Formulae And Relative Masses

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Notes de leçon

Formulae

Elements are represented by chemical symbols from the Periodic Table.
Seven elements exist as diatomic molecules (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂).
A molecular formula shows the type and number of atoms in a molecule (e.g., H₂O, CH₄).
Ionic compounds are formed from metals and non-metals; the formula must have no overall charge.
Use the swap-and-drop method to determine the formula when ions have different charges (e.g., Cu²⁺ and Cl⁻ → CuCl₂).
Common polyatomic ions: carbonate CO₃²⁻, sulfate SO₄²⁻, hydroxide OH⁻, nitrate NO₃⁻, ammonium NH₄⁺.

Swap-and-drop method for copper(II) chloride

Writing Equations

Word equations show reactants → products using full chemical names.
Symbol equations use chemical formulae and must be balanced to satisfy conservation of mass.
Include state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
Balance by adjusting coefficients in front of formulae; never change subscripts.
Ionic equations show only the species that change; spectator ions are removed.
Example: 2Al(s) + 3Cl₂(g) → 2AlCl₃(s) is a balanced symbol equation.

Relative Atomic Mass (Aᵣ)

Relative atomic mass (Aᵣ) compares the mass of an atom to 1/12 the mass of carbon-12.
Aᵣ values are found on the Periodic Table (e.g., H=1, C=12, O=16).
Carbon-12 is the standard with a fixed mass of 12 units.

Key for chemical symbols on the Periodic Table

Relative Formula Mass (Mᵣ)

Relative formula mass (Mᵣ) is the sum of the Aᵣ of all atoms in a formula.
For molecules, it is called relative molecular mass.
Calculate Mᵣ by multiplying each Aᵣ by the number of atoms and adding them (e.g., H₂O: 2×1 + 16 = 18).
Examples: NaCl = 58.5, Ca(OH)₂ = 74, (NH₄)₂SO₄ = 132.

Reacting Masses

Conservation of mass: total mass of reactants = total mass of products.
Use balanced equations and Mᵣ to calculate masses of reactants and products.
Example: 2Ca + O₂ → 2CaO: 80 g Ca reacts with 32 g O₂ to give 112 g CaO.
Mass ratios are fixed; doubling the reactant mass doubles the product mass.

Diapos

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1.Which of the following elements exists as a diatomic molecule?
Easy
- AHelium
- BChlorine
- CCarbon
- DSulfur
2.What is the molecular formula of water?
Easy
3.Calculate the relative formula mass of calcium hydroxide, Ca(OH)2. (Ar: Ca=40, O=16, H=1)
Medium
- A81
- B67
- C74
- D88
4.Complete the sentence.
Easy
In a balanced chemical equation, the total mass of reactants equals the total mass of products due to the law of ____.
5.The relative atomic mass of carbon is 12.
Easy
True or false?
6.Match each ion to its formula.
Easy
- Hydroxide
- Sulfate
- Ammonium
- OH-
- SO4²-
- NH4+
7.Arrange the following steps for writing an ionic equation in the correct order.
Medium
- Replace ionic compounds with component ions
- Write the full balanced symbol equation
- Remove spectator ions
8.Calculate the mass of carbon dioxide produced when 32 g of methane (CH4) burns completely in excess oxygen. (Ar: H=1, C=12, O=16) CH4 + 2O2 → CO2 + 2H2O
Hard
- A97
- B106
- C88
- D79

Plus de questions

9.What is the formula of iron(III) chloride?
Easy
10.Which of the following is the correct balanced equation for the reaction of aluminium with chlorine to form aluminium chloride?
Medium
- AAl + Cl2 → AlCl3
- B2Al + 3Cl2 → 2AlCl3
- CAl + 3Cl → AlCl3
- D2Al + Cl2 → 2AlCl
11.Calculate the relative formula mass of ammonium sulfate, (NH4)2SO4. (Ar: N=14, H=1, S=32, O=16)
Medium
- A158
- B119
- C132
- D145
12.In an ionic compound, the total positive charge must equal the total negative charge.
Easy
True or false?
13.Which of the following are polyatomic ions? (Select all that apply)
Medium
- ACO₃²⁻
- BCl⁻
- COH⁻
- DSO₄²⁻
- ENH₄⁺
14.Which of the following is the correct formula for ammonia?
Easy
- ANH₃
- BNH₄
- CN₂H₄
- DNH
15.The diagram shows a ball-and-stick model of a molecule. The black sphere represents a carbon atom and each white sphere represents a hydrogen atom. What is the molecular formula of this molecule?
Easy
- ACH4
- BCH3
- CC2H4
- DCH2
16.Match each ion to its correct formula.
Easy
- Hydroxide
- Carbonate
- Ammonium
- OH⁻
- CO₃²⁻
- NH₄⁺
17.Which of the following elements exist as diatomic molecules? (Select all that apply)
Easy
- AHydrogen
- BHelium
- CNitrogen
- DOxygen
18.What is the formula of iron(III) oxide?
Easy
- AFeO
- BFe₂O₃
- CFe₃O₄
- DFeO₂
19.A student reacts 4.0 g of calcium with excess oxygen to form calcium oxide. The balanced equation is: 2Ca + O₂ → 2CaO. What mass of calcium oxide is produced? (Ar: Ca=40, O=16)
Medium
- A5.6 g
- B11.2 g
- C2.8 g
- D8.0 g
20.Which of the following are diatomic molecules? (Select all that apply)
Easy
- AH₂
- BO₂
- CHe
- DCl₂
- ECO₂
21.In an experiment, 1.2 g of magnesium reacts with excess hydrochloric acid to produce hydrogen gas. The balanced equation is: Mg + 2HCl → MgCl₂ + H₂. What volume of hydrogen gas is produced at room temperature and pressure? (1 mole of gas occupies 24 dm³; Ar: Mg=24)
Hard
- A1.2 dm³
- B2.4 dm³
- C0.6 dm³
- D1.8 dm³
22.In a balanced chemical equation, the total number of atoms of each element is the same on both sides of the equation.
Easy
True or false?
23.The relative atomic mass of an element is the mass of an atom of that element compared to the mass of an atom of carbon-12, which is taken as 12.
Easy
True or false?
24.What is the relative formula mass (Mr) of water, H2O? (Ar: H=1, O=16)
Easy
- A18
- B17
- C20
- D16
25.In a balanced chemical equation, the total number of atoms of each element is the same on both sides.
Easy
True or false?
26.Match each diatomic element to its correct molecular formula.
Easy
- Hydrogen
- Nitrogen
- Oxygen
- Chlorine
- H2
- N2
- O2
- Cl2
27.In an experiment, 4.8 g of magnesium reacts with excess hydrochloric acid to produce hydrogen gas. The balanced equation is Mg + 2HCl → MgCl₂ + H₂. What mass of magnesium chloride is produced? (Ar: Mg=24, Cl=35.5)
Medium
- A19.0 g
- B9.5 g
- C23.8 g
- D14.3 g
28.Which of the following elements exists as a diatomic molecule at room temperature?
Easy
- AHelium
- BNitrogen
- CCarbon
- DSulfur
29.In the reaction 2Mg + O₂ → 2MgO, what mass of magnesium oxide is produced when 4.8 g of magnesium reacts completely? (Ar: Mg=24, O=16)
Medium
- A8.0 g
- B4.0 g
- C12.0 g
- D16.0 g
30.A student reacts 2.0 g of calcium with excess oxygen. The balanced equation is 2Ca + O₂ → 2CaO. What mass of calcium oxide is produced? (Ar: Ca=40, O=16)
Medium
- A2.8 g
- B5.6 g
- C56 g
- D28 g
31.Calculate the relative formula mass of calcium hydroxide, Ca(OH)₂. (Ar: Ca=40, O=16, H=1)
Medium
32.State the meaning of the term 'relative atomic mass'.
Easy
33.Calculate the mass of carbon dioxide produced when 32 g of methane (CH₄) burns completely in excess oxygen. (Ar: H=1, C=12, O=16) CH₄ + 2O₂ → CO₂ + 2H₂O
Hard
34.Calculate the relative formula mass of magnesium nitrate, Mg(NO₃)₂. (Ar: Mg=24, N=14, O=16)
Medium
35.Complete the sentence: In an ionic compound, the total positive charge must equal the total ___ charge.
Easy
In an ionic compound, the total positive charge must equal the total ____ charge.
36.Calculate the relative atomic mass of hydrogen. (Use the standard value.)
Easy
37.Complete the sentence about the law of conservation of mass.
Easy
In a chemical reaction, the total mass of the ____ equals the total mass of the ____.
38.Complete the sentence about the charge of ionic compounds.
Easy
In an ionic compound, the total positive charge must equal the total ____ charge, so the overall formula is ____.
39.Explain why the formula of an ionic compound is always neutral overall.
Medium
40.State the meaning of the term 'relative formula mass'.
Easy
41.Explain why the formula of an ionic compound is always neutral overall. Use the example of magnesium oxide (MgO) in your answer.
Medium
42.Calculate the relative formula mass of magnesium nitrate, Mg(NO₃)₂. (Ar: Mg=24, N=14, O=16)
Medium
- A133
- B148
- C163
- D178
43.Calculate the relative atomic mass of hydrogen. (Use the standard value.)
Easy
- A1
- B0
- C3
- D2

Cartes mémo

What is the relative atomic mass (Ar) of an element?

The relative atomic mass is the average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom.

How is relative molecular mass (Mr) calculated?

Mr is the sum of the relative atomic masses of all atoms in a molecule or formula unit.

Calculate the relative formula mass of water (H2O). (Ar: H=1, O=16)

Mr = (2×1) + 16 = 18.

Calculate the relative formula mass of magnesium nitrate, Mg(NO3)2. (Ar: Mg=24, N=14, O=16)

Mr = 24 + 2×(14+3×16) = 24 + 2×62 = 148.

What is the law of conservation of mass?

Mass cannot be created or destroyed; in a chemical reaction, the total mass of reactants equals the total mass of products.

If 80 g of calcium reacts with 32 g of oxygen to form calcium oxide, what mass of calcium oxide is produced?

112 g (80 + 32 = 112).

What is the molecular formula of a compound?

It shows the type and number of atoms of each element in a molecule.

Name the seven diatomic elements.

Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2), Iodine (I2).

What is the formula of iron(III) chloride?

FeCl3 (Fe3+ and Cl- combine in a 1:3 ratio).

What is the formula of copper(II) chloride?

CuCl2 (Cu2+ and Cl- combine in a 1:2 ratio).

What is the formula of ammonium sulfate?

(NH4)2SO4 (two NH4+ ions combine with one SO42- ion).

Write the balanced symbol equation for the reaction of aluminium with chlorine to form aluminium chloride.

2Al (s) + 3Cl2 (g) → 2AlCl3 (s).

Write the balanced symbol equation for the reaction of magnesium oxide with nitric acid.

MgO (s) + 2HNO3 (aq) → Mg(NO3)2 (aq) + H2O (l).

What are state symbols?

They indicate the physical state: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

Write the ionic equation for the reaction of chlorine with aqueous potassium iodide.

2I- (aq) + Cl2 (aq) → 2Cl- (aq) + I2 (aq).

Calculate the mass of carbon dioxide produced when 32 g of methane (CH4) burns completely in excess oxygen. (Ar: H=1, C=12, O=16)

From CH4 + 2O2 → CO2 + 2H2O, 16 g CH4 produces 44 g CO2, so 32 g produces 88 g CO2.

What is the difference between relative atomic mass and relative molecular mass?

Relative atomic mass (Ar) applies to atoms; relative molecular mass (Mr) applies to molecules or formula units.

What is the formula of calcium hydroxide?

Ca(OH)2 (Ca2+ and OH- combine in a 1:2 ratio).

What is the formula of potassium carbonate?

K2CO3 (two K+ ions combine with one CO32- ion).

Balance the equation: _Al + _CuO → _Al2O3 + _Cu

2Al + 3CuO → Al2O3 + 3Cu.

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