Extra Questions For Class 9 Science Chapter 3 Atoms and Molecules

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 (PO₄^3-) and Nitride (N^3-)

---

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 (Cr₂0₇^2-).

---

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 SO₄^2-

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.

---

In order to equip the students with the latest methodology of testing and evaluation in competitive exam segment in India Physics Gurukul has introduced successrouter.com for Online Testing and Assessment. Try our free tests for JEE and NEET Today!