Calculations Involving Chemical Formulae – Review Flashcards

These flashcards cover constructing chemical formulae, common ions, counting atoms, relative formula mass calculations, mole–mass conversions, empirical and molecular formula determination, percentage composition, theoretical and percentage yield, and related key definitions and examples.

What fundamental rule is used to construct an ionic formula?

The total positive charge must equal the total negative charge so the compound is electrically neutral.

Which diatomic molecule has the formula Cl₂?

Chlorine gas.

Give the chemical formula for hydrogen gas.

H₂

Give the chemical formula for nitric acid.

HNO₃

What is the chemical formula of sulphuric acid?

H₂SO₄

State the charge on a chloride ion.

1−

State the charge on a calcium ion.

2+

State the charge on a sulphate ion (SO₄).

2−

State the charge on an ammonium ion (NH₄).

1+

Why are brackets used in the formula Cu(NO₃)₂?

Because the polyatomic ion NO₃⁻ appears more than once; brackets show that the subscript 2 applies to the whole ion, not just to oxygen.

Write the correct formula for sodium chloride.

NaCl

Write the correct formula for magnesium sulphate.

MgSO₄

Write the correct formula for potassium oxide.

K₂O

Write the correct formula for calcium chloride.

CaCl₂

Write the correct formula for copper(II) nitrate.

Cu(NO₃)₂

How many oxygen atoms are present in Ca(OH)₂?

2

If the coefficient 3 precedes Ca(OH)₂, how many hydrogen atoms are present in 3 Ca(OH)₂?

6 hydrogen atoms.

Define relative formula mass (Mr).

The sum of the relative atomic masses of all atoms in a chemical formula.

Calculate the relative formula mass of H₂O.

(2 × 1) + 16 = 18

Calculate the relative formula mass of CuSO₄.

64 + 32 + (4 × 16) = 160

Calculate the relative formula mass of Ca(OH)₂.

40 + 2(16+1) = 74

State the formula that links mass, moles and molar mass.

Mass = number of moles × molar mass (m = n × M).

What mass of Mn is present in 3.00 moles? (Molar mass Mn = 54.9 g mol⁻¹)

3.00 mol × 54.9 g mol⁻¹ = 165 g

Calculate the mass of 0.25 mol of KOH. (M = 56 g mol⁻¹)

0.25 mol × 56 g mol⁻¹ = 14 g

State Avogadro’s number.

6.022 × 10²³ particles per mole.

Define molecular (molar) mass.

The mass in grams of one mole of a substance.

What is the difference between an empirical and a molecular formula?

The empirical formula gives the simplest whole-number ratio of atoms, whereas the molecular formula gives the actual number of each atom in a single molecule.

Give the empirical formula that corresponds to molecular benzene (C₆H₆).

CH

Determine the empirical formula of a compound containing 43.6 % P and 56.4 % O by mass.

P₂O₅

If a compound has empirical formula P₂O₅ and molar mass 284 g mol⁻¹, what is its molecular formula?

P₄O₁₀

Define percentage composition by mass.

The percentage by mass of each element present in a compound.

What is the percentage by mass of iron in Fe₂O₃?

(112 / 160) × 100 = 70.0 % Fe

Write the formula used to calculate percentage yield.

Percentage yield = (actual yield ÷ theoretical yield) × 100

In the reaction CuO + H₂SO₄ → CuSO₄ + H₂O, what mass of CuSO₄ can theoretically be obtained from 80 g of CuO?

160 g

From 20 g of CuO, what is the maximum mass of CuSO₄ that can be produced?

40 g

If 400 g of CuO are used, what is the theoretical yield of CuSO₄?

800 g

80 g of CuO should yield 160 g CuSO₄, but only 120 g are obtained. Calculate the percentage yield.

(120 g ÷ 160 g) × 100 = 75 %

Explain why actual yield is often less than theoretical yield.

Losses during separation, incomplete reactions, side reactions and measurement errors reduce the amount of product obtained.

Why is percentage composition useful when analysing metal ores?

It indicates the mass fraction of the desired metal, helping to assess ore quality and economic viability.

How many sodium atoms are present in the formula unit Na₂HPO₄?

2 sodium atoms.

How many oxygen atoms are present in 2 Na₂HPO₄?

2 × 4 = 8 oxygen atoms.

What does a subscript ‘3’ in NO₃⁻ signify?

There are three oxygen atoms bonded to one nitrogen atom in the ion.

When balancing ionic charges, how many nitrate ions are needed to balance one Al³⁺ ion?

Three NO₃⁻ ions, giving the formula Al(NO₃)₃.

State the molar mass of water to four significant figures.

18.02 g mol⁻¹

What term describes the mass of one mole of a compound?

Molar mass (also called molecular weight).