Chapter-3, 9th Class Atoms and Molecule, Subject Science

1. Name two scientists who established the laws of chemical combination?

2. Give an example of a triatomic molecule of an element.
3. Define atomicity.

4. Write the atomicity of the following molecules:

(i) Sulphur
(ii) Phosphorus

5. What is an ion? Give one example.

6. Write the cations and anions present (if any) in the following compounds:

(a) CH3COONa
(b) NaCl
(c) H2
(d) NH4NO3

7. Calculate the mass percentage of oxygen present in the following compounds and state the law of chemical combination associated. Given, H = 1, O = 16.

(i) Water (H2O) and
(ii) Hydrogen peroxide (H2O2)
8. Classify each of the following on the basis of their atomicity.
(a) F2 (b) NO2 (c) CO2−3 (d) C2H6
(e) CO (f) H2O2 (g) P4O10 (h) O3
(i) HCl (j) CH4 (k) He (l) Ag

9. Calculate the molecular mass of the following:

(a) H2CO3
(b) C2H5OH
(C) MgSO4

10. What are ionic and molecular compounds? Give examples.

11. Give three significance of mole.

12. Arrange the following in order of decreasing masses:

(i) 10 23 molecules of CO2 gas
(ii) 0.1 g atom of silver
(iii) 1 gram of carbon
(iv) 0.1 mole of H2SO4
(v) 10 23 atoms of calcium.
(Given Atomic masses: Ag = 108 u, S = 32 u, N = 14 u, Ca = 40 u)

13. Calculate the number of aluminum ions (Al3+) in 0.056 g of alumina (Al2O3).

14. Calculate the mass per cent of each element present in the molecule of calcium carbonate.

15. Verify by calculating that

(a) 5 moles of CO2 and 5 moles of H20 do not have the same mass.
(b) 240 g of calcium and 240 g of magnesium elements have a mole ratio of 3 : 5.

16. Find the ratio of mass of the combining elements in the following compounds:

(a) CaCO3 (b) MgCl2 (c) H2SO4
(d) C2H5OH (e) NH3 (f) Ca(OH)2

17. Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of same weight.

(a) Whose container is heavier?
(b) Whose container has more number of atoms?

18. What is a mole? What is the unit of mole? How many molecules are there in a certain mass of a

19. The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g.
Compute the mass of an electron.

20. The mass of one steel screw is 4.1 lg. Find the mass of one mole of these steel screws. Compare
this value with the mass of the Earth (5.98 × 10 24 kg). Which one of the two is heavier and by how
many times?

21. A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?

22. A silver ornament of mass ‘m’ gram is polished with gold equivalent to 1% of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.

23. A sample of ethane (C2H6) gas has the same mass as 1.5 × 1020 molecules of methane (CH4). How many C2H6 molecules does the sample of gas contain?

24. In photosynthesis, 6 molecules of carbon dioxide combine with an equal number of water molecules through a complex series of reactions to give a molecule of glucose having a molecular formula C6H12O6. How many grams of water would be required to produce 18 g of glucose? Compute the volume of water so consumed assuming the density of water to be lg cm-3.

25. A big drop of water has volume 1.0 mL. How many molecules of water are there in this drop, If the density of water is lg/mL?

26. What is the fraction of the mass of water due to neutrons?

27. You are provided with a fine white coloured powder which is either sugar or salt. How would youidentify it without tasting?

28. Calculate the number of electrons present in 15.4 g of carbon tetrachloride (CCl4).

29. How many atoms would be present in a black dot marked on the paper with graphite pencil as a full stop at the end of a sentence. [Given mass of a dot = 10-18 g]

30. Carbon dioxide produced by action of dilute hydrochloric acid on potassium hydrogen carbonate is moist whereas that produced by heating potassium hydrogen carbonate is dry. What would be the difference in the composition of carbon dioxide in the two cases? State the associated law.


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