To prepare 250ml of M/20 standard Mohr’s Salt Solution

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AIM: To prepare 250ml of M/20 standard Mohr’s Salt solution.

APPARATUS REQUIRED:

• 250ml volumetric flask
• Fractional weights
• Chemical balances
• Weighing bottle
• Funnel

CHEMICALS REQUIRED:

• Mohr's Salt crystals
• Distilled water
• 4N H₂SO₄

THEORY:

A standard solution is a solution whose exact concentration is known in order to prepare a standard solution of a substance; the salt must have a high degree of purity. It should neither be deliquescent nor hygroscopic. It should not decompose in the presence of light and it should not react with the solvent.

PROCEDURE:

A clean dry watch glass is taken and 4.9gm of Mohr's Salt has been weighed, accurately enough with the help of chemical balance. The crystals are then transferred into a clean dry standard 250ml volumetric flask with the help of a funnel. The watch glass is pre-weighed, with the help of wash bottle distilled water is added taking care that no particle of salt remains sticky to the sides of the funnel. The funnel is removed and water is added until the level of the solution in the flask is 1/4^th. Swirl the flask gently until the salt dissolves completely. About 3/4^th test tube of concentrated H₂SO₄ is added to the flask until the solution turns to light green in color, continue adding water till the level of the solution just lies below the edge mark with the help of a pipette add few drops of water so that it just coincides with the marked line the flask is then stoppered and shaken thoroughly six times until a homogenous mixture is obtained, this is known as a standard solution.

OBSERVATION:

Weight of empty watch glass (W₁) = ___gm (Weigh the watch glass when it is empty)

Weight of watch glass and salt (W₂) = ___gm (Now pour some salt very carefully to add 4.9gm of salt accurately into the watch glass, so that the difference between W₂and W₁ gets equal to 4.9gm)

Mass of salt transferred = W₂ - W₁ = ___gm (This should be equal to 4.9gm)

For example-

Weight of empty watch glass (W₁) = 20gm (Suppose the watch glass weigh 20gm)

Weight of watch glass and salt (W₂) = 24.9gm (After pouring the salt into it)

Mass of salt transferred = W₂ - W₁ = 4.9gm (This should be equal to 4.9gm)

CALCULATIONS:

Molarity (M) = mass of solute/(GMM x volume of the solution in litre)

INFERENCE:

The mass of salt taken is 4.9gm and molarity of Mohr's salt solution is 0.05M.

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Know more about stuff related to this practical experiment (Important for Boards):-

• To find the molarity and strength of the given KMnO4 solutionusing M/20 standard Mohr salt solution
• How to find out the mass of salt required to achieve therequired concentration of a solution
• Apparatus Used in Volumetric Analysis
• Some important terms related to concentrations of solutions
• Equivalent masses of oxidizing and reducing agents
• Preparing a standard solution
• Viva Questions related to Volumetric Analysis

Reference:-

• Verma, Dr. N.K.. Comprehensive Practical Chemistry, Class XII.
• NCERT. Chemistry Part I and II, Class XII.

For latest syllabus, both theory and practical papers for class XII, check out the CBSE’s official site http://cbseacademic.in/ for more info.

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