Chemical formulae

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Element Symbols

Each element has a unique symbol, e.g., C for carbon, Cu for copper.
The first letter of a symbol is always a capital letter; if a second letter exists, it is lower case.
Some symbols come from Latin names, e.g., Cu from×cuprum×.

Diatomic Molecules

Some elements exist as pairs of atoms (diatomic molecules).
Example: oxygen gas is written as O₂ – the subscript 2 means two oxygen atoms joined.
Other diatomic elements: H₂, N₂, F₂, Cl₂, Br₂, I₂.

Compounds and Formulae

A compound is a substance where two or more different elements are chemically joined.
Compounds have different properties from the elements they are made from.
A chemical formula shows the elements and the number of atoms of each in a compound.
Example: H₂O means two hydrogen atoms and one oxygen atom.

Distinguishing Elements and Compounds

Be careful: Co (cobalt, element) vs CO (carbon monoxide, compound).
In CO, both letters are capital because it is two elements: C and O.
The subscript numbers show the number of atoms: e.g., CO₂ has one C and two O atoms.

Common Compounds Table

Al₂O₃ – aluminium oxide: 2 Al, 3 O.
CO₂ – carbon dioxide: 1 C, 2 O.
HCl – hydrochloric acid: 1 H, 1 Cl.
MgO – magnesium oxide: 1 Mg, 1 O.
SiO₂ – silicon dioxide: 1 Si, 2 O.
NaCl – sodium chloride: 1 Na, 1 Cl.
H₂SO₄ – sulfuric acid: 2 H, 1 S, 4 O.
H₂O – water: 2 H, 1 O.

Writing Chemical Formulae

Use the element symbols from the periodic table.
Write the number of atoms as a subscript after the element symbol.
If there is only one atom, no subscript is written (e.g., O in H₂O).
The order of elements often follows standard conventions (e.g., metal first, then non-metal).

Examples from Workbook

NaOH – sodium hydroxide: 1 Na, 1 O, 1 H.
FeS – iron sulfide: 1 Fe, 1 S.
NH₃ – ammonia: 1 N, 3 H.
CaO – calcium oxide: 1 Ca, 1 O.
Li₂O – lithium oxide: 2 Li, 1 O.
CaCl₂ – calcium chloride: 1 Ca, 2 Cl.
CH₄ – methane: 1 C, 4 H.
Na₂CO₃ – sodium carbonate: 2 Na, 1 C, 3 O.
K₂SO₄ – potassium sulphate: 2 K, 1 S, 4 O.

Atomic structure of oxygen (O) – 8 protons, 8 neutrons, 2 electrons in inner shell, 6 in outer shell. Oxygen atoms often pair to form O₂.

Particle arrangement in solids, liquids, and gases. In compounds, atoms of different elements are chemically joined (e.g., H₂O molecules).

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Câu hỏi luyện tập

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1.Which of the following is the correct chemical formula for water?
Easy
- AH2O
- BHO2
- CH2O2
- DOH2
2.The symbol 'Co' represents which element?
Easy
- ACobalt
- BCarbon monoxide
- CCopper
- DCarbon
3.How many oxygen atoms are in the formula H2SO4?
Easy
- A1
- B2
- C4
- D6
4.Which of the following compounds has the formula NaCl?
Easy
- ASodium hydroxide
- BSodium chloride
- CSodium carbonate
- DSodium oxide
5.What is the chemical formula for aluminium oxide?
Medium
- AAlO
- BAl2O
- CAlO3
- DAl2O3
6.Which statement is true about compounds?
Easy
- ACompounds have the same properties as the elements they are made from.
- BCompounds have different properties to the elements they are made from.
- CCompounds are only made from one type of atom.
- DCompounds cannot be represented by chemical formulae.
7.What does the subscript 2 in O2 indicate?
Easy
- ATwo oxygen molecules
- BTwo oxygen atoms joined together
- CTwo different types of oxygen
- DTwo oxygen electrons
8.Which of the following is the correct formula for carbon dioxide?
Easy
- ACO
- BCo2
- CC2O

Thêm câu hỏi

9.The formula for sulfuric acid is H2SO4. How many hydrogen atoms are present?
Easy
- A1
- B2
- C4
- D0
10.Which of the following pairs have the same number of oxygen atoms?
Medium
- ACO2 and SiO2
- BH2O and H2O2
- CMgO and Al2O3
- DNaOH and NaCl
11.What is the correct symbol for copper?
Easy
- AC
- BCu
- CCo
- DCp
12.How many different elements are in the compound NaOH?
Easy
- A1
- B2
- C3
- D4
13.The chemical formula for a compound is MgO. Which row correctly describes the number and type of atoms in one molecule of this compound?
Medium
- A1 magnesium atom and 1 oxygen atom
- B1 magnesium atom and 1 oxygen molecule
- C1 magnesium molecule and 1 oxygen atom
- D1 magnesium molecule and 1 oxygen molecule
14.How many atoms are there in total in one formula unit of aluminium oxide, Al2O3?
Easy
- A5
- B2
- C3
- D6
15.Which of the following is the correct chemical formula for carbon monoxide?
Easy
- ACO
- BCO2
- CC2O
16.A compound has the formula Na2CO3. How many sodium atoms are present in one formula unit?
Easy
- A2
- B1
- C3
- D6
17.The formula for sulfuric acid is H2SO4. Which statement about this formula is correct?
Medium
- AIt contains 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms.
- BIt contains 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen molecules.
- CIt contains 1 hydrogen molecule, 1 sulfur atom, and 4 oxygen atoms.
- DIt contains 2 hydrogen atoms, 1 sulfur molecule, and 4 oxygen atoms.
18.A student writes the formula for sodium chloride as NaCl. Which of the following explains why the formula is correct?
Medium
- AIt contains one sodium atom and one chlorine atom.
- BIt contains one sodium ion and one chlorine ion.
- CIt contains one sodium molecule and one chlorine molecule.
- DIt contains one sodium atom and one chlorine molecule.
19.What is the chemical formula for silicon dioxide?
Easy
- ASiO2
- BSiO
- CSi2O
- DSi2O2
20.The formula for ammonia is NH3. How many hydrogen atoms are in one molecule of ammonia?
Easy
- A3
- B1
- C4
- D2
21.Which of the following compounds contains the largest number of atoms per formula unit?
Medium
- AH2SO4
- BNa2CO3
- CAl2O3
- DK2SO4
22.A compound is represented by the formula CaCl2. Which of the following is true?
Easy
- AIt contains one calcium atom and two chlorine atoms.
- BIt contains one calcium atom and one chlorine molecule.
- CIt contains one calcium molecule and two chlorine atoms.
- DIt contains two calcium atoms and one chlorine atom.
23.Which of the following is the correct formula for hydrochloric acid?
Easy
- AHCl
- BH2Cl
- CHCl2
- DH2Cl2
24.The formula for sodium hydroxide is NaOH. How many different elements are in this compound?
Easy
- A3
- B2
- C4
- D1
25.A student is given an unknown white powder. They heat it strongly and observe that it decomposes, releasing a gas that turns limewater milky. The residue left is a yellow solid when hot and white when cold. Which of the following is the most likely formula of the unknown powder?
Hard
- ACuCO3
- BZnCO3
- CCaCO3
- DNa2CO3
26.A student wants to determine the formula of a hydrated salt. They heat a sample of hydrated copper(II) sulfate and record the mass before and after heating. The initial mass is 2.50 g and the final mass is 1.60 g. What is the value of x in CuSO4·xH2O? (Mr: CuSO4 = 159.5, H2O = 18.0)
Hard
- A5
- B4
- C3
- D2
27.A student tests a colourless liquid with anhydrous copper(II) sulfate, which turns blue. They then add a small piece of sodium metal, which fizzes and produces a gas that burns with a 'pop'. The liquid is most likely:
Medium
- Aethanol
- Bwater
- Chydrochloric acid
- Dhydrogen peroxide
28.In an experiment to determine the empirical formula of magnesium oxide, a student heats a known mass of magnesium in a crucible. The following results are obtained: mass of crucible + lid = 25.00 g, mass of crucible + lid + magnesium = 26.20 g, mass of crucible + lid + magnesium oxide = 26.80 g. What is the empirical formula of magnesium oxide? (Ar: Mg = 24.3, O = 16.0)
Medium
- AMgO
- BMg2O
- CMgO2
- DMg2O3
29.Which of the following techniques is used to separate a mixture of sand and salt?
Easy
- AFiltration, then evaporation
- BDistillation
- CChromatography
- DCrystallisation
30.A student has a mixture of two liquids, P and Q. P boils at 78°C and Q boils at 100°C. Which method would best separate them?
Easy
- ASimple distillation
- BFractional distillation
- CEvaporation
- DFiltration
31.In a chromatography experiment, a student places a spot of ink on a baseline and develops the chromatogram using a solvent. After development, the solvent front is 10 cm from the baseline, and the ink spot has separated into two spots at distances 4 cm and 7 cm from the baseline. What are the Rf values of the two components?
Medium
- A0.4 and 0.7
- B0.4 and 0.3
- C0.6 and 0.3
32.Which of the following is a chemical change?
Easy
- AIce melting
- BSalt dissolving in water
- CBurning wood
- DCutting paper
33.A student adds a few drops of universal indicator to a colourless liquid. The indicator turns red. The liquid is most likely:
Easy
- Ahydrochloric acid
- Bsodium hydroxide
- Cwater
- Dethanol
34.In a titration, 25.0 cm³ of 0.10 mol/dm³ hydrochloric acid neutralises 20.0 cm³ of sodium hydroxide solution. What is the concentration of the sodium hydroxide? HCl + NaOH → NaCl + H₂O
Hard
- A0.125 mol/dm³
- B0.080 mol/dm³
- C0.100 mol/dm³
- D0.200 mol/dm³
35.A student is asked to prepare a pure, dry sample of copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid. Which of the following sequences of steps is correct?
Medium
- AAdd excess copper(II) oxide to warm acid, filter, evaporate filtrate to saturation, cool, filter, dry
- BAdd excess acid to copper(II) oxide, evaporate to dryness
- CMix copper(II) oxide and acid, then crystallise without filtering
- DAdd copper(II) oxide to acid, heat to dryness
36.Which piece of apparatus is used to measure an accurate volume of liquid, such as 25.0 cm³?
Easy
- AMeasuring cylinder
- BBurette
- CPipette
- DBeaker
37.Which of the following statements about chemical symbols is correct?
Easy
- AThe first letter of a symbol is always a capital letter.
- BThe first letter of a symbol is always a lower case letter.
- CIf a symbol has two letters, both are capital letters.
- DIf a symbol has two letters, the second is always a capital letter.
38.The chemical formula for carbon monoxide is CO. What does this formula represent?
Easy
- AOne carbon atom and one oxygen atom
- BOne cobalt atom
- COne carbon atom and two oxygen atoms
- DTwo carbon atoms and one oxygen atom
39.Which of the following is the correct chemical formula for calcium oxide?
Easy
- ACaO
- BCaO2
- CCa2O
- DCo
40.How many atoms of each element are present in one molecule of sulfuric acid, H2SO4?
Easy
- A2 hydrogen, 1 sulfur, 4 oxygen
- B1 hydrogen, 1 sulfur, 4 oxygen
- C2 hydrogen, 1 sulfur, 2 oxygen
- D2 hydrogen, 2 sulfur, 4 oxygen
41.Compounds have different properties to the elements that they are made from.
Easy
True or false?
42.The chemical formula for carbon monoxide is Co.
Easy
True or false?
43.What does the subscript '2' in O2 indicate?
Easy
- AThere are two oxygen atoms joined together.
- BOxygen has two electrons.
- CThe element is twice as heavy.
- DThere are two different types of oxygen.
44.Match each chemical formula to the correct name of the compound.
Medium
- CO
- Co
- CO2
- H2O
- Carbon monoxide
- Cobalt
- Carbon dioxide
- Water
45.Arrange these steps in the correct order to write the chemical formula for a compound from its name.
Medium
- Identify the symbols of the elements present.
- Write the symbol of the first element.
- Write the symbol of the second element.
- Add subscripts to show the number of atoms of each element.
46.Which of the following are correct chemical formulae? Select all that apply.
Medium
- ANaCl
- BMgO
- Ch2o
- DCO2
- ECaCl2
47.A compound has the formula FeS. How many atoms of each element are present in one formula unit?
Easy
- A1 iron atom and 1 sulfur atom
- B1 iron atom and 2 sulfur atoms
- C2 iron atoms and 1 sulfur atom
- D2 iron atoms and 2 sulfur atoms
48.What does the subscript '2' in H2O indicate?
Easy
- AThere are two water molecules.
- BThere are two hydrogen atoms in one water molecule.
- CThere are two oxygen atoms in one water molecule.
- DThere are two elements in water.
49.Which of the following is the correct chemical formula for potassium sulphate?
Medium
- AKSO4
- BK2SO4
- CK(SO4)2
- DK2SO3
50.A student is given an unknown white powder. They heat it strongly and observe that it decomposes, releasing a gas that turns limewater milky. The residue left is a white solid that does not melt. Which of the following is the most likely identity of the unknown powder?
Medium
- ASodium chloride
- BCalcium carbonate
- CMagnesium oxide
- DCopper(II) oxide
51.Fill in the missing name or formula.
Easy
The compound with the formula ____ is called sodium hydroxide.
52.What is the rule for writing chemical symbols of elements that have two letters?
Easy
53.Complete the table by filling in the missing name or formula.
Medium
Name: Sodium hydroxide | Formula: NaOH | Elements in compound: ____
54.Fill in the missing formula and elements for calcium oxide.
Medium
Name: Calcium oxide | Formula: ____ | Elements in compound: One calcium atom and one oxygen atom
55.How many atoms are there in total in one formula unit of potassium sulphate, K2SO4?
Medium
56.How many atoms are there in total in one formula unit of sodium carbonate, Na2CO3?
Medium
57.Complete the table by filling in the missing information for each compound.
Medium
Name of compound: Sodium hydroxide Chemical formula: NaOH Elements in compound: ____
58.Explain why the chemical formula of carbon monoxide is written as CO and not Co.
Hard
59.How many atoms are there in total in one formula unit of potassium sulphate, K2SO4?
Medium
- A6 atoms
- B9 atoms
- C8 atoms
- D7 atoms
60.How many atoms are there in total in one formula unit of sodium carbonate, Na2CO3?
Medium
- A7
- B6
- C5
- D8

Ghi chú ôn tập

## Chemical Formulae - **Elements** are represented by symbols: first letter capital, second letter lowercase (e.g., C for carbon, Cu for copper). - Some elements exist as **diatomic molecules** (e.g., oxygen gas is O₂, where the subscript 2 means two atoms joined). - **Compounds** are substances where two or more different elements are chemically joined. They have different properties from their constituent elements. - **Chemical formulae** show the elements and number of atoms in a compound. For example: - Water: H₂O (2 hydrogen atoms, 1 oxygen atom) - Carbon dioxide: CO₂ (1 carbon, 2 oxygen) - Sodium chloride: NaCl (1 sodium, 1 chlorine) - Be careful: element symbols and compound formulae can look similar. E.g., **Co** is cobalt (element), **CO** is carbon monoxide (compound). The difference is case: CO has a capital O. ### Common Compounds (from source) | Name | Formula | Elements | |------|---------|----------| | Aluminium oxide | Al₂O₃ | 2 Al, 3 O | | Carbon dioxide | CO₂ | 1 C, 2 O | | Hydrochloric acid | HCl | 1 H, 1 Cl | | Magnesium oxide | MgO | 1 Mg, 1 O | | Silicon dioxide | SiO₂ | 1 Si, 2 O | | Sodium chloride | NaCl | 1 Na, 1 Cl | | Sulfuric acid | H₂SO₄ | 2 H, 1 S, 4 O | | Water | H₂O | 2 H, 1 O | ### Key Points - Subscripts indicate the number of atoms of the element before them. - No subscript means one atom. - Compounds have different properties than their elements.

Element symbol: A one- or two-letter code for an element; first letter capital, second lowercase (e.g., C, Cu).
Chemical formula: A representation of a compound using element symbols and subscripts to show the number of atoms of each element (e.g., H₂O).
Compound: A substance where two or more atoms of different elements are chemically joined.
Subscript: A small number written after an element symbol indicating the number of atoms of that element in the formula.
Diatomic molecule: A molecule composed of two atoms of the same element (e.g., O₂).

Thẻ ghi nhớ

What is the rule for writing element symbols with more than one letter?

The first letter is always capital, the second letter is lower case. For example, 'Cu' for copper.

What does the subscript 2 in O₂ mean?

It shows that two oxygen atoms are joined together.

What is a compound?

A substance where two or more atoms of different elements are chemically joined.

How do the properties of a compound compare to the elements it is made from?

Compounds have different properties to the elements they are made from.

What is the chemical formula for water?

H₂O

How many hydrogen atoms and oxygen atoms are in one molecule of water?

Two hydrogen atoms and one oxygen atom.

What is the difference between the symbol 'Co' and the formula 'CO'?

'Co' is the element cobalt (capital C, lower case o), while 'CO' is carbon monoxide (both capital letters).

What is the chemical formula for carbon monoxide?

How many carbon and oxygen atoms are in carbon monoxide?

One carbon atom and one oxygen atom.

What is the chemical formula for aluminium oxide?

Al₂O₃

How many aluminium and oxygen atoms are in aluminium oxide?

Two aluminium atoms and three oxygen atoms.

What is the chemical formula for carbon dioxide?

CO₂

How many carbon and oxygen atoms are in carbon dioxide?

One carbon atom and two oxygen atoms.

What is the chemical formula for hydrochloric acid?

HCl

What elements are in hydrochloric acid?

Hydrogen and chlorine.

What is the chemical formula for magnesium oxide?

MgO

What is the chemical formula for silicon dioxide?

SiO₂

What is the chemical formula for sodium chloride?

NaCl

What is the chemical formula for sulfuric acid?

H₂SO₄

How many hydrogen, sulfur, and oxygen atoms are in sulfuric acid?

Two hydrogen atoms, one sulfur atom, and four oxygen atoms.

What is the chemical formula for sodium hydroxide?

NaOH

What is the chemical formula for ammonia?

NH₃

What is the chemical formula for calcium oxide?

CaO

What is the chemical formula for methane?

CH₄

What is the chemical formula for sodium carbonate?

Na₂CO₃

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