Preparation of 250 ml of M/20 Solution of Mohr's Salt

Mohr’s salt is also known as Ferrous Ammonium Sulphate (FAS), which is an inorganic substance with the formula $\left (NH_{4} \right )_{2}Fe\left ( SO_{4} \right )_{2}.6H_{2}O$. The ammonium cation and the ferrous cation are two major cations known to be present in Mohr's salt. Mohr's salt appears to be a bluish-green colour under standard temperature and pressure conditions. In these conditions, it is a crystalline solid.

The standard solution of FAS may be prepared by dissolving crystalline FAS in a known volume of solvent along with concentrated sulphuric acid. The FAS standard solution is a primary standard as its concentration does not change over time, and this standard solution of FAS can be used directly without any standardisation.

Table of Contents

• Aim of the Experiment

• Apparatus Required

• Theory

• Procedure

• Observations

• Result

• Precautions

Aim of the Experiment

To prepare 250 ml of M/20 solution of Mohr's salt.

Apparatus Required

• Chemical Balance

• Watch Glass

• Weight Box

• 250ml Beaker

• Glass Rod

• 250ml Measuring Flask

• Wash Bottle

• Mohr’s Salt

• Concentrated Sulphuric Acid

• Funnel

• Distilled Water

Theory

• The molecular formula of Mohr’s salt = $\left (NH_{4}  \right )_{2}Fe\left ( SO_{4} \right )_{2}.6H_{2}O$

• The molecular weight of  Mohr’s salt = 392 g/mol

• The equivalent weight of  Mohr’s salt is also 392 g/mol

• Mohr’s salt required for preparing 250 ml of M/20 or 0.05 M solution of Mohr’s salt can be calculated using (392 x 250 x 0.05) / 1000 = 4.9 g

Procedure 

1. Take a watch glass, rinse it with distilled water, and then dry it.

2. Weigh the clean and dried watch glass and record its weight in the notebook.

3. On the watch glass, weigh accurately 4.9 g of Mohr's salt crystals. Record its weight in the notebook.

4. In a clean 250 ml beaker, carefully transfer the weighted Mohr's salt from the watch glass. To check the hydrolysis of ferrous sulphate, add 5 ml of concentrated sulfuric acid to this beaker.

5. To completely transfer the sticking salt into the beaker, thoroughly rinse the watch glass with distilled water. Gently stir the beaker to help the salt dissolve.

6. Carefully transfer the entire solution with the help of a funnel into the 250 ml measuring flask.

7. With distilled water, clean the beaker. Pour the washings into the measuring flask.

8. With the help of a wash bottle, gently pour enough distilled water into the measuring flask to fill it to just below the etched mark on its neck.

9. Use a pipette to add the final few drops of distilled water until the lower level of the meniscus just touches the mark on the measuring flask.

10. Put a stopper on the measuring flask, give it a gentle shake to make the solution homogeneous (i.e., uniform throughout), and then label it as M/20 Mohr's salt solution.

Observation 

Weight of the watch glass = W1 g

Weight of the watch glass and Mohr’s salt = W2 g (W1 + 4.9)

Weight of Mohr’s salt = (W2 - W1) g = 4.9 g

Volume of concentrated sulphuric acid = 5ml

Volume of distilled water = 245 ml

Result 

250 ml of M/20 or  0.05 M of the standard Mohr’s salt solution is prepared.

Precautions

• Wash the watch glass thoroughly to ensure that not even a single Mohr’s salt crystal remains on it.

• To prevent adding additional distilled water over the mark on the standard flask's neck, add the final few drops using a pipette.

• Add the requisite amount of concentrated sulphuric acid to prevent ferrous sulphate hydrolysis.

• Heating is avoided when dissolving the Mohr’s salt in water, this is done to prevent the oxidation of ferrous ions (light green colour) to ferric ions (yellow colour).

Lab Manual Questions

1. Why is concentrated sulphuric acid added during the preparation of Mohr’s salt solution?

Ans: Concentrated sulphuric acid was added during the preparation of Mohr’s salt solution to prevent ferrous sulphate hydrolysis.

2. How to prepare Mohr’s salt solution of 1M concentration in 1 L of distilled water?

Ans: Dissolve 392 g of Mohr’s salt in 1L of distilled water along with concentrated sulphuric acid to prepare 1M, 1L solution of Mohr’s salt.

3. Calculate the number of moles in 392 g of sulphuric acid.

Ans: 1 mole of sulphuric acid = 98 g of sulphuric acid, then 392 g of sulphuric acid will contain 392/98 = 4 moles.

4. How to prepare 250 ml of an M/20 solution of Mohr’s salt?

Ans: Dissolve 4.9 g of Mohr’s salt in 250 ml of distilled water along with concentrated sulphuric acid to prepare 250 ml of M/20 solution of Mohr’s salt.

Viva Questions

1. What is the n-factor of Mohr’s salt?

Ans: n- factor of Mohr’s salt = 1

2. What are the 2 primary cations present in Mohr’s salt?

Ans: The 2 primary cations in Mohr’s salt are the ammonium cation and the ferrous cation 

3. What is added to avoid the hydrolysis of ferrous sulphate during the preparation of Mohr’s salt solution?

Ans: Concentrated Sulphuric acid is added to avoid the hydrolysis of ferrous sulphate during the preparation of Mohr’s salt solution.

4. Should the prepared Mohr’s salt solution be standardised?

Ans: No, since Mohr’s salt is a primary standard.

5. Is Mohr’s salt a primary standard or a secondary standard?

Ans: Mohr’s salt is a primary standard.

6. What is the basicity of sulphuric acid?

Ans: The basicity of sulphuric acid is 2 since it’s a dibasic acid.

7. Comment on Mohr's salt's equivalent weight and molecular weight.

Ans: The equivalent weight and molecular weight of Mohr’s salt is the same, i.e., 392g/mol, as its n-factor is 1.

8. How is the preparation of standard Mohr’s salt solution done?

Ans: The standard solution of Mohr’s salt may be prepared by dissolving crystalline Mohr’s salt in a known volume of solvent along with concentrated sulphuric acid. 

Practical-Based Questions

1. Mohr’s salt is also known as?

1. Ferric ammonium sulphate

2. Ferrous ammonium sulphate

3. Cupric ammonium sulphate

4. Cuprous ammonium sulphate

Answer: (b)

2. Which chemicals are not required for preparing the standard Mohr’s salt solution?

1. Conc. hydrochloric acid

2. Conc. sulphuric acid

3. FAS crystals

4. Distilled water

Answer: (a)

3. The molecular weight of Mohr’s salt is

1. 300 g/mol

2. 150 g/mol

3. 292 g/mol

4.  392 g/mol

Answer: (d)

4. Why is heating avoided during the preparation of standard Mohr’s salt solution?

1. To prevent oxidation of Fe(II) to Fe(III) ions

2. To prevent the reduction of Fe(III) to Fe(II) ions

3. To prevent the hydrolysis of Ferrous sulphate

4. None of the above

Answer: (a)

5. What is the n-factor of ferrous sulphate?

1. 1

2. 2

3. 3

4. 4

Answer: (a)

6. What is the shape and colour of Mohr’s salt crystal?

1. Light green and tetrahedral

2. Light green and octahedral

3. Yellow and tetrahedral

4. Yellow and octahedral

Answer: (b)

7. What is the equivalent weight of Mohr’s salt?

1. 350 g/mol

2. 250 g/mol

3. 192 g/mol

4.  392 g/mol

Answer: (d)

8. What amount of Mohr’s salt is required to prepare 0.1M of 250ml of Mohr’s salt solution?

1. 2.8 g

2. 5.8 g

3. 9.8 g

4. 7.8 g

Answer: (c)  

Summary

Mohr’s salt is a light green, crystalline inorganic salt whose molecular formula is $\left (NH_{4}  \right )_{2}Fe\left ( SO_{4} \right )_{2}.6H_{2}O$ and its molecular weight, as well as equivalent weight, is 392 g/mol. The amount of Mohr’s salt required to prepare 250ml of an M/20 standard solution of Mohr’s salt is calculated using the molarity formula.

Using the formula, the amount of Mohr’s salt required to make 250ml of 0.05 M Mohr’s salt solution is 4.9 g. 4.9 g of Mohr’s salt is accurately weighed and transferred into a 250 ml measuring flask with 5 ml concentrated sulphuric acid, and the crystals are dissolved using distilled water. The solution is made up to the mark in the 250ml measuring flask with distilled water.