Question

Concentrated hydrochloric acid is 36% (Weight/Volume) hydrochloric acid and has a density of 1.18 g.cm -3.
( i ) If 10 cm 3 of the concentrated acid is dissolved in water to produce 1 dm 3 of solution, calculate the concentration in mol. dm-3 of the dilute solution.
( ii ) If 5cm3 of the dilute solution in ( i ) above is made up to 20 cm3, what will be the new concentration in mol.dm-3.

Answer 1

i. The new concentration is 0.116 M

ii. The new concentration is 0.029 M

What is the concentration of the original hydrochloric acid acid?

The concentration of the original or stock hydrochloric acid is calculated as follows:

• Molarity = Percentage concentration * Density * 1000/Molar mass * 100

Molarity of stock HCl = 36 * 1.18* 1000 /36.5 * 100

Molarity of stock HCl = 11.6 mol/dm

i. Using the dilution formula: M₁V₁ = M₂V₂

M₂ = M₁V₁ /V₂

M₂ = 11.6 * 10/1000

M₂ = 0.116 M

The new concentration = 0.116 M

ii. Using M₁V₁ = M₂V₂

M₂ = M₁V₁ /V₂

M₂ = 0.116 * 5/20

M₂ = 0.029 M

The new concentration = 0.029 M

In conclusion, the new concentrations are found using the dilution formula.

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