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Class nine science Extra ques Atoms and Molecules

Chapter 3: Atoms and Molecules

VERY SHORT ANSWER TYPE QUESTIONS

Q. 1. State the law of conservation of mass.

Ans. Mass can neither be created nor be destroyed in a chemical reaction.

Q. 2. What is the law of constant proportions?

Ans. It states that a pure chemical compound is always found to be made-up of same elements Combined together in the same fixed ratio of mass.

Q. 3. Carbon dioxide collected from different sources contains carbon and oxygen in the same proportion. Which law of chemical combination governs this?

Ans. This is governed by the law of definite proportion.

Q. 4. Carbon dioxide produced by heating sodium hydrogen carbonate is dry whereas that Produced by the action of dilute hydrochloric acid on sodium hydrogen carbonate is moist. What do you think about the difference in the composition of carbon dioxide in the two cases?

Ans. There will be no difference in the composition of CO2 in two cases, i.e., in both cases, C and O will be present in the same ratio by mass.

Q. 5. A student performed an activity to verify the law of conservation of mass using solution X and Y. Identify the solution ‘Y’ which can’t react with ‘X’ namely sodium chloride.

Ans. Lead nitrate or silver nitrate.

Q. 6. What is an atom?

 Ans. An atom is defined as the smallest particle of an element which may or may not be capable of free existence. However, it is the smallest particle that takes part in a chemical reaction.

Q. 7. Why is it not possible to see an atom with naked eyes?

Ans. An atom is an extremely small particle whose radius is of the order to 10–10m. This size is so small that our eyes are not able to see it.

Q. 8. Name the smallest particle that has all the properties of an element.

Ans. Atom.

Q. 9. State the postulate of Dalton’s atomic theory which can explain the law of conservation of mass.

 Ans. According to a postulate of Dalton’s theory, atoms are indivisible particles which cannot be created or destroyed in a chemical reaction. This postulate explains the law of conservation of mass.

Q. 10. How do atoms exist?

Ans. Atoms exist in the form of atom, molecule or ions.

Class nine science Extra ques Atoms and Molecules

VERY SHORT ANSWER TYPE QUESTIONS

 Q. 11. Why is the symbol of silver written as Ag?

Ans. The symbol of silver is derived from the Latin name, ‘Argentums’ from which the symbol Ag has been derived.

Q. 12. What is meant by molar mass?

Ans. The mass of 1 mole of the substance (i.e., Avogadro’s number of particles) is called the molar mass of the substance.

Q. 13. Define the atomic mass unit.

Ans. One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12) the mass of one atom of carbon – 12.

Q. 14. Define mole.

Ans. A mole of particles (atoms, ions or molecules) is defined as that amount of the substance which contains the same number of particles as there are C–12 atoms in 12g i.e., 0.012 kg of C–12 isotope.

Q. 15. What is the mass of 1 mole of nitrogen atoms?

Ans. 1 mole of nitrogen atom = 1 × gram atomic mass of nitrogen atom.

= 1 × 14 g = 14 g.

Q. 16. By what name the quantity of any material that contains

6.0 × 1023 of chemical units, is called?

Ans. The quantity of material that contains 6.02 × 1023 chemical units is called a mole.

Q. 17. O2 gas has a molar mass 32 whereas SO2 gas has molar mass 64. What will be the relation between their molar volumes at STP?

Ans. Gram molecular volume is the volume occupied by one mole of the substance. In case of all gases, its value is 22.4 L at STP.

Q. 18. What is the value of Avogadro’s number?

Ans. 6.022 × 1023.

Q. 19. How many litres does 2 moles of H2 gas at STP contain?

Ans. 2 moles of H2 gas at STP contain 44.8L.

Q. 20. How many mole atoms of sulphur are present in two moles of sulphuric acid, H2SO4?

Ans. 2 moles.

Q. 21. How many molecules are present in 4g of H2O?

Ans.

½ × 6.023 × 1023 = 3.011 × 1023 molecules.

Q. 22. What is a molecule?

Ans. A molecule is the smallest particle of an element or a compound which can exist freely under ordinary conditions and shows all the properties of that substance.

Q. 23. What do you mean by atomicity of a molecule?

Ans. The number of atoms present in one molecule of substance is called its atomicity.

Q. 24. Give two examples of polyatomic molecules of elements.

Ans. (i) Phosphorus (P4), (ii) Sculpture (S8).

Q. 25. What is the difference between an atom and a molecule?

Ans. Atom is the smallest particle of an element which may or may not be capable of free existence whereas molecule is the smallest particle of an element or a compound which can exist freely.

Q. 26. Give an example of a triatomic molecule of an element.

Ans. Ozone (O3).

Q. 27. Which of the following represents a correct chemical formula? Name it.

(a) CaCl (b) BiPO4

(c) NaSO4 (d) NaS

Ans. (b) BiPO4 (Bisumth phosphate).

Here both the ions are trivalent (i.e., B + PO4)

Q. 28. What is meant by the term chemical formula?

Ans. The chemical formula of a compound is a symbolic representation of its composition and an actual number of atoms in one molecule of a pure substance, maybe an atom or a compound.

Q. 29. How do you define the valency of an element in a molecular compound?

Ans. In a molecular compound, the valency of an element is defined as its combining capacity and is equal to the number of hydrogen atoms or number of chlorine atoms or double the number of oxygen atoms with which one atom of the element combines.

Q. 30. How do you define the valency of an ion in an ionic compound?

Ans. The valency of an ion is defined as the units of positive or negative charge present on the ion.

Q. 31. Write the formula of the compound formed by the ions Al3+ and SO4-2

Ans. Al2 (SO4)3

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