2025 Y3 Chemistry Revision (Sem1 topics)

Calculate the volume occupied at STP by 0.5 mol of carbon dioxide gas.

• 11.35 dm³

• 22.7 dm³

• 0.5 dm³

• 44.8 dm³

Answer explanation

At STP, 1 mol of any gas occupies 22.4 dm³. Therefore, 0.5 mol of CO₂ occupies 0.5 x 22.4 dm³ = 11.2 dm³. The closest answer is 11.35 dm³, which accounts for rounding in calculations.

One mole of CO₂ contains how many oxygen atoms?

• 6.02 x 10²³

• 1.20 x 10²⁴

• 3.01 x 10²³

• 2.40 x 10²⁴

Answer explanation

One mole of CO2 contains 2 moles of oxygen atoms. Since 1 mole contains 6.02 x 10^23 particles, 2 moles contain 2 x 6.02 x 10^23 = 1.20 x 10^24 oxygen atoms. Thus, the correct answer is 1.20 x 10^24.

The molar volume of any gas at STP is:

• 24.0 dm³

• 22.7 dm³

• 20.0 dm³

• 25.0 dm³

Answer explanation

At standard temperature and pressure (STP), the molar volume of an ideal gas is approximately 22.7 dm³. This value is commonly used in calculations involving gases.

Which of the following acids is a weak acid?

• HCl

• HNO₃

• CH₃COOH

• H₂SO₄

Answer explanation

Among the given options, CH₃COOH (acetic acid) is a weak acid, as it does not completely dissociate in solution. In contrast, HCl, HNO₃, and H₂SO₄ are strong acids that fully dissociate.

Calculate the Mr of sodium oxide.

[Ar of Na = 23, Ar of O = 16]

• 39

• 55

• 62

• 78

Answer explanation

Na₂O is the formula of sodium oxide. Hence, 23 x 2 + 16 = 62

Determine the empirical formula of a compound with 52.2% carbon, 13% hydrogen, and 34.8% oxygen.

[Ar of O = 16, Ar of C = 12, Ar of H = 1]

• C₃H₈O

• C₂H₆O

• C₄H₁₀O₂

• C₂H₄O

Answer explanation

To find the empirical formula, convert the percentages to moles: C (52.2g/12.01), H (13g/1.008), O (34.8g/16). This gives approximately 4.35 C, 12.87 H, and 2.18 O. Dividing by the smallest (2.18) yields C2H4O, the correct empirical formula.

The empirical formula is defined as:

• Actual number of atoms in a molecule.

• Simplest ratio of atoms in a compound.

• The total mass of a compound.

• The volume occupied by a substance.

Answer explanation

The empirical formula represents the simplest ratio of atoms in a compound, not the actual number of atoms or the total mass. Therefore, the correct choice is 'Simplest ratio of atoms in a compound.'

Which calculation will give you the number of moles?

• Mass x Molar mass

• Volume of gas / Molar Volume

• Number of Mole / Volume of Solution

• Number of particles x Avogadro's number

Answer explanation

If 48 g of oxygen gas (O₂, Ar of O= 16] is present, the amount in moles will be:

• 1 mol

• 1.5 mol

• 2 mol

• 3 mol

Answer explanation

To find moles of O2, use the formula: moles = mass (g) / molar mass (g/mol). The molar mass of O2 is 32 g/mol. Thus, 48 g O2 = 48 g / 32 g/mol = 1.5 mol.

Ba + 2 H₂O --> Ba(OH)₂ + H₂

What is the volume of hydrogen gas formed at STP in dm³ when 0.25 mol of barium reacts with water?

• 0.25

• 0.5

• 5.68

• 11.4

Answer explanation

Ratio of Ba to H₂ is 1:1 hence 0.25 mol of gas formed.

Convert to vol by multiplying with Molar Volume at STP which is 22.7 dm³, hence 5.68 dm³.