CBSE Chemistry Experiment - Determination of Strength and Molarity of the Given HCl Solution - Introduction
Standard liquid solutions, like acids, are readily available and simple to prepare. The technique used in the quantitative analysis of any chemical is titration. A method for comprehending behaviour known as quantitative analysis makes use of statistical and mathematical modelling, observation, and investigation. Quantitative investigators use data to illustrate a particular fact.
The analysis often reveals any chemical substance's molarity or molality, or strength of solution and concentration. Compounds called indicators are introduced to analyte solutions along with titrant. Thus this chemistry experiment explains the strength and molarity of the given HCl solution with the 0.05M Na2CO3 solution.
Table of Content
Aim
Apparatus required
Theory
Procedure
Result
Aim
To determine the strength and molarity of the given HCl solution with the given 0.05M Na2CO3 solution.
Apparatus Required
Burette stand
Burette
Conical flask
Measuring flask
Pipette
Glass rod
Funnel
Watch glass
Wash bottle
Methyl orange
Theory
A standard solution is one whose concentrations are precisely known.
By titrating hydrochloric acid with a standard sodium carbonate solution while employing methyl orange as an indicator, one can measure the molarity of the acid.
By multiplying the acid's molarity by its molecular mass, which is 36.5, the strength of the solution given is calculated.
Procedure
Determination of Strength and Molarity of Given HCl solution with the given 0.05M Na2CO3
Using distilled water, clean the burette.
Fill the burette using the provided HCl solution after rinsing it.
Pipette 20 ml of the given Na2CO3 solution into a cleaned conical flask after rinsing the pipette with the provided Na2CO3 solution.
After adding 2-3 drops of methyl orange into the conical flask containing 20mL of Na2CO3, place it over the tile just below the tip of the burette.
Record the burette's initial value and gently run the drops of acid solution to the conical flask until the solution's colour shifts from yellow to pale pink.
Keep track of the final measurement and calculate the amount of HCl solution employed.
The procedure should be repeated to measure at a minimum of three concordant readings.
Observations
Result
The result of the chemistry experiment on the determination of strength and molarity of the given HCl solution with the given 0.05M Na2CO3 solution verified that:
The strength of the given HCl solution is ( _ gL-1).
Precautions
Handle chemicals and apparatus with caution.
Every apparatus should be rinsed with distilled water and dried before starting the experiment.
To prevent adding additional distilled water beyond the line on the measuring cylinder's neck, add the final few droplets using a pipette.
If any acid spills on your body, immediately wash it off with water. Apply a little ointment after that.
After the completion of the experiment, wash your hands thoroughly.
Lab Manual Questions
1. What is the aim of transferring solutions using a glass rod?
Ans: Solutions are poured using glass rods to prevent spillage. A glass rod placed against a beaker's pouring edge enables the solution to run down the glass rod and into the collecting vessel, as opposed to splashing beyond the lip.
2. Why does not the pipette blow out the final drop?
Ans: Due to the surface tension, a tiny drop of liquid remains in the pipette after the last drop is added, preventing the last drop from being blown out. Moreover, the design of pipettes ensures that this drop won't have an impact on the empirical value.
3. Why are concentrated HCl not used in this experiment?
Ans: When using concentrated acids, the reaction is extremely violent and the mixture gets hot. This speeds up the process much more and the liquid within may foam. The process is significantly under control when the acid is diluted.
4. What purpose does a burette serve?
Ans: A burette is a titration tool made of glass. It is utilised for the regulated distribution of any volume of liquid within a specific range.
Viva Questions
1. Mention a few common indicators?
Ans: Methyl orange, phenolphthalein, starch, etc.
2. Define molarity.
Ans: The number of moles of a solute per litre of a solution is known as molarity. Another term for molarity is the molar concentration of a solution.
3. What is the molarity formula?
Ans: The molarity formula is \[{M_1}{V_1} = {M_2}{V_2}\]
4. How to calculate the molecular mass of HCl?
Ans: Molecular mass of HCl= Molar mass of H+ + Molar mass of Cl-=1+35.5=36.5g. Hence, the molar mass of HCl is 36.5g.
5. What is the equivalent mass of HCl?
Ans: The equivalent mass of HCl is 85.1g.
6. How to calculate the strength of the solution of HCl?
Ans: Volume of HCl employed to neutralise 0.05 M of 20 ml of Na2CO3 solution is given by:
The volume strength formula is
\[\frac{{{M_1}{V_1}(HCl)}}{{{M_2}{V_2}(N{a_2}C{O_3})}}=\frac{{Stoichiometriccoeffecientof(HCl)}}{{Stoichiometriccoeffecientof(N{a_2}C{O_3})}}\]
\[\frac{{{M_1} \times a}}{{0.05 \times 20}} = \frac{2}{1}\]
\[{M_1} = \frac{{2 \times 20 \times 0.05}}{a} = \frac{2}{a}\]
\[Strength{\rm{ }}of{\rm{ }}HCl = \frac{2}{a} \times 36.5g{L^{ - 1}}\]
7. What is the endpoint?
Ans: The end point of a titration is the point where the reaction is almost finished. Once a chemical process reaches its end point, it is considered to be complete.
8. What does the term "concordant readings" refer to?
Ans: Volumetric analysis measurements that vary by lesser than 0.05 mL are said to be concordant readings.
9. Why should a pipette never be held by its bulb?
Ans: The glass bulb could expand due to body heat, which would cause a measuring inaccuracy.
10. Why are burette and pipette rinsed using their solutions filled?
Ans: To get rid of anything that can stick to their edges and limit the amount of liquids they can hold, the burette and pipette are rinsed using the solutions they were loaded with.
Practical-Based Questions
What is the common name of solution in burette?
Analyte
Solvent
Titrant
None of the above
Ans: The solution used in the burette is the titrant.
What is the common name of the solution in a conical flask?
Analyte
Solvent
Titrant
None of the above
Ans: The solution used in the conical flask is analyte.
What is the unit of molarity?
M or Moles L-1
N or eq L-1
Mol per kg
None of the above option
Ans: The unit of molarity is M or Moles L-1.
Why rinsing a conical flask is not recommended?
Rinsing might cause the pipetted volume in conical flask to decrease
Rinsing might cause the pipetted volume in the conical flask to increase
Rinsing might cause the pipetted volume in the conical flask to precipitate
None of the three option
Ans: Rinsing might cause the pipetted volume in the conical flask to increase, hence it is not recommended.
What is the hydrochloric acid molar mass?
34.5g
35.5g
36.5g
37.5g
Ans: The hydrochloric acid molar mass is 36.5g.
What type of instrument is used in this experiment to add the last drop to avoid the extra drops in the solution while transferring the solution in a volumetric flask?
Dropper
Wash bottle
Pipette
None of the above three options
Ans: Pipette is used to avoid extra drops during solution preparation.
What type of acid is HCl?
Strong
Moderate
Weak
None of the above three options
Ans: HCl is a strong acid.
How much volume of H2SO4 is required for neutralization using phenolphthalein with 20mL of solution containing Na2CO3 and NaHCO3?
5mL
10mL
15mL
20mL
Ans: 10mL of H2SO4 is required for neutralization using phenolphthalein with 20mL of solution containing Na2CO3 and NaHCO3.
How is the strength of a particular solution determined?
Law of mass action
Equivalent law
Henry’s law
None of the three options
Ans: Equivalent law is used to determine the strength of a particular solution.
How does distilled water act in chemical laboratories?
Indicator
Solvent
Universal solvent
None of the three options
Ans: Distilled water acts as a universal solvent in chemical laboratories.
Conclusion
This chemistry experiment on the determination of strength and molarity of the given HCl solution with the given 0.05M Na2CO3 solution infers that the molarity of the given HCl solution is determined using the titration of given HCl against the 20mL of 0.05M Na2CO3 solution using methyl orange indicator.
It is prepared to utilize only pure chemicals, and standardisation can be done to determine the precise concentration. An acid-base titration technique centred on the back titration employing a standard base after the sample has dissolved in excess of standard acid.
FAQs on Determination of Strength and Molarity of the Given HCl Solution with the Given 0.05M Na2CO3 Solution
1. What are the most important calculations to determine the strength of an HCl solution from a titration with 0.05M Na₂CO₃ for the Class 11 exam?
To find the strength of the HCl solution, you must first calculate its molarity using the titration results. The key formula, based on the reaction 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂, is:
(M₁V₁ / n₁)HCl = (M₂V₂ / n₂)Na₂CO₃
Where:
- M₁ is the molarity of HCl (what you need to find).
- V₁ is the volume of HCl used from the burette (your concordant reading).
- n₁ is the stoichiometric coefficient of HCl, which is 2.
- M₂ is the molarity of the standard Na₂CO₃ solution (given as 0.05M).
- V₂ is the volume of Na₂CO₃ solution taken in the flask (e.g., 20 mL).
- n₂ is the stoichiometric coefficient of Na₂CO₃, which is 1.
Strength (g/L) = Molarity of HCl × Molar Mass of HCl (36.5 g/mol).
2. For the titration of HCl with Na₂CO₃, why is methyl orange considered an important and suitable indicator over phenolphthalein?
The choice of indicator is critical for accuracy. This titration involves a strong acid (HCl) and a weak base (Na₂CO₃). The equivalence point, where neutralisation occurs, lies in the acidic pH range (around 3.1 to 4.4).
- Methyl orange is the ideal indicator because its pH range of colour change (3.1-4.4) perfectly overlaps with the equivalence point of this specific reaction.
- Phenolphthalein, on the other hand, changes colour in the basic pH range (8.2-10). Using it would result in an incorrect endpoint, leading to an inaccurate calculation of the acid's strength, as the colour change would happen long after the true neutralisation point.
3. What are three essential precautions a student must take in the CBSE Class 11 practical exam to ensure accurate titration results?
To minimise errors and score well in the practical exam, focusing on these three precautions is crucial:
- Proper Rinsing: The burette must be rinsed with the HCl solution, and the pipette must be rinsed with the Na₂CO₃ solution. The conical flask should only be rinsed with distilled water, not the Na₂CO₃ solution, to avoid introducing excess moles of the analyte.
- Removing the Air Bubble: Always ensure that the air bubble trapped in the nozzle of the burette is removed before starting the titration. If present, it will lead to an incorrect, larger-than-actual volume reading for HCl.
- Endpoint Detection: Add the acid from the burette drop by drop when you are close to the endpoint. Place a white tile under the conical flask to observe the colour change from yellow to the faintest persistent pale pink accurately.
4. How does the concept of a 'primary standard' make Na₂CO₃ crucial for this experiment?
In titration, accuracy depends on having a solution of a precisely known concentration. Sodium Carbonate (Na₂CO₃) is used as a primary standard because it has several key properties:
- It is available in a highly pure form.
- It is not hygroscopic (does not absorb moisture from the air).
- It has a high molecular weight, which minimises weighing errors.
- It is stable and does not decompose under normal conditions.
5. Why is it mandatory to take at least three concordant readings in the exam, and what does the term signify?
Concordant readings are a set of two or more titration values (volume of HCl used) that are very close to each other, typically differing by no more than 0.1 mL. It is mandatory in the CBSE practical exam to repeat the titration until you get at least three such readings because it demonstrates precision and reliability. A single titration can be affected by random errors. Taking multiple, consistent readings ensures that your result is reproducible and not a fluke, thereby leading to a more accurate average value for your final calculation.
6. How does the 2:1 stoichiometry of the HCl and Na₂CO₃ reaction directly impact the final molarity calculation?
The balanced chemical equation is 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂. This equation shows that two moles of HCl are required to neutralise one mole of Na₂CO₃. This 2:1 molar ratio is the most critical factor in the calculation. When using the molarity equation M₁V₁/n₁ = M₂V₂/n₂, the stoichiometric coefficients (n) must be correctly substituted. Here, n₁ (for HCl) is 2 and n₂ (for Na₂CO₃) is 1. Forgetting to include this 2:1 ratio is a common mistake that would lead to the calculated molarity of HCl being exactly half of the correct value.
7. What is the fundamental difference between the 'equivalence point' and the 'endpoint' in this acid-base titration?
While often used interchangeably, these terms have distinct meanings critical for understanding titration accuracy:
- The equivalence point is a theoretical point in the titration where the amount of titrant (HCl) added is chemically equivalent to the amount of analyte (Na₂CO₃) in the flask, according to the stoichiometry of the reaction. It is the point of complete neutralisation.
- The endpoint is the practical, observable point where the indicator (methyl orange) changes colour.
