Mohr’s salt
Aim- To prepare crystal of
double salt of ferrous ammonium sulphate (Mohr’s salt) [FeSO4(NH4)2SO4.6H2O]
250ml beaker, glass rod, china dish, funnel, funnel stand, filter paper, wash bottle, tripod stand, burner, wire gauze etc.
Chemical required- Ferrous sulphate, ammonium sulphate, distilled water, ethanol and dil. sulphuric acid etc.
Theory-
The additional compounds which
are stable in solid state only but are broken down into individual constituents
when dissolved in water are called double salts. Mohr’s salt is double salt
which is prepared by dissolving equimolar quantity of ferrous sulphate and
potassium sulphate and ammonium sulphate in the minimum quantity of water
containing dilute sulphuric acid. The solution is filtered and heated in a
china dish till point of crystallisation is reached and then cooled to obtain light
green monoclinic Mohr’s salt.
FeSO4. 7H2O
+ (NH4)2SO4
----------> FeSO4.
(NH4)2SO4. 6H2O + H2O
Procedure-
·
We take clean and dry a 250 ml beaker and add 7g Ferrous
sulphate 3.5g ammonium sulphate and 2-3ml dil. sulphuric acid into it. Dil. Sulphuric
acid is added to prevent hydrolysis of ferrous sulphate.
·
In another beaker, 50ml of water is taken in it
and boiled it to for 2-3 minutes to expel out dissolved air.
· Now, the boiled water is added to the content in first beaker and stir it to dissolve the crystal. .the solution is filtered to remove in dissolved impurities
· Gently heated with burner with constantly stirring the solution with a glass rode so that it may not boil.
·
When the content of the china dish is reduced to
50% of original volume then we dip the glass rod into the concentrated
solution.
·
Cool it, after sometime the crystals separate
out.
·
When sufficient crystals formed decant of the
mother liquor without disturbing the crystals.
·
Add small amount of ethanol and distilled water
(1:1 ratio) in china dish to wash the crystal and decant of the liquid.
Result- The yield of ferrous ammonium sulphate (Mohr’s
salt) is …….
And
expected yield is…….
Colour
of the crystal is light green
Shape
of the crystal is monoclinic.
Precaution
·
Mohr ‘salt yield should be light green in colour.
if it is pale yellow which means oxidation has occurred
·
Sulphuric acid should be added before adding
distilled water
·
Avoid over heat the content or from boiling the
content.
·
Don’t disturb the concentrated solution while
cooling