An Overview of Class 12 Chemistry Prepare 250 Ml N10 Solution Oxalic Acid Crystalline Oxalic Acid Experiment
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
Take a watch glass, rinse it with distilled water, and then dry it.
Weigh the clean and dried watch glass and record its weight in the notebook.
Weigh 1.575g of oxalic acid accurately on the watch glass and record this weight in the notebook.
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.
Use a clean glass rod to gently stir the crystals of oxalic acid in the beaker until they dissolve.
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.
With distilled water, clean the beaker. Pour the washings into the standard flask.
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.
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.
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.
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
Oxalic acid is also called?
Ethanedioic acid
Propanoic acid
Acetic acid
Ethanoic acid
Answer: (a)
The basicity of oxalic acid is found to be?
1
2
3
4
Answer: (b)
The apparatus not used to prepare standard oxalic solution is
Glass rod
Beaker
Standard flask
Burette
Answer: (d)
The molecular weight of oxalic acid is
108 g/mol
256 g/mol
126 g/mol
94 g/mol
Answer: (c)
The equivalent weight of oxalic acid is
63 g
43 g
53 g
33 g
Answer: (a)
What is the most common form of oxalic acid present in nature?
Monohydrate oxalic acid
Pentahydrate oxalic acid
Dihydrate oxalic acid
Trihydrate oxalic acid
Answer: (c)
What is the amount of oxalic acid required to prepare 1N of 250ml oxalic acid solution?
21.4 g
15.75 g
3.15 g
45 g
Answer: (b)
The preparation of N/10 oxalic acid solution with 63 gm of oxalic acid is done in?
1 L distilled water
1000 L distilled water
100 L distilled water
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.
FAQs on Class 12 Chemistry Prepare 250 Ml N10 Solution Oxalic Acid Crystalline Oxalic Acid Experiment
1. What is the exact calculation to determine the mass of crystalline oxalic acid required to prepare 250 ml of an N/10 standard solution for the CBSE Class 12 practical exam?
To find the mass of crystalline oxalic acid ($C_{2}H_{2}O_{4}\cdot 2H_{2}O$) needed, we use the normality formula. For the CBSE 2025-26 syllabus, the key steps are:
- Identify the Equivalent Weight: Oxalic acid is a dibasic acid, so its n-factor is 2. The molecular weight of crystalline oxalic acid is 126 g/mol. Therefore, Equivalent Weight = Molecular Weight / n-factor = 126 / 2 = 63 g.
- Use the Normality Formula: Normality = (Weight of Solute × 1000) / (Equivalent Weight × Volume in ml).
- Substitute the values: 0.1 N = (Weight × 1000) / (63 × 250).
- Calculate the weight: Weight = (0.1 × 63 × 250) / 1000 = 1.575 g.
Therefore, you must accurately weigh 1.575 g of crystalline oxalic acid to prepare the solution.
2. What types of questions are typically asked in the viva voce for the experiment on preparing a standard oxalic acid solution?
For the Class 12 Chemistry practical exam, important viva questions focus on the underlying theory and procedural accuracy. Expect questions such as:
- Why is oxalic acid considered a primary standard?
- What is the basicity of oxalic acid and how does it affect its equivalent weight?
- Why must you transfer the washings from the watch glass and beaker into the measuring flask?
- What is the IUPAC name for oxalic acid? (Ethanedioic acid).
- Why is the final volume adjusted using a pipette?
- What is the difference between molarity and normality for this specific solution?
3. Why is a solution of oxalic acid called a 'primary standard,' and why doesn't it require further standardisation?
A solution of oxalic acid is called a primary standard because the solute, crystalline oxalic acid, meets several key criteria: it is highly pure, chemically stable, non-hygroscopic (does not absorb moisture from the air), and has a high molecular weight to minimise weighing errors. Because its composition is pure and stable, a solution made by dissolving a precisely weighed amount in a specific volume has a highly accurate and known concentration. Therefore, it does not need to be standardised by titrating it against another known solution.
4. How would the calculation change if you were asked to prepare 100 ml of an N/20 solution of oxalic acid?
The core principle remains the same, but the values for normality and volume change. The equivalent weight of crystalline oxalic acid is still 63 g.
- New Normality (N): N/20 = 0.05 N
- New Volume (V): 100 ml
- Using the formula: Weight = (N × Equivalent Weight × V) / 1000
- Substituting new values: Weight = (0.05 × 63 × 100) / 1000 = 0.315 g.
You would need to weigh 0.315 g of oxalic acid for this preparation.
5. Why is it a critical mistake to use the molecular weight of anhydrous oxalic acid instead of crystalline oxalic acid for this experiment?
This is a common source of error in board practicals. The standard oxalic acid available in labs is the stable crystalline form, which is a dihydrate ($C_{2}H_{2}O_{4}\cdot 2H_{2}O$). Its molecular mass is 126 g/mol. Anhydrous oxalic acid ($C_{2}H_{2}O_{4}$) has a molecular mass of 90 g/mol. Using the incorrect mass (90 g/mol) would mean you add more moles of acid than required, resulting in a solution with a higher concentration than intended and leading to a significant calculation and experimental error, which would cost you marks.
6. How do you explain the relationship between normality and molarity for a dibasic acid like oxalic acid in the context of the Class 12 exam?
The relationship is based on the acid's basicity (the number of replaceable H+ ions). Oxalic acid is a dibasic acid, meaning one molecule can furnish two H+ ions.
- Molarity (M) is the number of moles of solute per litre.
- Normality (N) is the number of gram equivalents of solute per litre.
For oxalic acid, the n-factor is 2. The relationship is Normality = Molarity × n-factor. Therefore, for oxalic acid, Normality = Molarity × 2. An N/10 (0.1 N) solution of oxalic acid is equivalent to a 0.05 M solution.
7. What are the most crucial precautions to follow to score full marks in the preparation of a standard oxalic acid solution?
To ensure accuracy and score well in the practical exam, follow these precautions strictly:
- Use a chemical balance carefully and ensure the front door is shut while weighing to avoid errors from air currents.
- Wash the watch glass, beaker, and funnel thoroughly with distilled water and transfer all washings into the standard flask to ensure no solute is lost.
- Dissolve the crystals completely in the beaker before transferring the solution to the standard flask.
- When making up the volume in the standard flask, add the last few drops of distilled water with a pipette to avoid overshooting the mark.
- Ensure the lower meniscus of the solution touches the etched mark on the neck of the standard flask for an accurate volume of 250 ml.
