Practice 3: Solution Preparation

 This laboratory practice involved preparing two solutions of known concentration, observing the results, and learning the standard procedures for solution preparation. One solution was prepared using a solid solute (copper sulfate pentahydrate), while the other was prepared from a solution of known concentration (dilute hydrochloric acid).

Materials Used:

• Beakers: Used as containers for preparing the solutions. In the first experiment, a beaker was used to dissolve copper sulfate pentahydrate.

• Watch glass: Used to hold and transfer copper sulfate pentahydrate before mixing with water.

• Stirring rod: Used to mix copper sulfate pentahydrate with water during dissolution.

• Laboratory spatula: Used to accurately handle copper sulfate pentahydrate in the first experiment.

• Balance: Used to weigh the required amount of copper sulfate pentahydrate.

• Volumetric flask: Used for the first solution to contain the copper sulfate solution and add water to the required volume.

• Graduated cylinder: Used in the second experiment to measure water before adding hydrochloric acid.

• Pipette and pipette filler: Used to extract dilute hydrochloric acid and add it to the water in the second solution.

• Copper sulfate pentahydrate (CuSO₄·5H₂O): Solid solute used in the first solution.

• Dilute hydrochloric acid (HCl): Stock solution used in the second solution.

Key Concepts:

Solution:
A solution is a homogeneous mixture of two or more components, where each component mixes thoroughly with the others, losing its individual characteristics. The constituents become indistinguishable, and the mixture exists in a single well-defined phase (solid, liquid, or gas). A solution containing water as the solvent is called an aqueous solution. The composition of a solution remains constant throughout any sample.

In this context, the term "solution" or "dissolution" refers specifically to homogeneous mixtures in the liquid phase. Solid-phase homogeneous mixtures (e.g., alloys like steel, bronze, brass) or gas-phase mixtures (e.g., air, smoke) are not typically referred to as solutions. Salts, acids, and bases ionize when dissolved in water.

Characteristics of Solutions:
I) Components cannot be separated by simple physical methods such as decantation, filtration, or centrifugation.
II) Components can only be separated by distillation, crystallization, or chromatography.
III) The components of a solution are the solute and the solvent.

• Solute: The component present in smaller quantity, which dissolves in the solvent. Solutes can be solid, liquid, or gas (e.g., carbon dioxide in carbonated beverages).

• Solvent: The component present in larger quantity, which dissolves the solute. The solvent is the phase in which the solution exists. Although solvents can be gases, liquids, or solids, water is the most common solvent.

IV) In a solution, the solute and solvent interact at the molecular or ionic level, explaining the homogeneous nature of solutions and the inability to separate components by mechanical methods.

Concentration Levels:
Depending on their concentration, solutions are classified as dilute, concentrated, saturated, or supersaturated:

• Dilute: A small amount of solute relative to the solvent (e.g., 1 g of table salt in 100 g of water).

• Concentrated: A large proportion of solute relative to the solvent (e.g., 25 g of table salt in 100 g of water).

• Saturated: A solution that cannot dissolve more solute at a specific temperature (e.g., 36 g of table salt in 100 g of water at 20°C).

• Supersaturated: A solution containing more solute than allowed at a specific temperature, achieved through rapid cooling or sudden decompression (e.g., opening a carbonated drink bottle).

Concentration in mol/L:
The concentration of a solution can be expressed in various ways, with the two most common being grams per liter (g/L) and molarity (M):

• Grams per liter (g/L): Represents the mass of solute (in grams) contained in one liter of solution. For example, a sodium chloride solution with a concentration of 40 g/L contains 40 g of NaCl per liter of solution.

• Molarity (M): Defined as the amount of solute (in moles) per liter of solution: M = n/V. The number of moles of solute is calculated as the mass of solute divided by its molar mass. For example, a 2.5 M solution contains 2.5 moles of solute per liter of solution. Note that molarity considers the final volume of the solution, not the volume of the solvent.

PART 1: Preparation of a 0.01 mol/L Copper Sulfate Pentahydrate Solution

The objective was to prepare one liter of a copper sulfate pentahydrate (CuSO₄·5H₂O) solution in water. The steps were as follows:

1. Mass Calculation:
   The number of moles was calculated using the formula:
   $c=\frac{n}{V}$ → $n=c\cdot V=0.01\text{mol/L}\times 1\text{L}=0.01\text{mol}$.
   The mass was then determined using:
   $n=\frac{m}{M}$ → $m=n\cdot M=0.01\text{mol}\times 250\text{g/mol}=2.5\text{g}$.
   (Molar mass of CuSO₄·5H₂O = 250 g/mol)

2. Weighing:
   The calculated mass of copper sulfate was weighed on a watch glass using a balance.

3. Dissolution:
   The copper sulfate was transferred to a beaker, and 100 mL of water was added to dissolve it, with stirring.

4. Transfer and Dilution:
   The solution was poured into a 1 L volumetric flask and diluted to the mark. A dropper was used for precise dilution to achieve the meniscus as shown in Figure A.

PART 2: Preparation of a 0.1 mol/L Hydrochloric Acid Solution

The second solution was prepared from a 1 mol/L hydrochloric acid stock solution to obtain 50 mL of a 0.1 mol/L HCl solution.

1. Volume Calculation:
   The number of moles was determined using:
   $C=\frac{n}{V}$ → $0.1\text{mol/L}=\frac{n}{0.05\text{L}}$ → $n=0.005\text{mol}$.
   The volume of stock HCl required was calculated as:
   $V=\frac{n}{C}=\frac{0.005\text{mol}}{1\text{mol/L}}=0.005\text{L}=5\text{mL}$.

2. Water Measurement:
   25 mL of water was measured using a graduated cylinder.

3. Acid Addition:
   A pipette and pipette filler were used to transfer 5 mL of HCl into the graduated cylinder containing water. The acid was added second to prevent splashing, as HCl is corrosive.

4. Final Steps:
   The solution was mixed thoroughly to tilerummy ensure homogeneity.