ScienceCBSE Class 9 · Atomic Foundations of Matter

Chemical Formulae, Molecular Mass and Formula Unit Mass (Class 9)

Q: Do I write the signs of the charges in the final formula?

No. Signs and charges are used only to work out the subscripts. The final formula of a neutral compound has none.

Q: When do I use brackets in a formula?

Only when a polyatomic ion appears more than once, for example Mg(OH)2 and Al2(SO4)3. For a single ion such as Na2SO4 or CaCO3 no brackets are needed.

Q: Is the mass of a formula in u or in grams?

The formula or molecular mass is in u. A sample of the compound is weighed in grams, and the same ratio holds.

Q: Why does CaCl2 have two chlorine atoms?

One Ca2+ has two positive charges, and each Cl- carries one negative charge, so two Cl- ions are needed to balance.

Q: How is a ratio by mass different from a ratio of atoms?

A ratio by mass multiplies the number of atoms by the atomic mass. In CO2, C : O is 1 : 2 by atoms but 12 : 32 = 3 : 8 by mass.

Updated 17 October 2026 · Fact-checked

To write a chemical formula, write the cation first and the anion second, put the valency or charge number of each below its symbol, cross the numbers over as subscripts, reduce to the simplest ratio, and bracket a polyatomic ion that appears more than once. Molecular mass (for covalent compounds) and formula unit mass (for ionic compounds) are found by adding the atomic masses of all atoms in one molecule or one formula unit, in unified atomic mass units (u).

Swap the charge numbers (ignoring signs) to make subscripts, bracket the polyatomic ion, then add atomic masses: 2×27 + 3×32 + 12×16 = 342 u.

Illustration drawn for QuestionBuddy.

Understand Chemical Formulae and Molecular Mass

Why we need formulae. A formula tells you which elements are present and in what whole-number ratio of atoms. For a covalent compound it describes one molecule (H2O: 2 hydrogen atoms bonded to 1 oxygen atom). For an ionic compound it describes the simplest ratio of ions in the crystal (NaCl: one Na+ for one Cl-), because an ionic compound has no separate molecules.

Valency and charge. The valency of an atom is the number of electrons it gains, loses or shares to get a full outer shell. Metals with 1, 2 or 3 valence electrons lose them and become cations (Na+, Mg2+, Al3+). Non-metals with 5, 6 or 7 valence electrons gain electrons and become anions (N3-, O2-, Cl-). Common polyatomic ions are hydroxide OH-, nitrate NO3-, hydrogencarbonate HCO3-, carbonate CO3 2-, sulphate SO4 2- and ammonium NH4+. Iron and copper have two common valencies: ferrous Fe2+, ferric Fe3+, cuprous Cu+, cupric Cu2+.

Criss-cross method for ionic compounds. (1) Write the cation then the anion. (2) Write the charge numbers (ignore the signs). (3) Swap them so that the number of the cation becomes the subscript of the anion and the other way round. (4) If both subscripts have a common factor, divide by it. (5) Do not write a subscript of 1. (6) If a polyatomic ion needs a subscript greater than 1, put the ion in brackets first. Examples: Ca2+ with Cl- gives CaCl2; Al3+ with O2- gives Al2O3; Mg2+ with O2- gives Mg2O2, which reduces to MgO; Al3+ with SO4 2- gives Al2(SO4)3; NH4+ with SO4 2- gives (NH4)2SO4. The check is that total positive charge equals total negative charge: Al2(SO4)3 has 2 × 3 = 6 positive and 3 × 2 = 6 negative.

Covalent compounds. Use the valencies of the two non-metals in the same way: H (1) with S (2) gives H2S; C (4) with Cl (1) gives CCl4; N (3) with H (1) gives NH3. For names, use prefixes mono, di, tri, tetra, penta, hexa for the number of atoms, end the second element with -ide, and skip mono for the first element (carbon monoxide, dinitrogen pentoxide, phosphorus trichloride).

Molecular mass and formula unit mass. Use the atomic masses in u: H 1, C 12, N 14, O 16, Na 23, Mg 24, Al 27, P 31, S 32, Cl 35.5, K 39, Ca 40. List every atom with its count (multiply out brackets), multiply by atomic mass and add. H2SO4 = 2(1) + 32 + 4(16) = 98 u. Ca(OH)2 = 40 + 2(16 + 1) = 74 u, a formula unit mass. The same sums give the mass ratio of the elements in a compound, which is how a fixed ratio by mass (law of constant proportions) is checked.

Mass of an element in a given mass of compound. Mass of element = (mass of the element in one formula ÷ mass of the whole formula) × given mass of compound. For CaCO3 (100 u), Ca is 40 u, so 250 g of CaCO3 contains 250 × 40/100 = 100 g of calcium.

Chemical Formulae and Molecular Mass: formulas and key facts

Criss-cross rule: Cation charge number becomes the anion's subscript, and the anion's number becomes the cation's subscript; then reduce to the simplest ratio
Polyatomic ions need brackets: $A l 2 (S O 4) 3∣ (N H 4) 2 S O 4∣ C a (O H) 2∣ M g (N O 3) 2$
Molecular mass: Sum of (number of atoms × atomic mass) in one molecule; unit u
Formula unit mass: Same sum for the simplest ratio of ions in an ionic compound; unit u
Mass of element in a sample: (element's mass in one formula ÷ mass of formula) × mass of sample

How to solve Chemical Formulae and Molecular Mass questions

Use this method for formula-writing and mass-of-compound questions.

1Identify the cation and the anion (or the two non-metals) and write their symbols, cation first.
2Write the charge number or valency below each symbol, ignoring the signs.
3Cross the numbers over to become subscripts; divide by a common factor if there is one.
4Put brackets round a polyatomic ion that needs a subscript, and drop any subscript of 1.
5For a mass question, count each kind of atom (open the brackets), multiply by the atomic mass and add.
6Check the charges balance, and give the unit u for molecular mass and formula unit mass, g for masses of samples.

Chemical Formulae and Molecular Mass in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Formula unit mass and fertiliser content

Suppose a farmer buys a 50 kg bag of a plant food that is entirely ammonium sulphate, (NH4)2SO4, and wants to know how much nitrogen the bag holds. The label of this imagined bag gives no analysis.

Find the formula unit mass of (NH4)2SO4 and the mass of nitrogen in the bag (N = 14, H = 1, S = 32, O = 16).

Try it first, then show the solutionSolution

(NH4)2SO4 has 2 N, 8 H, 1 S, 4 O.
Mass = 2(14) + 8(1) + 32 + 4(16) = 28 + 8 + 32 + 64 = 132 u.
Nitrogen in one formula unit = 28 u, so nitrogen in 50 kg = 50 × 28/132 = 10.6 kg (to one decimal place).

Answer: 132 u; about 10.6 kg of nitrogen.

Memory hook: Add the whole formula, then take the share of the part you want.

Reading a formula as a recipe

Suppose a cleaning solution in a school lab store is labelled only with the symbol Al2(SO4)3 and a student has to say how many atoms of each element are in one formula unit.

State the number of Al, S and O atoms in one formula unit and the total number of atoms.

Try it first, then show the solutionSolution

Al: subscript 2, so 2 atoms.
The bracket (SO4) has 1 S and 4 O, and the subscript 3 multiplies both: S = 1 × 3 = 3, O = 4 × 3 = 12.
Total = 2 + 3 + 12 = 17 atoms.

Answer: 2 Al, 3 S and 12 O atoms; 17 atoms in all.

Memory hook: The number outside the bracket multiplies everything inside it.

Tricks to remember Chemical Formulae and Molecular Mass

Short memory aids. Say each one aloud once, then use it on a question the same day.

Swap the numbers, lose the signs
Write the two charge numbers without plus or minus, swap them and drop them down as subscripts. Then try to simplify.
Use it when: Writing any ionic formula quickly.
Bracket the group
A polyatomic ion is a team that always travels together. If you need two teams, put the team in a bracket and write 2 outside: Ca(OH)2, not CaOH2.
Use it when: Avoiding the most common formula error.
Count inside brackets by multiplying
Atoms in a bracketed group = atoms inside × the number outside. In Ca(NO3)2: N = 1 × 2 = 2, O = 3 × 2 = 6.
Use it when: Mass and atom-counting questions.
Name by counting
mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6. 'mono' is dropped for the first element only.
Use it when: Naming covalent compounds like CO, CO2, N2O5.

Chemical Formulae and Molecular Mass: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Writing formulae by the criss-cross method · 3 marks · Short answerBoard level

Write the formula of (a) aluminium oxide, (b) calcium hydroxide, (c) ammonium sulphate. Show the criss-cross working.

Try it first, then show the solutionSolution

(a) Al3+ and O2-: cross over to Al2O3. The charges balance (2 × 3 = 6, 3 × 2 = 6). Formula Al2O3. (1)
(b) Ca2+ and OH-: Ca has charge number 2 and OH has 1, so OH takes subscript 2. Bracket the OH group: Ca(OH)2. (1)
(c) NH4+ and SO4 2-: cross over to (NH4)2(SO4)1. The subscript 1 is not written and SO4 does not need brackets: (NH4)2SO4. (1)

Answer: (a) Al2O3; (b) Ca(OH)2; (c) (NH4)2SO4.

Example 2: Molecular mass and formula unit mass · 3 marks · Short answerBoard level

Calculate the mass in u of (a) H2SO4, (b) Ca(OH)2, (c) Al2(SO4)3. (H = 1, O = 16, Al = 27, S = 32, Ca = 40.) State which are molecular masses and which are formula unit masses.

Try it first, then show the solutionSolution

(a) H2SO4 = 2(1) + 32 + 4(16) = 2 + 32 + 64 = 98 u. (1)
(b) Ca(OH)2 = 40 + 2(16 + 1) = 40 + 34 = 74 u. (1)
(c) Al2(SO4)3 = 2(27) + 3[32 + 4(16)] = 54 + 3(96) = 54 + 288 = 342 u. (1)
H2SO4 is covalent, so 98 u is a molecular mass; Ca(OH)2 and Al2(SO4)3 are ionic, so 74 u and 342 u are formula unit masses.

Answer: (a) 98 u (molecular mass); (b) 74 u and (c) 342 u (formula unit masses).

Example 3: Mass of each element in a given mass of calcium carbonate · 5 marks · Long answerHarder board level

Calcium carbonate is CaCO3 (Ca = 40, C = 12, O = 16).
(a) Find its formula unit mass. (1)
(b) Find the ratio of Ca : C : O by mass in simplest whole numbers. (1)
(c) Find the mass of each element in 250 g of calcium carbonate. (2)
(d) What mass of calcium carbonate contains 20 g of calcium? (1)

Try it first, then show the solutionSolution

(a) 40 + 12 + 3(16) = 40 + 12 + 48 = 100 u. (1)
(b) Ca : C : O = 40 : 12 : 48 = 10 : 3 : 12 (divide by 4). (1)
(c) 250 ÷ 100 = 2.5. Ca = 2.5 × 40 = 100 g; C = 2.5 × 12 = 30 g; O = 2.5 × 48 = 120 g. Check: 100 + 30 + 120 = 250 g. (2)
(d) Calcium is 40 parts in 100, so mass of CaCO3 = 20 × 100/40 = 50 g. Check: 50 × 40/100 = 20 g. (1)

Answer: (a) 100 u; (b) 10 : 3 : 12; (c) Ca 100 g, C 30 g, O 120 g; (d) 50 g.

Example 4: Deduce a formula from electronic configurations · 5 marks · Long answerHarder board level

Atoms of three elements X, Y and W have electronic configurations X: 2, 8, 2; Y: 2, 8, 7; W: 2, 8, 3. Their atomic masses are X = 24 u, Y = 35.5 u, W = 27 u. Oxygen is 2, 6 with atomic mass 16 u.
(a) Give the ion formed by each of X, Y and W. (1)
(b) Write the formula of the compound of X with Y. (1)
(c) Find its formula unit mass. (1)
(d) Write the formula of the compound of W with oxygen and find its formula unit mass. (1)
(e) Why does the compound in (b) conduct electricity when molten but not as a solid? (1)

Try it first, then show the solutionSolution

(a) X loses its 2 outer electrons: X2+. Y gains 1 electron to complete 8: Y-. W loses 3: W3+. (1)
(b) Criss-cross X2+ and Y-: X1Y2, so XY2. (1)
(c) 24 + 2(35.5) = 24 + 71 = 95 u. (1)
(d) W3+ and O2-: cross over to W2O3. Mass = 2(27) + 3(16) = 54 + 48 = 102 u. (1)
(e) When molten, the ions are free to move and carry charge. In the solid, the ions are fixed in the crystal lattice and cannot move. (1)

Answer: (a) X2+, Y-, W3+; (b) XY2; (c) 95 u; (d) W2O3, 102 u; (e) ions are free to move only when molten.

Example 5: Identify two unlabelled substances from their formulae and behaviour · 4 marks · Case-basedHOTS / competency

A lab technician has two substances labelled only by formula: P is Ca(NO3)2, a white solid, and Q is CCl4, a colourless liquid. Atomic masses: C = 12, N = 14, O = 16, Cl = 35.5, Ca = 40.
(a) Classify each as ionic or covalent and give one reason from its formula. (1)
(b) Find the formula unit mass of P and the molecular mass of Q. (1)
(c) A student dissolves each in water and tests the conductivity. Predict the result for each with a reason. (1)
(d) Name Q, and name P using the rule for naming ionic compounds. (1)

Try it first, then show the solutionSolution

(a) P is ionic: it contains a metal (Ca) and a polyatomic ion (nitrate). Q is covalent: it is made of two non-metals, C and Cl, joined by shared pairs. (1)
(b) P = 40 + 2[14 + 3(16)] = 40 + 2(62) = 164 u. Q = 12 + 4(35.5) = 12 + 142 = 154 u. (1)
(c) The solution of P conducts, because P gives free Ca2+ and NO3- ions in water. Q is a covalent liquid made of molecules, is generally insoluble in water and gives no free ions, so it does not conduct. (1)
(d) Q is carbon tetrachloride (prefix tetra for four Cl; second element ends in -ide). P is calcium nitrate (cation first, then the polyatomic anion). (1)

Answer: (a) P ionic, Q covalent; (b) 164 u and 154 u; (c) P conducts, Q does not; (d) calcium nitrate and carbon tetrachloride.

Where students lose marks in Chemical Formulae and Molecular Mass

The slips examiners see most often, and what to write instead.

Mistake: Leaving out brackets round a repeated polyatomic ion.
Students write the subscript directly next to the last atom of the ion.
Wrong working
Calcium hydroxide = CaOH2
Right working
Calcium hydroxide = Ca(OH)2
Do this: If the ion needs a number, put the whole ion in brackets first.
Usually costs: 1 mark per formula.
Mistake: Not simplifying after criss-crossing.
Students stop right after swapping the numbers.
Wrong working
Magnesium oxide = Mg2O2
Right working
Mg2O2 reduces to MgO
Do this: Look for a common factor every time.
Usually costs: 1 mark.
Mistake: Writing the charge in the formula or writing a subscript of 1.
Confusion between the ion symbol and the formula of the compound.
Wrong working
Na+Cl-, or Ca1Cl2
Right working
NaCl, CaCl2
Do this: A compound is neutral, so signs and the subscript 1 are not written.
Usually costs: ½ to 1 mark.
Mistake: Multiplying only the atom next to the bracket when finding mass.
The subscript outside a bracket applies to every atom inside.
Wrong working
Ca(OH)2 = 40 + 16 + 1×2 = 58 u
Right working
Ca(OH)2 = 40 + 2(16 + 1) = 74 u
Do this: Open the bracket first and count each atom.
Usually costs: 1 to 2 marks.
Mistake: Calling the mass of an ionic compound a molecular mass.
Ionic compounds have no molecules.
Wrong working
The molecular mass of NaCl is 58.5 u.
Right working
The formula unit mass of NaCl is 58.5 u.
Do this: Ionic: formula unit mass. Covalent: molecular mass.
Usually costs: ½ mark.

Exam tips

In a formula question, show the charge numbers and the crossing over; the method carries marks even if the final bracket slips.
In a mass question, write the sum in full: each atom, its count, its atomic mass, and then the unit u.
Check charge balance for ionic formulae: total positive = total negative.
For 'mass of an element in a sample', find the fraction first (element's mass ÷ whole formula) and then multiply.
Say 'formula unit mass' for ionic compounds and 'molecular mass' for covalent ones.

Practice questions on Chemical Formulae and Molecular Mass

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
Write the formulae of (a) sodium carbonate and (b) aluminium hydroxide, showing the crossing over of the charge numbers.
Hint
Carbonate is CO3 with charge 2-; hydroxide is OH-.
Answer
(a) Na+ and CO3 2-: cross over to Na2CO3 (1 mark). (b) Al3+ and OH-: Al(OH)3, with brackets because OH is repeated (1 mark).
Q2 · 3 marks · Short answerHarder board level
Find the formula unit mass of sodium carbonate, Na2CO3 (Na = 23, C = 12, O = 16). How much sodium is present in 53 g of it?
Hint
Sodium contributes 2 × 23 u to the total.
Answer
Mass = 2(23) + 12 + 3(16) = 46 + 12 + 48 = 106 u (1 mark). Sodium in 53 g = 53 × 46/106 = 23 g (2 marks). Check: 53 g is half a formula unit's worth of 106, so half of 46 = 23 g.
Q3 · 2 marks · Assertion-reasonBoard level
Assertion (A): Calcium oxide has the formula CaO and not Ca2O2. Reason (R): Criss-crossing the charges of Ca2+ and O2- gives Ca2O2, which is divided by the common factor 2 to give the simplest ratio. Choose: (a) A and R both true, R explains A; (b) A and R both true, R does not explain A; (c) A true, R false; (d) A false, R true.
Hint
What do you do when both subscripts have a common factor?
Answer
(a). Both statements are true: criss-crossing gives Ca2O2, and the formula of an ionic compound is written as the simplest ratio of ions, which is CaO.
Q4 · 4 marks · Short answerHarder board level
Ammonia is a compound of nitrogen (valency 3) and hydrogen (valency 1). (a) Write its formula by the criss-cross method. (b) Find its molecular mass (N = 14, H = 1). (c) Find the masses of nitrogen and hydrogen in 34 g of ammonia.
Hint
Use the mass of each element per 17 u.
Answer
(a) N3 H1 crossed over gives NH3 (1 mark). (b) 14 + 3(1) = 17 u (1 mark). (c) 34 ÷ 17 = 2 formula masses, so nitrogen = 2 × 14 = 28 g and hydrogen = 2 × 3 = 6 g; check 28 + 6 = 34 g (2 marks).
Q5 · 4 marks · Case-basedHOTS / competency
A student wrote these names and formulae: (i) carbon monoxide = CO2; (ii) dinitrogen pentoxide = N2O5; (iii) phosphorus trichloride = P3Cl. Identify and correct the errors, find the molecular mass of each correct compound (C = 12, N = 14, O = 16, P = 31, Cl = 35.5), and arrange the three in order of increasing molecular mass.
Hint
The prefix tells the number of atoms of the element that follows it.
Answer
(i) Wrong: carbon monoxide is CO (one O); CO = 12 + 16 = 28 u. (ii) Correct: N2O5 = 2(14) + 5(16) = 28 + 80 = 108 u. (iii) Wrong: trichloride means three Cl, so PCl3 = 31 + 3(35.5) = 31 + 106.5 = 137.5 u (3 marks for identifying and calculating). Order: CO (28 u) < N2O5 (108 u) < PCl3 (137.5 u) (1 mark).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the formula of aluminium sulphate?Show answer
Al2(SO4)3.
2.What is the unit of molecular mass?Show answer
Unified atomic mass unit, u.
3.Why is NaCl given a formula unit mass and not a molecular mass?Show answer
It does not exist as separate molecules; it is a crystal of Na+ and Cl- ions.
4.What is the molecular mass of H2O?Show answer
18 u.
5.Write the formula of ammonium sulphate.Show answer
(NH4)2SO4.
6.What is the name of N2O5?Show answer
Dinitrogen pentoxide.

Chemical Formulae and Molecular Mass: frequently asked questions

Do I write the signs of the charges in the final formula?

No. Signs and charges are used only to work out the subscripts. The final formula of a neutral compound has none.

When do I use brackets in a formula?

Only when a polyatomic ion appears more than once, for example Mg(OH)2 and Al2(SO4)3. For a single ion such as Na2SO4 or CaCO3 no brackets are needed.

Is the mass of a formula in u or in grams?

The formula or molecular mass is in u. A sample of the compound is weighed in grams, and the same ratio holds.

Why does CaCl2 have two chlorine atoms?

One Ca2+ has two positive charges, and each Cl- carries one negative charge, so two Cl- ions are needed to balance.

How is a ratio by mass different from a ratio of atoms?

A ratio by mass multiplies the number of atoms by the atomic mass. In CO2, C : O is 1 : 2 by atoms but 12 : 32 = 3 : 8 by mass.