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To grasp the concept of osmotic pressure in solutions, it is imperative to understand molarity calculation. Molarity is defined as the number of moles of solute dissolved in one liter of solution. It is crucial because it determines how much of a substance is present in a given volume, influencing various properties, including osmotic pressure.
In this exercise, we calculated the molarity of two solutions: Solution A, containing magnesium chloride (\( \text{MgCl}_2 \)), and Solution B, containing sodium chloride (\( \text{NaCl} \)).
Let's break it down:

• For Solution A: Given 7 g of \( \text{MgCl}_2 \) per liter, its molar mass is approximately 95.21 g/mol. Thus, the molarity is the number of moles per liter, calculated as \( \frac{7}{95.21} \approx 0.0735 \text{ M} \).
• For Solution B: With 7 g of \( \text{NaCl} \) per liter, and a molar mass of about 58.44 g/mol, the molarity becomes \( \frac{7}{58.44} \approx 0.1197 \text{ M} \).

This calculation is foundational to understanding how these ions behave in the solutions, affecting their respective osmotic pressures.

The van't Hoff factor (\( i \)) is a crucial component when considering solutions' colligative properties, such as osmotic pressure. It signifies the number of particles into which a solute dissociates in a solution.
For ionic compounds like \( \text{MgCl}_2 \) and \( \text{NaCl} \), determining \( i \) helps in comprehending how many ions are present in the solution. This directly impacts properties like osmotic pressure.
Here's how \( i \) relates to our solutions:

• For \( \text{MgCl}_2 \), which dissociates completely into three ions (one \( \text{Mg}^{2+} \) and two \( \text{Cl}^- \)), the van’t Hoff factor is 3.
• For \( \text{NaCl} \), dissociating into two ions (one \( \text{Na}^+ \) and one \( \text{Cl}^- \)), the van’t Hoff factor is 2.

Understanding \( i \) is essential, as the more ions a compound breaks into upon dissolving, the more it affects colligative properties like osmotic pressure.

Osmotic pressure is a vital property of solutions, especially when comparing solutions with different solutes. It is defined as the pressure required to stop the flow of solvent molecules through a semipermeable membrane from a region of low solute concentration to high solute concentration.
To calculate osmotic pressure (\( \Pi \)), the formula used is \( \Pi = i \times M \times R \times T \), where:

• \( i \) is the van’t Hoff factor, representing the number of particles the solute dissociates into.
• \( M \) is the molarity of the solution.
• \( R \) is the ideal gas constant, approximately 0.0821 L·atm/mol·K.
• \( T \) is the temperature in Kelvin.

In our example:

• For Solution A (\( \text{MgCl}_2 \)): \( \Pi_A = 3 \times 0.0735 \times R \times T = 0.2205 \times R \times T \)
• For Solution B (\( \text{NaCl} \)): \( \Pi_B = 2 \times 0.1197 \times R \times T = 0.2394 \times R \times T \)

Comparing these values indicates that Solution B has a higher osmotic pressure than Solution A. This is because Solution B has a larger molarity, despite having a lower van’t Hoff factor. The formula highlights how both the concentration and dissociation of solutes impact the overall osmotic pressure of a solution.