Prepare 250 ml of an N/10 Solution of Oxalic Acid From Crystalline Oxalic Acid

The IUPAC name for oxalic acid is ethanedioic acid, and it has the chemical formula $C_{2}H_{2}O_{4}$. It is regarded as the most basic dicarboxylic acid. It is a white, crystalline substance that dissolves in water to produce a colourless solution. Most commonly, oxalic acid is found to exist naturally as a dihydrate which is the crystalline form of oxalic acid with the chemical formula $C_{2}H_{2}O_{4}\cdot 2H_{2}O$.

The molecular weight of oxalic acid is 126 g/mol, while the equivalent weight of oxalic acid is 63 g. The standard oxalic acid solution may be prepared by dissolving crystalline oxalic acid in a known volume of solvent. The oxalic acid standard solution is a primary standard as its concentration does not change over time, and this standard solution of oxalic acid can be used directly without any standardisation. 

Table of Contents

• Aim 

• Apparatus required

• Theory

• Procedure

• Observations

• Result

• Precautions

Aim 

To prepare 250 ml of an N/10 solution of oxalic acid from crystalline oxalic acid.

Apparatus Required

• Chemical Balance

• Watch Glass

• Weight Box

• 250ml Beaker

• Glass Rod

• 250 ml Standard Flask

• Wash Bottle

• Oxalic Acid Crystals

• Funnel

• Distilled Water

Theory

• Crystalline oxalic acid formula = $C_{2}H_{2}O_{4}\cdot 2H_{2}O$ (a hydrated form of oxalic acid is most common in nature).

• The molecular weight of oxalic acid = 126 g/mol.

• The number of protons that 1 mole of an acid replaces is known as the n factor for that acid. Since oxalic acid is a dibasic acid as it has two replaceable protons. Hence,  the n factor for oxalic acid dihydrate is 2.

• Equivalent weight of oxalic acid = molecular mass of oxalic acid/n factor of oxalic acid  = 126g/2 = 63 g.

• Using the formula of normality, the amount of oxalic acid required to prepare 250ml standard was found to be 1.575g.

Normality = (weight x 1000) / (equivalent weight x volume)

0.1 = (weight x 1000) / (63 x 250)

Weight = 1.575g

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. Weigh 1.575g of oxalic acid accurately on the watch glass and record this weight in the notebook.

4. Oxalic acid should be carefully and gently transferred from the watch glass into a clean 250 ml beaker. With a wash bottle, wash the watch glass with distilled water to transfer any material stuck to it into the beaker.  No more than 50 ml of distilled water should be used for this use.

5. Use a clean glass rod to gently stir the crystals of oxalic acid in the beaker until they dissolve.

6. With a funnel, gently transfer the entire solution from the beaker into a 250 ml standard flask once the oxalic acid in the beaker has completely dissolved.

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

8. Transfer the solution, which is stuck, to the funnel into the standard flask by thoroughly washing the funnel with distilled water using a wash bottle.

9. With the help of a wash bottle, carefully pour enough distilled water into the standard flask to fill it up to just below the etched mark.

10. 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 standard flask.

11. Place a stopper on the measuring flask, then gently shake it to distribute the solution evenly. Label it as N/10 or 0.1 N  oxalic acid solution.

Observation 

• Weight of the watch glass = W1 g

• Weight of the watch glass and oxalic acid = W2 g (W1 + 1.575)

• Weight of oxalic acid = (W2 - W1) g = 1.575 g

• The volume of distilled water = 250 ml

Result 

250 ml of N/10 or  0.1 N of the standard oxalic acid solution is prepared.

Precautions

• Oxalic acid crystal weighing requires weights of 2g, 1g, 100mg, and 5mg.

• Wash the watch glass thoroughly to ensure that not even a single oxalic acid 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 diluted sulphuric acid and heat the flask to between 60 and 70 °C if titrating oxalic acid or oxalate is necessary.

Lab Manual Questions

1. How to prepare 100 ml of an N/10 solution of oxalic acid from crystalline oxalic acid?

Ans: Dissolve 0.63 g of oxalic acid in 100 ml of distilled water in order to prepare 100 ml of N/10 solution of oxalic acid.

2. Why is a pipette used to add the final few drops of distilled water in the standard flask?

Ans: A pipette is used to add the final few drops of distilled water in the standard flask. This is to prevent adding additional distilled water over the mark on the standard flask's neck while preparing a standard solution.

3. Why are the funnel, beaker and watch glass washings transferred into the standard flask?

Ans: Wash the watch glass thoroughly to ensure that not even a single oxalic acid crystal remains on the glassware and the entire amount is transferred to the standard flask.

4. How to prepare 250 ml of an N/100 solution of oxalic acid from crystalline oxalic acid?

Ans: Dissolve 0.157 g of oxalic acid in 250 ml of distilled water to prepare 250 ml of N/100 solution of oxalic acid.

Viva Questions

1. What is a standard solution?

Ans: Standard solutions contain the specified and accurate amount of the substance or compound. These solutions are frequently used to identify unknown substances and establish their concentration.

2. What is a normal solution?

Ans: A normal solution has one gram-equivalent mass of the solute per litre of the solution.

3. Is oxalic acid a primary standard?

Ans: Yes, oxalic acid is a primary standard.

4. What is the basicity of oxalic acid?

Ans: The basicity of oxalic acid is 2, as it’s a dibasic acid.

5. Are “molarity” and “normality” the same?

Ans: No, molarity is the number of moles of solute dissolved in 1 litre of the solution while normality is the number of gram equivalent of solute dissolved in 1 litre of solution.

6. Why is the front door of the weighing balance shut during weighing?

Ans: The operator's breath generates vibrations in the pan when the front door of the weighing balance is opened, which produces inaccurate readings.

7. What is the maximum weight that can be weighed using a chemical balance?

Ans: Maximum weight that can be weighed using a chemical balance is 100 g.

8. What is the relationship between normality and equivalent weight?

Ans:  Normality is the number of gram equivalent of solute dissolved in 1 litre of solution. Hence, normality is directly proportional to the equivalent weight of the solute.

9. How is the preparation and standardisation of oxalic acid done?

Ans: Oxalic acid solution can be prepared by dissolving crystalline oxalic acid in a known volume of the solvent. Standardisation of this solution is not required because it is a primary standard.

Practical-Based Questions

1. Oxalic acid is also called?

1. Ethanedioic acid

2. Propanoic acid

3. Acetic acid

4. Ethanoic acid

Answer: (a)

2. The basicity of oxalic acid is found to be?

1. 1

2. 2

3. 3

4. 4

Answer: (b)

3. The apparatus not used to prepare standard oxalic solution is

1. Glass rod

2. Beaker

3. Standard flask

4. Burette

Answer: (d)

4. The molecular weight of oxalic acid is

1. 108 g/mol

2. 256 g/mol

3. 126 g/mol

4. 94 g/mol

Answer: (c) 

5. The equivalent weight of oxalic acid is 

1. 63 g

2. 43 g

3. 53 g

4. 33 g

Answer: (a)

6. What is the most common form of oxalic acid present in nature?

1. Monohydrate oxalic acid

2. Pentahydrate oxalic acid

3. Dihydrate oxalic acid

4. Trihydrate oxalic acid

Answer: (c)

7. What is the amount of oxalic acid required to prepare 1N of 250ml oxalic acid solution?

1. 21.4 g

2. 15.75 g

3. 3.15 g

4. 45 g

Answer: (b)

8. The preparation of N/10 oxalic acid solution with 63 gm of oxalic acid is done in?

1. 1 L distilled water

2. 1000 L distilled water

3. 100 L distilled water

4. 10 L distilled water

Answer: (d)

Summary

Oxalic acid is a white, crystalline dicarboxylic acid, and the basicity of this acid is two; hence, its molecular weight is 126 g/mol while its equivalent weight is 63 g. The amount of oxalic acid required to prepare 250ml of an N/10 standard solution of oxalic acid is calculated using the normality formula.

Using the formula, the amount of oxalic acid required to make 250 ml of 0.1N oxalic acid is 1.575 g. 1.575 g of oxalic acid is accurately weighed and transferred into a 250 ml standard flask, and the crystals are dissolved using distilled water. The solution is made up to the mark in the 250ml standard flask with distilled water.