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Fundamentals of chemistry: Class 9 Chemistry
What chemistry studies, elements, compounds and mixtures, atoms, molecules and ions, relative atomic mass, the mole and Avogadro's number, with worked numericals.
Key points
Chemistry is the study of the composition, structure, properties and changes of matter. Matter is anything that has mass and occupies space.
Matter is classified as:
- An element is a pure substance made of one kind of atom, for example hydrogen, iron and sodium.
- A compound is a pure substance made of two or more elements joined chemically in a fixed ratio, for example water (H₂O) and sodium chloride (NaCl).
- A mixture contains two or more substances that are not chemically joined. It can be separated by physical methods, for example salt and sand.
An atom is the smallest particle of an element that takes part in a chemical reaction. A molecule is a group of two or more atoms joined together, such as H₂, O₂ and H₂O. An ion is an atom or group of atoms with an electric charge. A cation is positive and an anion is negative.
Atoms are very small, so their masses are compared with a standard. The relative atomic mass of an element is the average mass of its atom compared with one twelfth of the mass of a carbon-12 atom. Relative atomic masses are written without units. Some values used in calculations are H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, S = 32, Cl = 35.5 and Ca = 40.
The relative molecular mass of a compound is the sum of the relative atomic masses of all atoms in its molecule. For water it is (2 × 1) + 16 = 18. For ionic compounds the term formula mass is used.
A mole is the amount of substance that contains 6.022 × 10²³ particles. This number is called Avogadro's number. The mass of one mole of a substance in grams is its molar mass. The molar mass of water is 18 g/mol, and one mole of any gas at standard temperature and pressure (STP) occupies 22.4 dm³.
Study tip: For mole questions, first find the molar mass, then write which formula connects the given quantity to moles.
Definitions and flashcards
Say the meaning first, then open the card to check.
Matter
Anything that has mass and occupies space.
Element
A pure substance made of one kind of atom.
Compound
A pure substance of two or more elements chemically joined in a fixed ratio.
Mixture
Two or more substances present together without a chemical bond between them.
Atom
The smallest particle of an element that can take part in a chemical reaction.
Molecule
Two or more atoms joined together.
Ion
An atom or group of atoms that carries an electric charge.
Relative atomic mass
The average mass of an atom of an element compared with one twelfth of a carbon-12 atom.
Mole
The amount of substance that contains 6.022 × 10²³ particles.
Avogadro's number
6.022 × 10²³, the number of particles in one mole.
Molar mass
The mass of one mole of a substance in grams.
Formula sheet
| Name | Formula | What it means |
|---|---|---|
| Number of moles from mass | n = mass ÷ molar mass | Mass in grams, molar mass in g/mol. |
| Number of particles | N = n × 6.022 × 10²³ | Particles in n moles. |
| Moles from particles | n = N ÷ 6.022 × 10²³ | Moles in N particles. |
| Volume of a gas at STP | V = n × 22.4 dm³ | One mole of any gas occupies 22.4 dm³ at STP. |
| Relative molecular mass | Sum of the relative atomic masses of all atoms | For example H₂O: 2(1) + 16 = 18. |
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Define an element and give two examples. (2 marks)
An element is a pure substance made of one kind of atom that cannot be split into simpler substances by chemical methods. Hydrogen and iron are elements.
What is the difference between a compound and a mixture? (3 marks)
A compound is made of elements chemically joined in a fixed ratio and has properties different from its elements. A mixture contains substances that are only mixed, in any proportion, and keeps the properties of its parts. A mixture is separated by physical methods but a compound only by chemical methods.
What is a molecule? Give an example of a molecule of an element and of a compound. (2 marks)
A molecule is a group of two or more atoms joined together. Oxygen (O₂) is a molecule of an element and water (H₂O) is a molecule of a compound.
Define the mole. (2 marks)
A mole is the amount of a substance that contains 6.022 × 10²³ particles, which may be atoms, molecules or ions.
Why is the relative atomic mass written without a unit? (2 marks)
It compares the mass of an atom with one twelfth of the mass of a carbon-12 atom. A ratio of two masses has no unit.
State Avogadro's number and explain its importance. (3 marks)
Avogadro's number is 6.022 × 10²³. It links the very small masses of atoms with the masses we can weigh. One mole of any substance contains this many particles, so we can count particles by weighing.
Explain the difference between an atom and an ion. (3 marks)
An atom is electrically neutral because the number of protons equals the number of electrons. An ion forms when an atom gains or loses electrons. A cation has lost electrons and is positive, and an anion has gained electrons and is negative.
Numericals with worked steps
How many moles are in 36 g of water? (H = 1, O = 16)
- Molar mass of H₂O = 2(1) + 16 = 18 g/mol.
- n = mass ÷ molar mass = 36 ÷ 18.
Answer: 2 mol
How many molecules are present in 2 mol of water?
- N = n × 6.022 × 10²³.
- = 2 × 6.022 × 10²³.
Answer: 1.2044 × 10²⁴ molecules
Find the mass of 0.5 mol of carbon dioxide. (C = 12, O = 16)
- Molar mass of CO₂ = 12 + 2(16) = 44 g/mol.
- Mass = n × molar mass = 0.5 × 44.
Answer: 22 g
What volume do 2 mol of oxygen gas occupy at STP?
- One mole of a gas occupies 22.4 dm³ at STP.
- V = 2 × 22.4.
Answer: 44.8 dm³
How many moles are present in 5.8 g of sodium chloride? (Na = 23, Cl = 35.5)
- Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol.
- n = 5.8 ÷ 58.5 = 0.0991.
Answer: About 0.099 mol
Common mistakes
- Using atomic mass instead of molecular mass for gases such as O₂ and H₂ (the molar mass of O₂ is 32, not 16).
- Forgetting to write the unit, for example g/mol for molar mass and dm³ for gas volume.
- Mixing up the number of molecules and the number of atoms. One mole of water has 6.022 × 10²³ molecules but three times as many atoms.
- Calling a mixture a compound because the parts look uniform.