The equation for calculating molarity is the ratio of the moles of solute whose molarity is to be calculated and the volume of solvent used to dissolve the given solute.
M =
[tex]\frac{n}{v}[/tex]
Here,
M is the molality of the solution that is to be calculated
n is the number of moles of the solute
V is the volume of solution given in terms of litres
Solved Examples
Example 1: A solution is prepared by bubbling 1.56 grams of hydrochloric acid in water. Here, the volume of the solution is 26.8 mL. Calculate the molarity of the solution.
Solution:The chemical formula of hydrochloric acid = HCl
The chemical formula for Water = H2O
The molecular weight of HCl = 35.5 ×1 + 1×1 = 36.5 moles/gram
The molecular weight of H2O = 1 ×2 + 16 ×1 = 18 moles/gram
Given, mass of hydrochloric acid in the solution = 1.56 g
The number of moles of hydrochloric acid =
nt = ⇒ mass in grams molecular weight
nt = [tex]\frac{1.56}{36.5}[/tex] = 4.2 x 10 - 2 m
o
l
e
Now, given volume of the solution = 26.8 mL
Expressing the volume in terms of litres,
v
o
l
u
m
e = [tex]\frac{26.8}{1000}[/tex] = 2.68 x 10 - 2 L
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t
r
e
Now, we calculate the molarity of the solution using the formula given above.
M
o
l
a
r
i
t
y = number of moles of element
volume of solution in litres
M
o
l
a
l
i
t
y = [tex]\frac{4.27x10-2}{2.68x10-2}[/tex] = 1.59m
The molarity of the solution is 1.59 M.
