Year 8 Science · Unit 2 · Lesson 20
Challenge Worksheet
Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. One Al₂(SO₄)₃ contains 4 oxygen atoms.
Correct it:
2. CO is a compound of carbon and oxygen, but Co is a single element (cobalt).
Correct it:
3. HCl is named “chlorine hydride”, because hydrogen is not a metal.
Correct it:
4. N₂ is a binary compound, because each molecule contains two atoms.
Correct it:
5. One (NH₄)₃PO₄ contains 12 hydrogen atoms.
Correct it:
6. In potassium iodide, KI, iodine gets the -ide ending because it is the non-metal written second.
Correct it:
Find the errors
Exactly three entries in this homework contain an error.
Scenario: a student’s homework
| Formula | Student’s atom count | Student’s name |
|---|---|---|
| H₂S | 2 H + 1 S = 3 | hydrogen sulfide |
| MgO | 1 Mg + 1 O = 2 | magnesium oxygen |
| CaF₂ | 2 Ca + 2 F = 4 | calcium fluoride |
| NaCl | 1 Na + 1 Cl = 2 | chlorine sodide |
| AlBr₃ | 1 Al + 3 Br = 4 | aluminium bromide |
1. Write the three corrections. For each, name the rule the student broke.
2. Mei is building ball-and-stick models of aluminium nitrate, Al(NO₃)₃. She wants to build 4 formula units, and she has 30 red balls for oxygen. (a) How many atoms are in one Al(NO₃)₃? (b) How many balls does she need altogether for 4 formula units? (c) Does she have enough red balls? Show your working.
(c) Oxygen balls needed = balls
| Fertiliser | Formula | N atoms | Total atoms | Fraction that is N |
|---|---|---|---|---|
| Ammonium nitrate | NH₄NO₃ | 2 | 9 | 2/9 |
| Urea | CO(NH₂)₂ | |||
| Ammonium sulfate | (NH₄)₂SO₄ |
Source: real fertiliser formulas. The last column compares numbers of atoms, not masses.
3. Complete Table 1. A shop assistant says: “Each has 2 nitrogen atoms, so they are equally rich in nitrogen.” Use the last column to evaluate this.
4. Evaluate: “Any compound at all can be named by adding -ide to its final element.” Cite three formulas from this sheet, and explain why chemists all agree on one naming system.
True or false
1. False: the 3 outside the bracket multiplies SO₄, so there are 3 × 4 = 12 oxygen atoms. 2. True. 3. False: HCl is hydrogen chloride; hydrogen is written first and keeps its name, and the second element takes -ide. 4. False: N₂ is a diatomic element (one element), not a compound. 5. True. 6. True.
Find the errors
1. (i) MgO is magnesium oxide: the second element takes -ide. (ii) CaF₂ is 1 Ca + 2 F = 3: a subscript counts only the symbol just before it. (iii) NaCl is sodium chloride: the first element keeps its name and -ide goes on the second. [3 marks: 1 per correction with its rule]
Your Turn
2. (a) 1 Al + 3 N + 9 O = 13 atoms. (b) 4 × 13 = 52 balls. (c) Oxygen: 4 × 9 = 36 red balls needed; she has 30, so she is 6 short. [4 marks: 1 (a), 1 (b), 1 for 36, 1 conclusion]
Go Deeper
3. Table: urea 2; 1 C + 1 O + 2 N + 4 H = 8; 2/8 (= 1/4). Ammonium sulfate 2; 2 N + 8 H + 1 S + 4 O = 15; 2/15. Evaluation: each has 2 N atoms per formula, but the totals differ, so the share differs: urea 2/8 (25%), ammonium nitrate 2/9 (about 22%), ammonium sulfate 2/15 (about 13%). Urea has the largest share, so they are not equally rich in nitrogen. (This compares atoms; by mass the ranking is the same.) [5 marks: 2 table rows, 1 same N count but different totals, 1 comparison of fractions, 1 urea]
Show What You Know
4. Marking guide (points expected): 1 the rule works for binary compounds: first element unchanged, second + -ide (for example MgO = magnesium oxide, NaCl = sodium chloride, H₂S = hydrogen sulfide); 1 the -ide goes on the non-metal written second, which is only “the last element” because a binary compound has just two; 1 it fails for compounds of three or more elements, for example NH₄NO₃, CO(NH₂)₂, (NH₄)₂SO₄, Al(NO₃)₃; 1 so it cannot name every compound (those need other rules); 1 one shared rule means a name tells every chemist exactly which substance is meant, avoiding confusion in labs and on labels; 1 judgement: an overclaim, the rule is reliable for binary compounds only. [6 marks]
Common error to watch for: in TF 5 and Table 1, forgetting that a number after a bracket multiplies every atom inside it, so (NH₄)₂ gives 8 hydrogen atoms, not 4.