Class 12 Biology PBA: Solution Preparation & Glucose Investigation | FBISE Federal Board

Preparation and Dilution of Solutions

This topic covers the essential skills for preparing specific concentrations of reagents and performing dilutions for laboratory experiments.

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1. Solution Preparation

• Concentration Types: Solutions are typically prepared and specified in two ways:
  • Percentage Concentration: Expressed as % (w/v), representing the weight of solute per volume of solvent.
  • Molar Concentration: Expressed in mol dm⁻³ (molarity).
• Reagent Management: It is necessary to determine and describe suitable volumes and concentrations of reagents required for specific procedures.

2. Dilution Methods

To obtain a desired concentration from a stronger stock solution, two primary methods are used:

1. Simple (Proportional) Dilution: Preparing a specific concentration by mixing a known volume of stock solution with a known volume of solvent.
2. Serial Dilution: A sequence of stepwise dilutions where the concentration decreases by the same factor at each step (e.g., a ten-fold or two-fold dilution series).

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3. Investigating Reducing Sugars

• Objective: To determine the concentration of a reducing sugar, specifically glucose.
• Methodology: Use the Benedict’s Test in conjunction with a colorimeter.
• Key Requirement: This investigation requires the preparation of different sugar concentrations using the dilution techniques mentioned above to create a standard for comparison.

Practical Laboratory Procedures

Based on SLO-B-12-X-02, these procedures detail the steps for reagent preparation, dilution, and quantitative sugar analysis.

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1. Preparation of % (w/v) Solutions

This method is used to prepare a solution where a solid solute is dissolved in a liquid solvent.

1. Calculation: Determine the mass required. For a 1% (w/v) solution, you need 1g of solute for every 100ml of final solution.
2. Weighing: Use a digital balance to weigh the exact amount of glucose or solute.
3. Dissolving: Transfer the solute to a beaker and add a small volume of distilled water (less than the final volume). Stir until the solid is fully dissolved.
4. Volume Adjustment: Pour the solution into a volumetric flask. Add distilled water until the bottom of the meniscus reaches the graduation mark.
5. Homogenization: Stopper the flask and invert it several times to ensure the concentration is uniform.

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2. Dilution Techniques

To obtain lower concentrations from a "Stock" solution, use one of the following:

A. Simple (Proportional) Dilution

• Mix a calculated volume of stock solution with a specific volume of solvent in a single step.
• Example: To make a 50% dilution, mix 5ml of stock with 5ml of distilled water.

B. Serial Dilution

• A stepwise process where each tube is a dilution of the previous one.
• Step 1: Add 9ml of solvent to four labeled test tubes.
• Step 2: Transfer 1ml of stock to Tube 1 (10^-1 dilution).
• Step 3: Transfer 1ml from Tube 1 to Tube 2 (10^-2 dilution).
• Step 4: Repeat until the desired range is reached.

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3. Investigating Glucose Concentration

This experiment combines the Benedict’s Test with Colorimetry to quantify reducing sugars.

1. Standard Preparation: Use serial dilution to create a range of known glucose concentrations (e.g., 0.2%, 0.4%, 0.6%, 0.8%, 1.0%).
2. Reaction: Add 2ml of Benedict’s reagent to each tube. Heat in a boiling water bath for 5 minutes.
3. Filtration: If a precipitate forms, filter the solution or use a centrifuge to obtain a clear liquid for the colorimeter.
4. Colorimetry:
   • Calibrate the colorimeter using a "blank" (distilled water).
   • Measure the absorbance or transmission of each known concentration.
5. Calibration Curve: Plot the results on a graph (Concentration on X-axis, Absorbance on Y-axis).
6. Analysis: Test your "Unknown" sample, record its absorbance, and use the curve to find its exact concentration.

Exam Practice: Short Questions & Answers

Based on SLO: B-12-X-02 (iv) & (v)

No.	Question	Answer
1	Define % (w/v) concentration.	It refers to the mass of a solid solute (in grams) dissolved in 100 ml of a liquid solvent.
2	What is the standard unit for molar concentration?	The standard unit is mol dm⁻³.
3	What is a stock solution?	A concentrated solution of known concentration that is used to prepare weaker (diluted) solutions.
4	Define simple dilution.	A method where a specific volume of stock is added to a solvent to reach a final desired concentration in one step.
5	What is serial dilution?	A stepwise dilution of a substance in solution where the dilution factor is kept constant at each step.
6	Which instrument is used to measure the intensity of color in the Benedict's test?	A colorimeter.
7	What is the primary objective of investigating glucose via Benedict’s test in this SLO?	To investigate and determine the concentration of the reducing sugar.
8	Why is glucose called a reducing sugar?	Because it has the ability to reduce the copper(II) ions in Benedict's reagent to copper(I) oxide.
9	When preparing reagents, what two factors must be appropriately described?	Suitable volumes and concentrations.
10	How do you prepare a 1% (w/v) glucose solution?	By dissolving 1 gram of glucose in enough distilled water to make a final volume of 100 ml.
11	What is proportional dilution?	Another term for simple dilution where components are mixed in a specific ratio to achieve a target concentration.
12	If you dilute 1 ml of stock into 9 ml of solvent, what is the dilution factor?	This is a 1:10 dilution.
13	In a serial dilution, if each step is a 10-fold dilution, what is the concentration of the 3rd tube relative to the stock?	It would be 0.001 (or 10⁻³) of the original concentration.
14	What determines the concentration in a colorimeter reading?	The amount of light absorbed (absorbance) or transmitted (transmission) by the colored solution.
15	What type of solutions are mentioned in SLO-02 (iv)?	Weight/Volume solutions (% or molar).
16	Can concentrations be specified in mol dm⁻³?	Yes, the text explicitly lists mol dm⁻³ as a required specification.
17	What is the role of the solvent in dilution?	The solvent is used to decrease the concentration of the solute without changing the total amount of solute present.
18	Why is serial dilution preferred over simple dilution for very low concentrations?	It is more accurate for measuring very small volumes and reduces the risk of pipetting errors.
19	What is the relationship between sugar concentration and Benedict's test color?	Higher concentrations produce a more intense color change (from blue to green, yellow, orange, or brick-red).
20	What must a student be able to "describe" according to SLO-02 (v)?	How different concentrations are prepared using serial or proportional dilution.

FBISE Exam-Style MCQs: Biology Practical (SLO-B-12-X-02)

No.	Question Statement	Correct Option
1	According to the SLO, what is the standard unit for molar concentration?	mol dm⁻³
2	Which method is used for making a series of stepwise dilutions from a stock solution?	Serial dilution
3	The concentration expressed as % (w/v) specifically refers to:	Weight/Volume
4	Which instrument is required to investigate the concentration of glucose in Benedict's test?	Colorimeter
5	In the context of the Benedict’s test, glucose is categorized as a:	Reducing sugar
6	To prepare a specific concentration in one single step from a stock, we use:	Proportional dilution
7	What is the primary purpose of a colorimeter in sugar analysis?	Measuring concentration
8	Which of the following is a type of solution mentioned in SLO:B-12-X-02 (iv)?	Molar solution
9	In a dilution, the solution with the highest initial concentration is called the:	Stock solution
10	If a student needs to describe reagent preparation, they must specify:	Volume and Concentration
11	Serial dilution is best described as:	Stepwise dilution
12	Which reagent is used specifically to test for reducing sugars?	Benedict’s reagent
13	Simple dilution is also referred to in the SLO as:	Proportional dilution
14	The unit mol dm⁻³ is used to represent:	Molarity
15	When using a colorimeter, what is actually being investigated?	Light absorbance/transmission
16	How many grams of solute are in 100ml of a 5% (w/v) solution?	5g
17	What remains constant in each step of a "serial" dilution?	Dilution factor
18	Why are multiple concentrations of glucose prepared for this experiment?	To investigate concentration levels
19	The SLO B-12-X-02 (iv) focuses on the preparation of:	Weight/Volume solutions
20	Which dilution method is most practical for achieving extremely low concentrations?	Serial dilution