Question 1
Why is 24 + 16 + 2(1) incorrect for Mg(OH)₂?
Need a hint for Question 1?
Use the relationships above and check particle ratios and units.
Check Question 1
The 2 multiplies O as well as H: use 24 + 2(16 + 1).
STUDENT SPACE · TOPIC 1
Formulae and relative masses
Write ionic formulae and calculate relative molecular or formula masses.
The essentials
- NaCl is a giant ionic lattice: one Na⁺ ion for each Cl⁻ ion.
- A molecular formula gives the number and type of atoms in one molecule.
- Al³⁺ and O²⁻: 2(+3) + 3(−2) = 0 → Al₂O₃.
- Ca²⁺ and NO₃⁻: use brackets → Ca(NO₃)₂.
Key vocabulary
| English | Chinese |
|---|---|
| formula | 化学式 |
| relative atomic mass | 相对原子质量 |
| relative formula mass | 相对式量 |
Quick practice
Try each question before opening its hint or answer. Use paper for calculations and diagrams.
Question 2
Write the formula of aluminium sulfate using Al³⁺ and SO₄²⁻.
Need a hint for Question 2?
Find the smallest numbers of ions whose charges cancel.
Check Question 2
Al₂(SO₄)₃: total charge +6 and −6.
Question 3
Calculate Mᵣ of Mg(OH)₂. Aᵣ: Mg = 24, O = 16, H = 1.
Need a hint for Question 3?
The subscript outside brackets multiplies both O and H.
Check Question 3
24 + 2(16 + 1) = 58; no unit.
Use Lesson 1 in your worksheet for written practice. Your teacher will discuss those answers in class.