Question - 1 : -
In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.
Sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

Answer - 1 : -

Total mass of reactants = mass of sodium carbonate + mass of ethanoic acid

= 5 – 3 g + 6 g = 11.3 g

Total mass of products = mass of sodium ethanoate + mass of carbon dioxide + mass of water

= 8.2 g + 2.2 g + 0.9 g = 11.3 g

Thus, the mass of reactants is equal to the mass of products, therefore the observations are in agreement with the law of conservation of mass.

Question - 2 : -
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?

Answer - 2 : -

∵ 1 g of hydrogen reacts with = 8 g of oxygen

∴ 3 g of hydrogen reacts with = 8 × 3 = 24 g of oxygen

Thus, 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

Question - 3 : -
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer - 3 : -

The postulate that “atoms can neither be created nor destroyed in a chemical reaction” is the result of the law of conservation of mass.

Question - 4 : -
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer - 4 : -

The postulate that “A chemical compound always consists of the same elements combined together in the same proportion by mass” is the law of definite proportions.

Question - 5 : -
Define the atomic mass unit.

Answer - 5 : -

Atomic mass unit is defined as the mass unit equal to exactly one-twelfth (1/12^th) of the mass of one atom of carbon-12. It is denoted by u (unified mass).

i.e. 1 u = 1.66 × 10-²⁴ g

Question - 6 : -
Why is it not possible to see an atom with naked eyes?

Answer - 6 : -

It is not possible to see an atom with naked eye because of its extremely small size (atomic radius is of the order of 10-¹⁰ m).

Question - 7 : -
Write down the formula of

sodium oxide’
aluminium chloride
sodium sulphide
magnesium hydroxide.

Answer - 7 : - Sodium oxide

Aluminium chloride

Sodium sulphide

Magnesium hydroxide

Question - 8 : - Write down the names of compounds represented by the following formulae:
1.   AI₂(S0₄)₃
2.   CaCI₂
3.   K₂S0₄
4.   KNO₃
5.   CaCO₃

Answer - 8 : -

(i) Aluminium sulphate

(ii) Calcium chloride

(iii) Potassium sulphate

(iv) Potassium nitrate

(v) Calcium carbonate

Question - 9 : -
What is meant by the term chemical formulae?

Answer - 9 : -

The chemical formula of a compound is a symbolic representation of its composition. e.g., formula of calcium oxide.

Thus, chemical formula of calcium oxide is CaO.

Question - 10 : - How many atoms are present in a
(i) H₂S molecule and
(ii) ion?

Answer - 10 : - (i) 3 atoms because H₂S molecule has two atoms of hydrogen and one atom ofsulphur.
(ii) 5 atoms because ion has one atom of phosphorus and four atoms ofoxygen.

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