts when explaining chemical quantities.
- Use balanced chemical equations to support conservation-of-mass explanations. (WS 1.2)
Relative formula mass7 objectives
- Define relative formula mass, Mr, as the sum of the relative atomic masses of atoms in the numbers shown in a formula.
- Calculate the relative formula mass of a compound from its chemical formula and relative atomic masses.
- Interpret subscripts in a formula when calculating relative formula mass.
- Explain that in a balanced chemical equation the total relative formula mass of reactants in the stated quantities equals that of the products.
- Calculate the percentage by mass of an element in a compound given the relative formula mass and relative atomic masses.
- Use formulae and Ar values accurately when completing percentage-by-mass calculations.
- Distinguish relative formula mass from relative atomic mass in quantitative chemistry calculations.
Mass changes when a reactant or product is a gas7 objectives
- Explain why some reactions appear to involve a change in mass when a gas is a reactant or product.
- Explain why the mass of a metal oxide may be greater than the mass of the metal when oxygen reacts with the metal.
- Explain why thermal decomposition of a metal carbonate may appear to lose mass when carbon dioxide escapes.
- Use a balanced symbol equation to explain observed mass changes in a non-enclosed system.
- Explain apparent mass changes in terms of the particle model.
- Link gas escape or gas uptake to whether mass appears to decrease or increase in an open apparatus.
- Interpret investigations of mass changes using suitable apparatus. (AT 1, AT 2, AT 6)
Chemical measurements6 objectives
- Explain that every measurement has some uncertainty in the result obtained.
- Represent the distribution of repeated measurement results.
- Make estimations of uncertainty from a set of chemical measurements.
- Use the range of a set of measurements about the mean as a measure of uncertainty.
- Interpret repeated results to judge measurement uncertainty. (WS 3.4)
- Distinguish uncertainty from mistakes or anomalous results when discussing chemical measurements.
Key terms
Conservation of massChemical reactionreactantproductbalanced symbol equationreactantssymbol equationbalancingmultiplierbalanced chemical equationsubscriptconservation of mass
Exam tips
- Understand Conservation of Mass: Calculate carefully by remember that in any chemical reaction, the total mass of reactants equals the total mass of products. No atoms are lost or created. Link your answer to Conservation of mass and balanced chemical equations in Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations, and show formula, substitution, calculation, final answer, and unit where relevant.
- Understand Mass Conservation: Always remember that in a chemical reaction, the mass of the products must equal the mass of the reactants. Use balanced equations to verify this.
Common mistakes
- Misunderstanding Atom Conservation: Emphasize that the law of conservation of mass states that no atoms are lost or made; they are simply rearranged.
- Mass Misunderstanding: Emphasize that according to the law of conservation of mass, the total mass of reactants must equal the total mass of products in a closed system.
Practice preview
- What does the law of conservation of mass state?
- For Conservation of mass and balanced chemical equations, which option best supports this Unit 4.3 objective: State that the law of conservation of mass means no atoms are lost or made during a chemical reaction?
- For Conservation of mass and balanced chemical equations, which option best supports this Unit 4.3 objective: State that the law of conservation of mass means no atoms are lost or made during a chemical reaction?
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