Questions and Answers
What is the substance known as when it is dissolved in a solution?
Which of the following steps is NOT part of the solution preparation process?
How is molarity defined?
Which characteristic describes a concentrated solution compared to a diluted solution?
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What is the primary purpose of rinsing the beaker with solvent during solution preparation?
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What is the molar mass of sodium chloride (NaCl)?
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What weight of NaCl is required to prepare 50 ml of a 0.5 M solution?
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How is normality (N) defined?
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What weight of sodium carbonate (Na2CO3) is needed to prepare a 0.5 N solution in 50 ml?
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What is the equivalent weight of a compound?
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Study Notes
Objectives of Practical Laboratory
- Define solutions and their preparation methods.
- Understand concentration and expression methods.
- Learn dilution techniques for prepared solutions.
Definitions
- Solute: Substance that dissolves in a solvent (usually solid).
- Solvent: Substance that dissolves the solute, typically a liquid like water or alcohol; must be greater than 50% of the solution.
- Solution: Homogeneous mixture of two or more substances.
Solution Preparation Steps
- Weigh the appropriate solute and add it to a beaker.
- Add sufficient solvent and stir until the solute dissolves completely.
- Transfer the solution to a volumetric flask, rinsing the beaker with additional solvent.
- Add more solvent until the final calibrated volume is achieved.
- Stopper the flask, shake well, and label appropriately.
Concentration Definitions
- Concentration refers to the amount (mass/volume) of solute within a specified amount (mass/volume) of solution.
Methods to Express Concentration
Qualitative Methods
- Concentrated solutions are darker, stronger in taste/odor, and have more solute and less solvent.
- Diluted solutions are lighter in color, milder in taste/odor, and contain less solute and more solvent.
Quantitative Methods
- Concentrations commonly expressed as molarity, normality, and percentage.
Molarity (M)
- Defined as moles of solute per liter of solution (mol/L).
- Requires the molecular weight of the solute for calculations.
- Example: To prepare 50 ml of 0.5 M NaCl (m.w. NaCl = 58.5 g/mol), weigh 1.46 g of NaCl and dissolve in water to 50 ml.
Normality (N)
- Indicates the number of gram equivalent weights of solute per liter of solution.
- Equivalent weight is the molecular weight divided by the number of replaceable hydrogen or hydroxyl ions.
- Example: To prepare 50 ml of 0.5 N Na₂CO₃ (m.w. = 106 g/mol), weigh 1.325 g of Na₂CO₃ and dissolve in water to 50 ml.
Percentage Concentration Expressions
- Percent weight/volume (w/v): grams of solute in 100 ml of solution.
- Percent volume/volume (v/v): milliliters of solute in 100 ml of solution.
- Percent weight/weight (w/w): grams of solute in 100 g of solution/mixture.
Percent Example
- 0.9% NaCl solution contains 0.9 g of sodium chloride in 100 ml of solution.
- 5% v/v methanol solution involves 5 ml of methanol in 100 ml.
Dilution and Concentration Law
- To dilute a concentrated solution: pipette an appropriate amount of stock solution into a volumetric flask, adding solvent up to the mark.
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Dilution Law: C1V1 = C2V2
- C1: initial concentration (stock solution).
- C2: final concentration (desired solution).
- V1: initial volume from stock.
- V2: final volume desired.
Example of Dilution
- To prepare 250 ml of 4% w/v glucose from 20% w/v glucose, measure 5 ml of 20% solution and dilute with distilled water to reach 250 ml.
Experimental Work
- Prepare the following solutions using a volumetric flask:
- 100 ml of 0.5 M NaCl
- 100 ml of 2 N NaCl
- 100 ml of 0.1 N Na₂CO₃
- 100 ml of 0.1 M Na₂CO₃
- 100 ml of 2% w/v NaCl
- 100 ml of 5% v/v Alcohol
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Description
This quiz focuses on the practical aspects of Pharmaceutical Calculations for second-year pharmacy students. It covers the definitions and preparation of solutions and concentration expressions, as well as the methods for diluting solutions and mixtures. Test your understanding of these essential lab skills!
