Atoms and Molecules Worksheet-8

Atoms and Molecules Worksheet-8

21. What is the mass of 0.2 mole of oxygen atoms?

22. Calculate the number of molecules of sulphur (S₈) present in 16 g of solid sulphur.

23. Calculate the number of aluminium ions in 0.051 g of aluminium oxide (Al₂O₃).

24. The mass of an atom of element (X) is 2.0 × 10^–23 g. Calculate its atomic mass.

25. Calculate the molar mass of Nitric acid.

26. What is number of molecules present 1.5 mole of ammonia (NH₃) ?

27. Explain why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas.

28. If the valency of carbon is 4 and that of sulphur is 2, what is the chemical formula and name of the compound formed between carbon and sulphur atoms?

29. A mole is quite often known as chemist’s. dozene. Why it is so named ?

30. The mass of one molecule of a substance is 4.65 × 10^–23 g. What is its molecular mass? The compound is made up of carbon and oxygen. Name the compound

31. An element forms an oxide A₂O₅.

(i) What is the valency of the element A?

(ii) What will be the formula of the chloride of the element?

32. On analyzing an impure sample of sodium chloride, the percentage of chlorine was found to be 45.5. What is the percentage of pure sodium chloride in the sample?

33. What weight of calcium contains the same number of atoms as are present in 3.2 g of sulphur ?

Answer:

21. a) 0.2 mole of oxygen atoms

Mass of 1 mole of oxygen (O) atoms = 16 u

Mass of 0.2 mole of oxygen (O) atoms = 0.2 × 16 = 3.2 u

22. Gram molecular mass of sulphur (S₈) = 8 × gram atomic mass of sulphur

          = 8 × 32 g = 256 g

No. of S₈ molecules in 256 g of sulphur = 6.022 × 10²³

No. of S₈ molecules in 16 g of sulphur

23. Step I : Number of moles of Al₂O₃ in 0.051 g of Al₂O₃

Gram molecular mass of Al₂O₃ = 2 × gram atomic mass of Al + 3 × gram atomic mass of I = (2 × 27 g) + (3 × 16 g) = 102 g

Now, 102 g of Al₂O₃ = 1 mol

Step II : Calculation of no. of Al ions in 0.0005 mole of Al₂O₃

1 mole of Al₂O₃ contain Al atoms = 2 × N₀

0.0005 mole of Al₂O₃ contain Al atoms = 2 × 0.0005 × N₀

          = 2 × 0.0005 × 6.022 × 10²³ = 6.022 × 10²⁰ atoms

In Al₂O₃ the valency of Al = 3 +

No. of aluminium ions (Al³⁺) present is the same as the no. of Al atoms

          ∴  No. of Al³⁺ ions = 6.022 × 10²⁰ ions.

24. One atom of element (X) has mass = 2.0 × 10^–23 g.

6.022 × 10²³ atoms of element (X) will have mass

= (2.0 × 10^–23 g) × (6.022 × 10²³)

= 12.044 ≈ 12.0 g

25. Chemical formula of Nitric acid is (HNO₃)

Molar mass of HNO₃ = (1 × atomic mass of H) + (1 × atomic mass of N) + (3 × atomic mass of O)

= (1 × 1u) + (1 × 14u) + (3 × 16u) = 63u

26. 1.0 mole of ammonia (NH₃) contains = 6.022 × 10²³ molecules

1.5 mole of ammonia (NH₃) contain

          = 9.033 × 10²³ molecules.

27. Hydrogen gas is diatomic in nature (H₂) while helium gas is monoatomic (He). As a result, the number of atoms in one mole of hydrogen is (2 × N_A) which is double as compared to number of atoms in one mole of helium (N_A)

28. The chemical formula of compound can be written by exchanging the valencies (cross-over). Therefore, the expected formula is C₂S₄ or CS₂. The compound is called carbon disulphide.

29. A dozon represents a fixed number of articles i.e. 12. Similarly, a mole represents a fixed number of particles i.e. Avogadro’s number (N_A) or 6.022 × 10²³

30. Given: Mass of one molecule of substance = 4.65 × 10^–23 g

Mass of 6.022 × 10²³ molecules of substance = 6.022 × 10²³ × (4.65 × 10^–23) = 28 g

The compound is Carbon monoxide (CO)

Molecular weight (12u + 16 u = 28 u) Gram molecular weight = 28g

31. Formula of oxide of the element = A₂O₅

(I)     The valency of the element A in the oxide = 5

(ii)    The formula of its chloride will be ACl₅

32. Molecular mass of pure NaCl = Atomic mass of Na + Atomic mass of Cl

          = 23 + 35.5 = 58.5u

Percentage of chlorine in pure

if chlorine is 60.6 parts, NaCl = 100 parts

if chlorine is 45.5 parts,

Thus, percentage of pure sodium chloride in the sample is 75%.

33. Step I. Number of atoms in 3.2 g of sulphur

Gram atomic mass of S = 32 g

32 g of sulphur contain = 6.022 × 10²³ atoms

3.2 g of sulphur contain atoms

Step II. Weight of 6.022 × 10²² atoms of calcium

Gram atomic mass of Ca = 40 g

6.022 × 10²³ atoms of Ca weigh = 40 g

6.022 × 10²² atoms of Ca weigh .