1.
State what is meant by relative formula mass.
[1 mark]
- sum of relative atomic masses in a formula
JD SCIENCE
Key concepts in chemistry
Indicative marking points. Award marks for equivalent scientific or mathematical wording. Do not issue this sheet with the student worksheet.
2.
State what is meant by a mole.
[1 mark]
- the amount of substance that contains 6.02 × 10²³ particles
3.
State what is meant by conservation of mass.
[1 mark]
- total mass of reactants equals total mass of products in a closed system
4.
State what is meant by a limiting reactant.
[1 mark]
- the reactant that is used up first
5.
State what is meant by concentration.
[1 mark]
- amount of solute in a given volume of solution
6.
State what is meant by yield.
[1 mark]
- how much product is obtained
7.
State what is meant by atom economy.
[1 mark]
- Mr of desired product / Mr of all products × 100
8.
State what is meant by a balanced equation.
[1 mark]
- shows the same number of each atom on both sides
9.
State what is meant by an empirical formula.
[1 mark]
- simplest whole-number ratio of atoms
10.
State what is meant by Avogadro's constant.
[1 mark]
- 6.02 × 10²³ mol⁻¹
11.
Devon is asked about relative formula mass in a Edexcel Chemistry paper.
(a) [2 marks]
- Mr has no units
(b) [3 marks]
- why Mr(H₂O) is 18
12.
Devon is asked about a mole in a Edexcel Chemistry paper.
(a) [2 marks]
- Avogadro's constant
(b) [3 marks]
- why we use moles in chemistry
13.
Devon is asked about conservation of mass in a Edexcel Chemistry paper.
(a) [2 marks]
- atoms are rearranged not created
(b) [3 marks]
- why a mass drop can occur when a gas escapes
14.
Devon is asked about a limiting reactant in a Edexcel Chemistry paper.
(a) [2 marks]
- stops the reaction
(b) [3 marks]
- why adding more of the other reactant does not increase the product
15.
Devon is asked about concentration in a Edexcel Chemistry paper.
(a) [2 marks]
- mol/dm³ or g/dm³
(b) [3 marks]
- why diluting halves the concentration
16.
Devon is asked about yield in a Edexcel Chemistry paper.
(a) [2 marks]
- percentage yield = actual/theoretical × 100
(b) [3 marks]
- why yield is less than 100%
17.
Calculate Mr of CaCO₃ (Ca=40, C=12, O=16).
[3 marks]
- Final answer: 100
18.
How many moles in 9 g of NaOH (Mr=40)?
[3 marks]
- n = m/Mr
- Final answer: 0.23 mol
19.
What mass of 0.25 mol of H₂O (Mr=18)?
[3 marks]
- Final answer: 4.5 g
20.
A solution is 0.20 mol/dm³ and has volume 250 cm³. How many moles of solute?
[3 marks]
- n = cV, V in dm³
- Final answer: 0.050 mol
21.
2Na + 2HCl → 2NaCl + H₂. What mass of H₂ from 4.6 g Na (Aᵣ 23)?
[3 marks]
- 0.2 mol Na → 0.1 mol H₂
- Final answer: 0.20 g
22.
Actual yield 8.0 g, theoretical 10.0 g. Percentage yield?
[3 marks]
- Final answer: 80%
23.
Devon carries out a titration of acid with alkali. This is a typical Edexcel required-practical style question.
(a) [2 marks]
- Independent: volume of acid
- Dependent: volume of alkali to the end-point
(b) [2 marks]
- same indicator, same concentration
- so the test is fair / only one variable is changed
(c) [2 marks]
- overshooting the end-point
24.
Devon carries out making a soluble salt from an acid and an insoluble base. This is a typical Edexcel required-practical style question.
(a) [2 marks]
- Independent: n/a
- Dependent: mass of crystals
(b) [2 marks]
- same acid volume
- so the test is fair / only one variable is changed
(c) [2 marks]
- not evaporating carefully / spitting
25.
Devon carries out measuring mass change when Mg burns or CaCO₃ is heated. This is a typical Edexcel required-practical style question.
(a) [2 marks]
- Independent: starting mass
- Dependent: mass of product
(b) [2 marks]
- same heating time
- so the test is fair / only one variable is changed
(c) [2 marks]
- product lost as smoke
26.
Devon carries out preparing a standard solution. This is a typical Edexcel required-practical style question.
(a) [2 marks]
- Independent: mass of solid
- Dependent: concentration
(b) [2 marks]
- volumetric flask to the mark
- so the test is fair / only one variable is changed
(c) [2 marks]
- not dissolving fully before making up
27.
Compare percentage yield with atom economy.
[4 marks]
- yield is about how much you actually get
- atom economy is about how much of the atoms are in the desired product
28.
Compare a concentrated solution with a dilute solution.
[4 marks]
- concentrated: more solute per dm³
- dilute: less solute per dm³
29.
Compare an empirical formula with a molecular formula.
[4 marks]
- empirical is simplest ratio
- molecular is actual numbers of atoms
30.
Compare a closed system with an open system.
[4 marks]
- closed: mass appears constant
- open: gas can escape so mass can fall
31.
Apply your knowledge of atom economy to this situation: less waste
[3 marks]
- less waste
32.
Apply your knowledge of a balanced equation to this situation: mole ratios come from the equation
[3 marks]
- mole ratios come from the equation