Page 151 - Chemistry ICSE Class X

Mole Concept and Stoichiometry 137
Thus,
32 g of oxygen reacts completely with 4 g of hydrogen
4
1 g of oxygen reacts completely with g of hydrogen
32
6.4 × 4
6.4 g of oxygen reacts completely with g = 0.8 g of hydrogen
32
Thus, 6.4 g of oxygen gas reacts completely with 0.8 g of hydrogen.
EXAMPLE 5.25. Hydrazine (N H ) and hydrogen peroxide (H O ) are used
4
2
2
2
together as a rocket fuel. The products of the reaction are nitrogen and water.
How many grams of H O are needed per kg (1000 g) of hydrazine carried by the
2
2
rocket?
Solution: The reaction is,
Hydrazine + Hydrogen peroxide o Nitrogen + Water
The balanced chemical equation is,
N H 4 + 2H O o N 2 + 4H O
2
2
2
2
1 mol 2 mol
From the chemical equation of the reaction,
1 mol of N H = 2 mol of H O 2
2
2
4
or (2 × 14 + 4 × 1.0) g of N H = 2 × (2 × 1.0 + 2 × 16.0) g of H O 2
2
4
2
(28 + 4) g of N H = 2 × (34) g of H O 2
4
2
2
32 g of N H = 68 g of H O 2
2
2
4
Thus, 32 g of N H requires 68 g of H O 2
4
2
2
68
1 g of N H requires 32 g of H O 2
2
2
4
1000 × 68
1000 g of N H requires 32 g = 2125 g of H O 2
2
2
4
Therefore, mass of H O required in the reaction is 2125 g, or 2.125 kg.
2
2
Calculations based on mass-volume and volume-volume
relationships
The following relationships are useful for solving problems based on mass-
volume, and volume-volume relationships:
–1
ƒ Molar volume of a gas at NTP = 22.4 L mol (= 22400 mL)
i.e., 1 mol of each gas under NTP conditions occupies a volume of 22.4 L.
These calculations are illustrated through the following examples:
EXAMPLE 5.26. 2.3 g of metallic sodium reacts with excess of water. Calculate
the mass of sodium hydroxide formed. What is the volume of hydrogen evolved
under NTP conditions?
Solution: Sodium reacts with water according to the equation,
2Na(s) + 2H O o 2NaOH(aq) + H (g)
2
2
2 mol 2 mol 1 mol
2 × 23 g 2 (23 + 16 + 1) 2 g
46 g 80 g 22.4 L (at NTP)
2.3 × 80
Thus, 2.3 g of sodium gives = 4.0 g of NaOH
46
22.4 × 2.3
and 2.3 g of sodium gives = 1.12 L of H gas
46 2
Therefore, Mass of NaOH formed = 4.0 g
Volume of H evolved at NTP = 1.12 L
2