Page 35 - Chemistry ICSE Class IX

The Language of Chemistry 23
So,
Mass percentage of hydrogen (H) in water = 2 u × 100 = 11.11
18 u
16 u
and Mass percentage of oxygen (O) in water = × 100 = 88.89
18 u
Therefore, water (H O) contains 11.11% of hydrogen (H), and 88.89% of oxygen (O).
2
Illustrative Questions

Q.1. Hydrogen and oxygen react to form water. Write the balanced chemical equation for this reaction.
Ans. The chemical equation for the reaction between hydrogen and oxygen is written as follows:
Step 1. The reactants and product are identi ed as follows:
Reactants: Hydrogen and Oxygen Product: Water (H O)
2
Step 2. Write the formulae of reactants and product on either side of an arrow. The molecular formula of
hydrogen is H and that of oxygen is O . So, the skeleton equation is,
2
2
H (I) + O (I) o H O(N)
2
2
2
Step 3. Counting of atoms of each element on both the sides gives,
No. of atoms on the
Element
reactant side (left-hand side) product side (right-hand side)
H 2 2
O 2 1
The number of atoms of oxygen (O) is not equal on both the sides, XK\., there are two atoms on the reactant side
(left-hand side), while there is only one atom on the product side (right-hand side). The number of oxygen atoms
on both the sides can be made equal by placing a coef cient of 2 before H O (on the product side). Thus,
2
H (I) + O (I) o 2H O(N)
2
2
2
But, by doing so, the number of hydrogen atoms on the right-hand side has become four. On the left-hand side,
there are only two hydrogen atoms. Hydrogen atoms can be equalized by placing a coef cient of 2 before H (on
2
the reactant side). Then, the chemical equation becomes,
2H (I) + O (I) o 2H O(N)
2
2
2
Now, the counting of the number of atoms of each element shows that there are four hydrogen atoms and two
oxygen atoms on both the sides. Thus, the balanced chemical equation for the reaction between hydrogen and
oxygen is,
2H (I) + O (I) o 2H O(N)
2
2
2
Q.2. Rewrite the following information in the form of a balanced chemical equation: “Magnesium burns in carbon
dioxide to form magnesium oxide and carbon.”
Ans. The reaction can be written as follows:
Magnesium + Carbon dioxide o Magnesium oxide + Carbon
Mg(U) + CO (I) o MgO(U) + C(U)
2
Inspection of this equation shows that it is not a balanced equation. The equation may be balanced by the hit-
and-trial method, as follows.
(a) The balancing of this chemical equation may be started by balancing the oxygen atoms. This can be done by
placing a coef cient 2 before MgO. So, the chemical equation may be rewritten as,
Mg(U) + CO (I) o 2MgO(U) + C(U)
2
(b) Now, Mg can be balanced by placing a coef cient 2 before Mg. So, one gets,
2Mg(U) + CO (I) o 2MgO(U) + C(U)
2
This is the balanced equation.
Q.3. Rewrite the following equation in a balanced form showing in it that Al(OH) is an insoluble product:
3
Al (SO ) + NaOH o Al(OH) 3 + Na SO 4
2
2
4 3