Atoms And Molecules

Q1. Chemical symbol of sodium is Na. What is its Latin name?

Ans. Natrium

Q2. Name the scientist who gave atomic theory of matter.

Ans. John Dalton

Q3. What are the building blocks of matter?

Ans. Building blocks of all matter are atoms.

Q4. Write the full form of IUPAC.

Ans. IUPAC (International Union of Pure and Applied Chemistry)

Q5. Name the scientist who gave law of conservation of mass.

Ans. Law of conservation of mass – Antonie Lavoisier

Q6. Name the scientist who gave law of constant proportions.

Ans. Law of constant proportions – Joseph Proust

Q7. What does the symbol ‘u’ represent?

Ans. The symbol ‘u’ represents unified mass.

Q8. A change in the physical state can be brought about

(a) only when energy is given to the system

(b) only when energy is taken out from the system

(c) when energy is either given to, or taken out from the system

(d) without any energy change

Ans. (c) when energy is either given to, or taken out from the system

Q9. Why are Dalton's symbols not used in chemistry?

Or

What were the problems with Dalton's element symbols?

Ans. Dalton’s symbol for elements were difficult to draw and inconvenient to use.

Q10. What is atomicity?

Ans. The number of atoms constituting a molecule is known as its atomicity.

Q11. The chemical symbol for nitrogen gas is

(a) Ni (b) N₂ (c) N⁺ (d) N

Ans. (b) Nitrogen molecule is diatomic molecule, therefore, it exists as N₂ molecules.

Q12. The chemical symbol for sodium is

(a) So (b) Sd (c) NA (d) Na

Ans. (d) The chemical symbol for sodium is derived from its Latin name ‘Natrium’. Therefore, its symbol is ‘Na’.

Q13. What is an atom?

Ans. The building blocks of all matter are atoms. Atoms are very small, they are smaller than anything that we can imagine or compare with.

Q14. State the ‘law of conservation of mass.’

Ans. Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.

Q15. Who gave the name Parmanu to the smallest particle of matter?

Ans. An Indian philosopher Maharishi Kanad, gave the name Parmanu to the smallest particle of matter.

Q16. What is the difference between 2H and H₂?

Ans. 2H means two atoms of hydrogen, H₂ means a molecule of hydrogen which contains two atoms.

Q17. Write any 2 laws of chemical combination.

Ans. 1. Law of conservation of mass 

 2. Law of constant proportions

Q18. What was the basis for the Dalton's atomic theory?

Or

What formed the basis for John Dalton to formulate Dalton's atomic theory?

Ans. Dalton's atomic theory was based on the laws of chemical combination.

Q19. What theory did Dalton propose about the structure of matter?

Ans. According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

Q20. Which part of the Dalton’s atomic theory came from the law of constant proportions given by Proust?

Ans. The elements consist of atoms having fixed mass, and that the number and kind of atoms of each element in a given compound is fixed.

Q21. What is meant by the term chemical formula?

Ans. The chemical formula of a compound is a symbolic representation of its composition. Example: chemical formula of sodium chloride is NaCl.

Q22. How many atoms are present in a

(i) H₂S molecule and

(ii) PO₄^3– ion?

Ans. (i) H₂S molecule – 3 (2 hydrogen atoms and 1 sulphur atom)

(ii) PO₄^3– ion – 5 (1 phosphorous atom and 4 oxygen atoms)

Q23. Mass of one atom of oxygen is

Ans. (a)

Explanation:

Mass of 1 atom of oxygen = atomic mass / 6.022 x 10²³

                                             = 16/6.022 x 10²³

Q24. Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. Molar atomic mass of magnesium is 24g mol^–1.

Ans. Number of moles of Mg = 12g/24g mol^–1

                                          = 0.5 mol

Q25. Define the atomic mass unit.

Ans. One atomic mass unit is a mass unit equal to exactly one twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.

Q26. Why is it not possible to see an atom with naked eyes?

Ans. Atoms are so small. Secondly, atoms of most elements are not able to exist independently. Thus, it is not possible to see an atom with naked eyes.

Q27. The visible universe is estimated to contain 10²² stars. How many moles of stars are present in the visible universe?

Ans. 1 mole of star = 6.022 x 10²³ stars

No. of moles of stars = 10²²/6.022 x 10²³

                              = 1.67 x 10^-2

Q28. Does the solubility of a substance change with temperature? Explain with the help of an example.

Ans. In most of the cases, the solubility of a substance in a particular solvent such as water increases with the rise in temperature. Example: More sugar can be dissolved in hot water as compared to cold water.

Q29. What is mole?

Ans. The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.

Q30. What are polyatomic ions? Give examples.

Ans. Clusters of atoms that act as an ion are called polyatomic ions.

They carry a fixed charge on them. Example: carbonate ion CO₃^2-, sulphate ion SO₄^2-

Q31. How the size of atoms is measured?

Or

Write the relation between nanometer and metre.

Ans. The size of an atom is indicated by its radius which is called ‘atomic radius’. Atomic radius is measured in nanometres.

1/10⁹ m = 1 nm

1 m = 10⁹ nm

Q32. How big are atoms?

Ans. Atoms are very small, they are smaller than anything that we can imagine or compare with. More than millions of atoms when stacked would make a layer barely as thick as this sheet of paper.

Q33. What is molecular mass?

Ans. The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).

Q34. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas? 

Ans. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water.

1 g of hydrogen requires 8 g of oxygen to form water.

Thus, 3g of hydrogen requires (3 x 8 = 24 g) of oxygen to form water.

Q35. Write the molecular formulae of all the compounds that can be formed by the combination of following ions

Cu₂⁺, Na⁺, Fe³⁺, Cl^–, SO₄^2- , PO₄^3-

Ans. For Cu₂⁺ ion : CuCl₂, CuSO₄, Cu₃(PO₄)₂

For Na⁺ ion : NaCl, Na₂SO₄, Na₃PO₄

For Fe₃⁺ ion :FeCl₃, Fe₂(SO₄)₃, FePO₄

Q36. A sample of vitamic C is known to contain 2.58 ×10²⁴ oxygen atoms. How many moles of oxygen atoms are present in the sample?

Ans. No. of oxygen atoms in given sample = 2.58 ×10²⁴

6.022 x 10²³ oxygen atoms = 1 mol

2.58 ×10²⁴ oxygen atoms = (1/6.022 x 10²³) x 2.58 ×10²⁴

                                      = 4.28 mol

Q37. Compute the number of ions present in 5.85 g of sodium chloride.

Ans. Molar mass sodium chloride (NaCl) = 23 + 35.5 = 58.5 g

58.5 g of NaCl has number of ions = 6.022 x 10²³ x 2

5.85 g of NaCl has number ions = (6.022 x 10²³ x 2 x 5.85)/58.5

                                              = 12.044 x 10²² = 1.2044 x 10²³