Page 150 - Chemistry ICSE Class X
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136 ICSE Chemistry – 10
How to solve numerical problems based on chemical
equations
The calculations based on chemical equations can be done as follows.
Write down the given amount, mass or volume of the substance, and
identify the substance whose amount, mass or volume is to be found
out.
Write down the balanced chemical equation.
Write down the amount, mass or volume of the concerned reactant
and product, as indicated by the balanced chemical equation.
Calculate the unknown quantity by applying the unitary method.
Calculations based on mole-mole relationship
+P C DCNCPEGF EJGOKECN GSWCVKQP VJG EQGHſEKGPVU RNCEGF DGHQTG CP[ U[ODQN
or formula gives the number of moles of that substance involved in the
TGCEVKQP 6JGUG EQGHſEKGPVU VJGTGHQTG IKXG VJG TGNCVKXG COQWPVU PQ QH
moles) of the reactants and products. This relationship is illustrated
through the following examples:
EXAMPLE 5.23. A reaction proceeds in accordance with the chemical equation.
3CaCl 2 + 2K PO o Ca (PO ) + 6KCl
4
3
3
4 2
calcium chloride potassium phosphate calcium phosphate potassium chloride
How many moles of potassium phosphate are needed to produce 0.076 moles of
potassium chloride?
Solution: The balanced chemical equation is
3CaCl 2 + 2K PO o Ca (PO ) + 6KCl
4 2
4
3
3
2 mol 6 mol
This gives the relationship,
6 mol of KCl are obtained from 2 mol of K PO 4
3
2
1 mol of KCl is obtained from mol of K PO
6 3 4
2 × 0.076
0.076 mol of KCl is obtained from mol of K PO = 0.025 mol of K PO
6 3 4 3 4
Thus, to produce 0.076 mol of potassium chloride, 0.025 mol of potassium
phosphate (K PO ) is required.
3
4
Calculations based on mole-mass and mass-mass
relationships
For making these calculations, the number of moles of the reactants and
products are converted into mass by using their molar masses. Some
typical calculations are illustrated below.
EXAMPLE 5.24. Hydrogen reacts to form water as follows.
2H 2 + O o 2H O
2
2
How many grams of hydrogen are needed to react completely with 6.4 g of
oxygen gas? Atomic masses are : H = 1 u, O = 16 u.
Solution: The chemical equation for the reaction, and the mass-mole relationship
is,
2H 2 + O o 2H O
2
2
2 mol 1 mol 2 mol
2 × (2 × 1) g 1 × (2 × 16) g 2 × (2 × 1 + 16) g
4 g 32 g 36 g
