Atoms and Molecules NCERT Solution, study material, CBSE Notes

NCERT Solution: Atoms and Molecules

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?

One gram of gold sample will contain 90 /100 = 0.9 gram of gold

Number of moles of gold =mass of gold / atomic mass of gold

0.9 / 197 = 0.0046
One moles of gold contains NA atoms = 6.022 x 10²³
∴0.0046 mole of gold will contain = 0.0046 x 6.022 x 10²³
= 2.77 x 10²¹

What are ionic and molecular compounds? Give examples.

Atoms of different elements join together in definite proportions to form molecules of compounds. Examples— water, ammonia, carbondioxide. Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an anion and the positively charged ion is called cation. Examples— sodium chloride, calcium oxide

Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is 9.1x10^-28 g). Which one is heavier?

Mass of 1 mole of aluminium atom = the molar mass of aluminium = 27 g mol^-1
An aluminium atom needs to lose three electrons to become an ion, AI³⁺
For one mole of AI³⁺ ion, three moles of electrons are to best lost.

The mass of three moles of electrons 3 x (9.1x10^-28) x 6.022 x10²³ g

= 27.3 x 6.022 x10^-5 g

=164.400 x 10⁵ g
=.00164 g

Molar mass of Al3+ = (27 - 0.00164) gmol^-1
= 26.998gmol^-1

Difference = 27 – 26.9984 = 0.0016 g

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.

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?

Fill in the blanks

(a) In a chemical reaction, the sum of the masses of the reactants and products remains
unchanged. This is called __________.
Ans. Law of conservation of mass
(b) A group of atoms carrying a fixed charge on them is called __________.
Ans. Polyatomic ion
(c) The formula unit mass of Ca₃(PO₄)₂ is __________.
Ans. (3 × atomic mass of Ca) + (2 × atomic mass of phosphorus) + (8 × atomic mass of oxygen) = 310
(d) Formula of sodium carbonate is and that of ammonium sulphate is __________.
Ans. Na₂ CO₃; (NH4)₂SO4

Write the formulae for the following and calculate the molecular mass for each one of them.

(a) Caustic potash
Ans. KOH
(39 + 16 + 1) = 56 g mol^-1
(b) Baking powder
Ans. NaHCO₃
23 + 1 + 12 + (3 × 16) = 84 g mol^-1
(c) Lime stone
Ans. CaCO₃
40 + 12 + (3 × 16) = 100 g mol^-1
(d) Caustic soda
Ans. NaOH
23 + 16 + 1 = 40 g mol^-1
(e) Ethanol
Ans. C₂H₅OH = C2H6O
2 × 12 + (6 × 1) + 16 = 46 g mol^-1
(f) Common salt
Ans. NaCI
23 + 35.5 = 58.5 g mol^-1

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 1 g cm^-3.