CBSE Class 9 Chemistry Atoms and Molecules Worksheets Set 02

Short and Long Answer Questions on Atoms and Molecules

Question 1. Write the names of the symbols of five elements where the symbols are taken from their names in English.
Answer:
Hydrogen (H), Carbon (C), Oxygen (O), Nitrogen (N) and Calcium (Ca).
In simple words: These symbols come from the first letter or first two letters of the English name.

Teacher's Note:
1. Use the first letter, capital, or the first two letters if needed.
2. The first letter is always capital and the second small.

Question 2. Write the names of the symbols of five elements where the symbols are taken from a language other than English.
Answer:
Sodium (Na, from Latin natrium)
Potassium (K, from Latin kalium)
Iron (Fe, from Latin ferrum)
Copper (Cu, from Latin cuprum)
Silver (Ag, from Latin argentum)
In simple words: Some symbols come from Latin names of old elements.

Teacher's Note:
1. Learn the Latin names of Na, K, Fe, Cu, Ag, Au, Pb and Hg.

Question 3. Name the smallest particle that has all the properties of an element.
Answer:
The atom is the smallest particle of an element that takes part in chemical reactions. (A molecule is the smallest particle of an element or compound that can exist independently.)
In simple words: An atom is the smallest unit of an element.

Teacher's Note:
1. Atoms are the building blocks of matter.

Question 4. How many ions are present in one mole of ions?
Answer:
One mole of ions contains 6.022 × 10²³ ions (Avogadro's number).
In simple words: A mole of anything contains Avogadro's number of particles.

Teacher's Note:
1. Mole is a counting unit like a dozen.

Question 5. Which postulate of Dalton's atomic theory can explain the law of conservation of mass?
Answer:
Atoms can neither be created nor destroyed in a chemical reaction. They only rearrange, so the total mass stays the same.
In simple words: Atoms are only rearranged, so the mass cannot change.

Teacher's Note:
1. Match each law with the postulate that explains it.

Question 6. Which postulate of Dalton's atomic theory can explain the law of definite proportions?
Answer:
The relative numbers and kinds of atoms in a given compound are constant. Since each atom has a fixed mass, the elements are always present in the same ratio by mass.
In simple words: Same atoms in the same numbers give a fixed ratio by mass.

Teacher's Note:
1. Definite proportions follow from the fixed number of atoms in a compound.

Question 7. Write down the formulae of (i) sodium oxide, (ii) aluminium chloride, (iii) sodium sulphide, (iv) magnesium hydroxide.
Answer:
(i) Na₂O
(ii) AlCl₃
(iii) Na₂S
(iv) Mg(OH)₂
In simple words: Cross the valencies to write the formula.

Teacher's Note:
1. Valencies: Na 1, Al 3, Mg 2, O 2, Cl 1, S 2, OH 1.
2. Write the symbol of the metal first.

Question 8. Write down the names of the compounds represented by the following formulae: (i) Al₂(SO₄)₃, (ii) CaCl₂, (iii) K₂SO₄, (iv) KNO₃, (v) CaCO₃.
Answer:
(i) Aluminium sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium nitrate
(v) Calcium carbonate
In simple words: Name the metal first and then the radical.

Teacher's Note:
1. Learn the names of common radicals: SO4 sulphate, NO3 nitrate, CO3 carbonate.

Question 9. How many atoms are present in (i) an H₂S molecule and (ii) a PO₄³⁻ ion?
Answer:
(i) 3 atoms (2 hydrogen and 1 sulphur).
(ii) 5 atoms (1 phosphorus and 4 oxygen).
In simple words: Add the subscripts.

Teacher's Note:
1. Atoms in an ion are counted the same way as in a molecule.

Question 10. Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃ and CH₃OH.
Answer:
H₂ = 2 u; O₂ = 32 u; Cl₂ = 71 u
CO₂ = 12 + 32 = 44 u; CH₄ = 12 + 4 = 16 u
C₂H₆ = 24 + 6 = 30 u; C₂H₄ = 24 + 4 = 28 u
NH₃ = 14 + 3 = 17 u; CH₃OH = 12 + 3 + 16 + 1 = 32 u
In simple words: Add the atomic masses of all atoms in the molecule.

Teacher's Note:
1. Atomic masses: H 1, C 12, N 14, O 16, Cl 35.5.
2. Multiply by the subscript before adding.

Question 11. Which separation techniques will apply for the separation of the following? (a) Sodium chloride from its solution in water. (b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride. (c) Small pieces of metal in the engine oil of a car. (d) Different pigments from an extract of flower petals. (e) Butter from curd. (f) Oil from water. (g) Tea leaves from tea. (h) Iron pins from sand. (i) Wheat grains from husk. (j) Fine mud particles suspended in water.
Answer:
(a) Evaporation
(b) Sublimation
(c) Filtration
(d) Chromatography
(e) Centrifugation
(f) Separating funnel
(g) Filtration
(h) Magnetic separation
(i) Winnowing
(j) Centrifugation (or sedimentation and decantation, then filtration)
In simple words: Choose the method from the difference in a property of the two parts.

Teacher's Note:
1. Use the physical property that differs: volatility, solubility, magnetism or density.

Question 12. What is meant by the term chemical formula?
Answer:
A chemical formula is the symbolic representation of a compound or molecule. It shows the elements present and the number of atoms of each element in one molecule or formula unit, for example H₂O.
In simple words: A formula is a short way of writing a substance.

Teacher's Note:
1. Formula gives both kinds and numbers of atoms.

Question 13. The formula of the carbonate of a metal M is M₂CO₃. Write the formula of its chloride.
Answer:
Carbonate carries a charge of 2−, so M has valency 1 (M⁺). The chloride is MCl.
In simple words: Valency of M is 1, so one chloride ion combines with it.

Teacher's Note:
1. Find the valency from the carbonate, then cross it with chloride.

Question 14. What are the postulates of Dalton's atomic theory of matter?
Answer:
1. All matter is made of very small indivisible particles called atoms.
2. Atoms of a given element are identical in mass and properties; atoms of different elements differ in mass and properties.
3. Atoms can neither be created nor destroyed in a chemical reaction.
4. Atoms combine in small whole-number ratios to form compounds.
5. The relative numbers and kinds of atoms in a given compound are constant.
In simple words: Atoms are indivisible, they are only rearranged, and they combine in fixed ratios.

Teacher's Note:
1. Know which postulate explains each law of chemical combination.
2. Isotopes show that postulate 2 is not fully correct.

Question 15. How many moles are present in 11.5 g of sodium?
Answer:
Moles = mass / atomic mass = 11.5/23 = 0.5 mol.
In simple words: Divide the mass by the atomic mass of sodium, 23.

Teacher's Note:
1. Moles = given mass / molar mass.

Question 16. The mass of an atom of element X is 2.0 × 10⁻²³ g. Calculate its atomic mass.
Answer:
Mass of 1 mole = 2.0 × 10⁻²³ × 6.022 × 10²³ = 12.04 g.
Atomic mass = 12 u (about). The element is carbon.
In simple words: Multiply the mass of one atom by Avogadro's number.

Teacher's Note:
1. Atomic mass in grams per mole = mass of one atom × N_A.

Question 17. How many particles are represented by 0.25 mole of an element?
Answer:
Number of particles = 0.25 × 6.022 × 10²³ = 1.5 × 10²³ atoms.
In simple words: One quarter of Avogadro's number.

Teacher's Note:
1. Number of particles = moles × 6.022 × 10²³.

Question 18. Out of 4 g of methane (CH₄) and 11 g of CO₂, which has more molecules?
Answer:
Moles of CH₄ = 4/16 = 0.25 mol.
Moles of CO₂ = 11/44 = 0.25 mol.
Both have the same number of molecules, 0.25 × 6.022 × 10²³ = 1.5 × 10²³ each.
In simple words: Equal moles mean equal numbers of molecules.

Teacher's Note:
1. Convert masses to moles before comparing.

Question 19. What is valency? Mention the use of valency.
Answer:
Valency is the combining capacity of an atom of an element, that is, the number of electrons it loses, gains or shares to complete its outer shell.
Use: it helps in writing the chemical formula of a compound by crossing the valencies of the combining atoms.
In simple words: Valency tells how many bonds an atom can form.

Teacher's Note:
1. Examples: Na 1, Mg 2, Al 3, O 2.

Question 20. Define the atomic mass unit.
Answer:
One atomic mass unit (u) is exactly one-twelfth (1/12) of the mass of one carbon-12 atom, which is about 1.66 × 10⁻²⁴ g.
In simple words: It is the standard unit for the mass of atoms.

Teacher's Note:
1. Atomic masses are relative to carbon-12.

Question 21. What are the laws of chemical combination?
Answer:
1. Law of conservation of mass: mass is neither created nor destroyed in a chemical reaction.
2. Law of constant (definite) proportions: a pure compound always contains the same elements in the same ratio by mass.
3. Law of multiple proportions: when two elements form more than one compound, the masses of one that combine with a fixed mass of the other are in a ratio of small whole numbers.
4. Gay-Lussac's law of gaseous volumes and Avogadro's law relate gas volumes in reactions.
In simple words: Mass is conserved, composition is fixed, and ratios are simple whole numbers.

Teacher's Note:
1. Give one example for each law in an exam.
2. Dalton's theory explains the first three.

Question 22. What is the full form of IUPAC? What is the present accepted norm of IUPAC for naming symbols of elements?
Answer:
IUPAC stands for the International Union of Pure and Applied Chemistry.
The first letter of the symbol is written in capital and the second letter (if any) in small, usually taken from the English name of the element; newly discovered elements get systematic names and symbols approved by IUPAC.
In simple words: IUPAC decides the official names and symbols.

Teacher's Note:
1. Capital first letter, small second letter.

Question 23. Define and explain the atomic mass of an element.
Answer:
The atomic mass of an element is the average mass of its atom compared with 1/12 of the mass of a carbon-12 atom. It is expressed in atomic mass units (u).
Example: the atomic mass of oxygen is 16 u, which means an oxygen atom is 16 times heavier than 1/12 of a carbon-12 atom. Elements with isotopes have a fractional atomic mass because it is an average.
In simple words: It tells how heavy an atom is compared with the carbon-12 standard.

Teacher's Note:
1. Atomic mass is a relative number without a true unit.

Question 24. Distinguish between a molecule and a compound.
Answer:
Molecule: the smallest particle of an element or compound that can exist independently and shows its properties, for example O₂, H₂O.
Compound: a pure substance made of two or more different elements chemically combined in a fixed ratio, for example H₂O, CO₂.
A molecule can be of an element (O₂) but a compound always has more than one kind of atom.
In simple words: Every compound is made of molecules, but not every molecule is a compound.

Teacher's Note:
1. Give one example of each.

Question 25. Define (a) atomicity, (b) ion, (c) molecular mass, (d) relative formula mass.
Answer:
(a) Atomicity: the number of atoms in one molecule of an element, for example 2 in O₂.
(b) Ion: a charged atom or group of atoms formed by gain or loss of electrons.
(c) Molecular mass: the sum of the atomic masses of all atoms in a molecule.
(d) Relative formula mass: the sum of the atomic masses of all atoms in one formula unit of an ionic compound, for example NaCl = 58.5 u.
In simple words: Each term is a short definition.

Teacher's Note:
1. Molecular mass is for molecules and formula mass is for ionic compounds.