Chemical Formulae Writing and Meaning - UNSOLVED PRACTICE SET

Q7. What information does a chemical formula convey? Write the chemical formula of water and explain what it tells us.

Q8. Write the chemical formulae of the following compounds using the criss-cross method: (i) Magnesium chloride, (ii) Aluminium sulphate, (iii) Calcium nitrate, (iv) Ammonium phosphate.

Q9. Your younger sibling asks you what the formula CO₂ means. Explain it in simple language that a 10-year-old can understand.

Q10. What is meant by the valency of an element? Write the valencies of sodium, magnesium, aluminium, and chlorine. How does valency help in writing chemical formulae?

Q11. Write the formulae of the following compounds and identify the elements present in each: (i) Sulphuric acid, (ii) Nitric acid, (iii) Hydrochloric acid, (iv) Phosphoric acid.

Q12. Explain why the formula of sodium chloride is NaCl and not Na₂Cl or NaCl₂. Use the concept of valency in your answer.

Q13. Explain the rules for writing chemical formulae of compounds. Describe the criss-cross method with a detailed example. Write the chemical formulae of: (i) potassium sulphate, (ii) calcium carbonate, (iii) ammonium nitrate, and (iv) magnesium hydroxide, showing all steps clearly.

Q14. A student is given the following ions and asked to write the formulae of compounds formed by their combinations:

Table

Cation Valency Anion Valency

Na⁺ 1 Cl⁻ 1

Ca²⁺ 2 O²⁻ 2

Al³⁺ 3 SO₄²⁻ 2

NH₄⁺ 1 PO₄³⁻ 3

(a) Write the formula of the compound formed by each cation-anion pair.

(b) Name each compound.

(c) Explain what the brackets in Al₂(SO₄)₃ and (NH₄)₃PO₄ signify.

(d) Why can't Na⁺ and NH₄⁺ form a compound with each other?

Q15. "Chemical formulae are like recipes in a cookbook — they tell you exactly what ingredients are needed and in what proportions." Discuss this analogy by explaining: (i) how a formula tells us the types of atoms, (ii) how it tells us the number of each type of atom, (iii) how it helps predict the properties of a compound, and (iv) how it is used in chemical industries to manufacture products in correct proportions.

Q16. A chemistry teacher gives students a set of elements and asks them to predict the formulae of compounds:

Element A: Valency = 1

Element B: Valency = 2

Element C: Valency = 3

Element D: Valency = 4

(a) Write the formulae of all possible binary compounds (compounds with two elements) formed by combining these elements.

(b) If Element A is hydrogen, Element B is oxygen, Element C is aluminium, and Element D is carbon, write the actual formulae and names of these compounds.

(c) Calculate the molecular mass of each compound formed.

(d) Which of these compounds are commonly found in your home? Name two uses of each.

Q17. A pharmaceutical company manufactures calcium supplements. The active ingredient is calcium carbonate (CaCO₃).

(a) Write the chemical formula and identify the elements present.

(b) Calculate the molecular mass of calcium carbonate.

(c) If each tablet contains 500 mg of calcium carbonate, how many tablets are needed to provide 2 g of the compound?

(d) The company also produces calcium phosphate (Ca₃(PO₄)₂) as another supplement. Write its formula and calculate its molecular mass.

(e) Which compound provides more calcium per gram? Show your calculation.

Q18. In a school science fair, a student creates a display showing how chemical formulae are used in everyday products:

Table

Product Main Chemical Compound

Baking powder NaHCO₃

Vinegar CH₃COOH

Table salt NaCl

Bleach NaOCl

Fertilizer NH₄NO₃

(a) Write the name of each compound.

(b) For each compound, list the elements present and the number of atoms of each element in one molecule.

(c) Calculate the molecular mass of baking powder (NaHCO₃) and table salt (NaCl).

(d) If a recipe requires 10 g of baking powder, how many molecules of NaHCO₃ are present?

(e) Why is it important for consumers to understand chemical formulae, even if they are not chemists?