Atomic structure
Define an isotope and state one way isotopes of the same element are similar and one way they differ.
Same element (same protons); different mass numbers (different neutrons).
Write the electron configurations for Na and Cl atoms, then for Na⁺ and Cl⁻ ions.
Na 2,8,1 → Na⁺ 2,8; Cl 2,8,7 → Cl⁻ 2,8,8.
Define element, compound, and mixture; give one example of each.
Element—one type of atom (e.g., O₂); Compound—chemically bonded atoms (e.g., H₂O); Mixture—physically combined (e.g., air).
Calculate Mr of CaCO₃. (Ar Ca = 40, C = 12, O = 16)
40 + 12 + (3×16) = 100.
How many moles are in 18 g of water? (Ar H = 1, O = 16)
n = m/Mr = 18/18 = 1.00 mol.
Element X has mass number 35 and atomic number 17. For neutral X and for X⁻, state the number of protons, neutrons, and electrons.
Neutral X: p=17, n=18, e=17.
X⁻: p=17, n=18, e=18.
An atom has configuration 2,8,3. State its period, (main) group, and whether it is a metal or non-metal.
Period 3, Group 13 (III), metal (Al).
State the separation method to obtain:
(a) salt from salty water,
(b) water from salty water,
(c) sand from a sand–salt–water mixture.
(a) Evaporation/crystallisation, (b) Simple distillation, (c) Filtration.
Balance: Al + O₂ → Al₂O₃
4Al + 3O₂ → 2Al₂O₃.
Magnesium burns in oxygen: 2Mg + O₂ → 2MgO. If 6.0 g of Mg react completely, calculate the mass of MgO formed. (Ar Mg = 24, O = 16)
n(Mg)=6.0/24=0.25 mol → n(MgO)=0.25 mol → mass=0.25×40=10.0 g.
Chlorine exists as Cl-35 (75%) and Cl-37 (25%). Calculate the relative atomic mass, Aᵣ(Cl), to 1 d.p.
(35×0.75)+(37×0.25)=35.5.
Which is larger: Mg or Mg²⁺? Explain briefly in terms of shells/attraction.
Mg is larger; Mg²⁺ has lost its outer shell and has stronger attraction nucleus→electrons.
In paper chromatography, a dye travels 4.0 cm while the solvent front travels 8.0 cm. Calculate Rf.
Rf = 4.0/8.0 = 0.50.
Calculate % by mass of calcium in CaCO₃. (Ar Ca = 40, C = 12, O = 16)
(40/100)×100% = 40%.
Heptane burns completely: C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O. What minimum mass of O₂ is needed to burn 30.0 g of heptane? (Ar C = 12, H = 1, O = 16)
n(C₇H₁₆)=30/100=0.30 mol → n(O₂)=11×0.30=3.3 mol → mass=3.3×32=105.6 g.
For the ion ²⁷₁₃Al³⁺, give: protons, neutrons, electrons.
p=13, n=14, e=10.
Explain why reactivity increases down Group 1 but decreases down Group 7.
G1: more shells → more shielding → outer e⁻ lost more easily (↑reactivity). G7: more shells → harder to gain e⁻ (↓reactivity).
You have sand, salt, and water mixed. Outline the sequence of methods to separate and obtain dry samples of both sand and salt.
Filter (sand residue); Evaporate/Crystallise filtrate to get salt; Dry both solids.
A compound contains C 40.0 g, H 6.7 g, O 53.3 g. Find the empirical formula. (Ar C = 12, H = 1, O = 16)
Moles: C 40/12=3.33, H 6.7/1=6.7, O 53.3/16=3.33 → ratio 1:2:1 → CH₂O.
2H₂ + O₂ → 2H₂O. If 20.0 g of O₂ is used up, what mass of H₂O is formed? (Ar H = 1, O = 16)
n(O₂)=20.0/32=0.625 mol
n(H₂O)=2×0.625=1.25 mol → mass = 1.25×18 = 22.5 g.
An ion Y²⁻ has electron configuration 2,8,8. If the nucleus contains 18 neutrons, identify Y and give its mass number.
Y atom has 16 e⁻ → S; mass no. = p+n = 16+18=34 → ³⁴S.
Atoms A and B have electron configurations A: 2,8,2 and B: 2,8,7. State the ion each is most likely to form.
A → A²⁺, B → B⁻
You must separate and collect pure liquids from the miscible solution:
Ethanol (bp 78 °C) + water (bp 100 °C)
State the best separation method and explain how it works.
Fractional distillation.
Substances with different boiling points travel up the fractionating column and boil off at different temperatures. The vapor is collected and then condensed in the condenser.
A compound has empirical formula C₃H₈O and Mr = 120. Determine its molecular formula. (Ar C = 12, H = 1, O = 16)
Empirical mass = 60 → factor 120/60=2 → C₆H₁₆O₂.
4Al + 3O₂ → 2Al₂O₃. What is the minimum mass of O₂ needed to react with 25.0 g of Al? (Ar Al = 27, O = 16)
n(Al)=25.0/27=0.9259 mol → n(O₂)=(3/4)×0.9259=0.694 mol → mass = 0.694×32 = 22.2 g (3 s.f.).