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Preparation of an Acid Solution of Desired Molarity from a Concentrated One

Acids like hydrochloric acid (HCI), tetraoxosulphate(VI) acid (HSO4), trioxonitrate(V) acid (HNO3) and acetic acid (CH3COOH) which are commonly used in laboratory are originally available as concentrated aqueous solutions. The solutions required in laboratory are Prepared by diluting the concentrated solutions with water. Knowing the concentration of the concentrated acid we can calculate how much volume of this acid must be diluted to prepare a definite volume of dilute acid of certain required concentration. The approximate concentrations of some acids as commercially available are given in Table 23.2.

Table 23.2. Approximate Concentrations of Some Acids

Concentrated Acid                          Approximate Molarity

Hydrochloric acid (HCI)                                12M

Tetraoxosulphate(VI) acid (H2SO4)                18M

Trioxonitrate(V) acid (HNO3)                        16M

Glacial acetic acid (CHFOOH)                      17M

Knowing the molarity of concentrated acid, it is possible to calculate the volume of this acid required for preparing a definite volume of dilute solution of known molarity. This can be done with the help of molarity equation:

\M1V1 = M2 V2

where,                         M1 is the molarity of concentrated acid,

V1 is the volume of concentrated acid,

M2 is the molarity of dilute acid,

V2 is the volume of dilute acid.

For example, let us calculate the volume of concentrated tetraoxosulphate(VI) acid (molarity = 18 M) required to prepare 250 cm3 of 2 M acid.

According to molruity equation:

M1VI = M2V2

Now. molarity of concentrated acid (M1)

=18M

volume of concentrated acid required (V 1) = molarity of dilute acid to be prepared (M2)

=2M

volume of dilute acid to be prepared (V 2)

= 250 cm3.

Substituting these values in molarity equation, we get

18 x V1 = 2 x 250

V1 = 2 x 250 / 18 = 27.8 cm3

Therefore, 27.8 cm3 of 18M H2SO4 must be diluted with water to make the volume 250 cm3. The resulting solution will be 2 M H2SO4.

 

 PRECAUTIONS

  1. While diluting acid with water, always add acid to water and not the water to acid.
  2. Pour the concentrated acid into water very slowly.
  3. When H2SO4 is diluted, dilution is done in instalments and the mixture is cooled under tap water after each addition. This is done because dilution of H2SO 4 is exothermic and a large amount of heat is liberated during its dilution.
  4. Concentrated acids are highly corrosive and therefore, should be handled with great care.
  5. Measure the concentrated acid in a small measuring cylinder. If pipette is to be used, make use of a pipette filler. Do not suck it.
  6. Go on stirring the solution with a glass rod when you are adding the concentrated acid to water. This ensures uniform rise of temperature throughout the solution. ·This avoids local heating which may otherwise break the glass vessel.
  7. lf any acid falls on any part of the body, wash it with water thoroughly. Then apply some ointment.