Page 79 - Chemistry ICSE Class IX

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of sodium chloride means that 5 grams of sodium chloride are present in
100 grams of the solution. dŚĞ ƐŝŵƉůĞƐƚ ǁĂLJ ƚŽ ĞdžƉƌĞƐƐ ƚŚĞ
ĐŽŶĐĞŶƚƌĂƟŽŶ ŽĨ Ă ƐŽůƵƟŽŶ ŝƐ ŝŶ
Mathematically, mass per cent of a solute in solution can be expressed ƚĞƌŵƐ ŽĨ ƉĞƌĐĞŶƚĂŐĞ ŽĨ ƐŽůƵƚĞ ŝŶ ŝƚ͘
as: WĞƌĐĞŶƚĂŐĞ ďLJ ŵĂƐƐ ŝƐ ĐĂůůĞĚ mass
Mass of solute per cent ĂŶĚ ƉĞƌĐĞŶƚĂŐĞ ďLJ ǀŽůƵŵĞ
Mass per cent of the solute = × 100
Mass of solution ŝƐ ĐĂůůĞĚ ǀolume per cent͘
Both, the mass of solute and that of the solution must be expressed in
the same mass units, viz., both in grams or both in kilograms, etc.
However, when a solute is present in trace quantity (very small
quantity), the concentration is expressed in the units of parts per million
(abbreviated as ppm). For example, a sample containing 5 grams of solute
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in 10 grams of a solution is said to have a concentration of 5 ppm. The
composition of gaseous mixtures is also expressed in ppm units based on
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the volume ratio. For example, 10 mL of solute gas in 10 mL of gaseous
mixture corresponds to 10 ppm.

Volume per cent (or per cent by volume)
The number of units of volume of the solute per 100 units of volume of the
solution is known as the volume per cent (% v/v) in water. For example, dŚĞ ĂƚŵŽƐƉŚĞƌŝĐ ƉŽůůƵƟŽŶ ŝƐ ĂůƐŽ
a 5% (v/v) solution of ethyl alcohol in water contains 5 mL of alcohol in ŵĞĂƐƵƌĞĚ ŝŶ ƚŚĞ ŶƵŵďĞƌ ŽĨ ƵŶŝƚƐ ŽĨ
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100 mL of the solution. Mathematically, volume per cent is expressed as: ƉŽůůƵƚĂŶƚ ŝŶ ϭϬ ƵŶŝƚƐ ŽĨ ƚŚĞ Ăŝƌ͕ ŝ͘Ğ͕͘
ŝŶ ƉĂƌƚƐ ƉĞƌ ŵŝůůŝŽŶ ;ƉƉŵͿ ƵŶŝƚ͘
Volume of the solute
Volume per cent of the solute = × 100
Volume of the solution

EXAMPLE 3.1. 15.0 g of sodium chloride is dissolved in 200.0 g of water. What
is the mass per cent of sodium chloride in this solution?
Solution: Mass of sodium chloride dissolved = 15.0 g
Mass of water taken = 200.0 g
So, Total mass of the solution = 200.0 g + 15.0 = 215.0 g
From the de nition,

Mass per cent of sodium ⎤ Mass of sodium chloride
chloride in solution ⎥ = × 100
⎦ Mass of the solution
Mass per cent of sodium ⎤ 15.0 g
Or = × 100 = 6.98
chloride in solution ⎥ 215.0 g
⎦
So, the concentration of sodium chloride in solution is 6.98% by mass.

EXAMPLE 3.2. 10 mL of alcohol is added into water to make a solution having
a total volume of 250 mL. Calculate the percentage (by volume) of alcohol in the
solution.
Solution: Volume of alcohol = 10 mL
Volume of the solution = 250 mL
10 mL
Percentage (by volume) of = × 100 = 4%
alcohol in the solution 250 mL
Thus, the given solution contains 4% (by volume) of alcohol.

What is an unsaturated solution
A solution in which more solute can be dissolved at any xed temperature
is called an unsaturated solution. Fox example, a solution of sugar in which
more sugar can be dissolved without changing its temperature is called an
unsaturated solution of sugar.