Extra Questions For Class 9 Science Chapter 3 Atoms and Molecules

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Extra Questions For Class 9 Science Chapter 5 Fundamental Unit of Life

Very Short Answer Type Questions (One Mark Each)

Q.1. What is Avogadro’s constant ?

Answer Ans. Avogadro’s constant is the number of particles (atoms, ions or molecules) present in mole of the substance. Its value is 6.022 x 1023.

Q.2. The atomic number of three elements A, B and Care 9, 10 and 13 respectively. Which of them will form a cation ?

Answer Ans. The element with atomic number 13 will form a cation. It has the electronic configuration 2, 8, 3. It loses three electrons to attain stable octet and forms the cation

Q.3. Name the isotope of an element which is used in the treatment of cancer.

Answer Ans. An isotope of cobalt is used in the treatment of cancer.

Q.4. What is amu ? Which quantity is measured in terms of amu ?

Answer Ans. amu stands for Atomic Mass Unit.
Atomic mass is measured in terms of amu.

Q.5. What do you understand by an octet of electrons in the valence shell ?

Answer Ans. If there are eight electrons in the outermost shell of an atom, we say that it has an octet of electrons. All atoms have a tendency to acquire octet of electrons in the valence shell.

Q.6. Who gave the laws of chemical combination ?

Answer Ans. Lavoisier and Proust.

Q.7. Carbon dioxide collected from different sources contains carbon and oxygen in the same proportion. Which law of chemical combination governs this ? State the

Answer Ans. This is governed by law of definite proportion which states:
In a chemical substance, its constituents are always present in fixed ratio by mass.

Q.8. State the law of conservation of mass.

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

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

Answer 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. Two elements X and Y combine to form XY in the ratio 1:35.5 by mass. What mass of Y will combine with 2 grams of X ?

Answer Ans. 2 x 35.5 = 71 g.

Q.11. Copper is represented by the symbol Cu’ while there is no letter V’ in the name. Why?

Answer Ans. Latin name of copper is cuprum. The symbol of an element is taken as the first two letters of the Latin name commonly. So copper is represented as Cu.

Q.12. What do you understand from the statement “relative atomis mass of sulphur is 32” ?

Answer Ans. The statement means that an atom of sulphur is 32 times heavier than 1/12 of mass of one carbon-12 atom.

Q.13. How many moles are present in 4.6 g of Na ? (Atomic mass of Na = 23).

Answer Ans. Number of moles = Given mass/Molar mass = 4.6 g/23 g/mole = 0.2 mole

Q.14. Give two examples of polyatomic molecules of elements.

Answer Ans. (i) Phosphorus (Pd) (ii) Sulphur (Sg).

Q.15. Give two examples of polyatomic anions.

Answer Ans. Anions are negatively charged atoms or group of atoms. Examples : Polyatomic anions are carbonate and bicarbonate.

Q.16. Give an example of a polyatomic cation.

Answer Ans. Cations are positively charged radicals (atoms or group of atoms). Ammonium ion (NH) is an example of a polyatomic cation.

Q.17. Explain the form in which noble gases like helium, neon and argon occur in nature.

Answer Ans. Monoatomic.

Q.18. Name two trivalent elements having positive valency.

Answer Ans. Aluminium (Al3+) and iron in ferric form (Fe+)3.

Q.19. Name two trivalent anions.

Answer Ans. Phosphate (PO43-) and Nitride (N3-)

Q.20. What is the chemical formula for zinc phosphate ?

Answer Ans. Zn3(PO4)2

Q.21. Calcium salt of a hypothetical anion Z has the molecular formula Cazz, What is the valency of Z and what would be the molecular formula of aluminium salt of Z?

Answer Ans. Valency of Z is -3. So, formula of aluminium salt is AlZ.

Q.22. An element M has valency 2. Write the formulae of its hydroxide and nitride.

Answer Ans. M(OH)2, and M3N2 respectively

Q.23. The formula of magnesium sulphate is MgSO4. Write the formula of magnesium bicarbonate and magnesium nitride.

Answer Ans. Formula of magnesium bicarbonate : Mg(HCO3)2.
Formula of magnesium nitride : Mg3N2

Q.24. Write the relation between 1 mole of a compound and Avogadro number.

Answer Ans. 1 mole of compound = 6.022 x 1023 molecules.

Q.25. Name the scientist who introduced the term ‘mole’.

Answer Ans. Wilhem Ostwald.

Q.26. Name a cation which does not contain a metal atom.

Answer Ans. Ammonium ion, NHA

Q.27. Name an anion which contains a metal atom.

Answer Ans. Permanganate ion (MnO4) or dichromate ion (Cr2072-).

Q.28. The valency of potassium is +1. Write the molecular formulae for potassium dichromate and potassium permanganate.

Answer Ans. Potassium dichromate
Potassium permanganate

Q.29. How many molecules are present in 9 g of H2O ?

Answer Ans. No. of molecules = 6.022 x 1023 x 9 / 18 = 3.011 x 1023.

Q.30. Write the formula of the compound formed by the ions Al3+ and SO42-

Answer Ans. Al2(SO4)3.

Q.31. Convert 20 picometre into metre.
Ans. 1 pm = 1 x 10-12 m
20 pm = 20 x 10-12 m =2 x 10-11 m.

Answer Ans. 1 pm = 1 x 10-12 m
20 pm = 20 x 10-12 m =2 x 10-11 m.

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