Problem 65 - Chemistry Matter and Change

Question

How do 0.5\(M\) and 2.0\(M\) aqueous solutions of NaCl differ?

Step-by-Step Solution

Answer

A 2.0M solution of NaCl is more concentrated than a 0.5M solution, affecting boiling point, freezing point, and osmotic pressure.

Step 1: Understanding Molarity

Molarity (M) is defined as the number of moles of solute (in this case, NaCl) per liter of solution. It provides a measure of the concentration of the solute in the solution.

Step 2: Comparing Concentrations

For a 0.5M NaCl solution, there are 0.5 moles of NaCl per liter of solution. For a 2.0M NaCl solution, there are 2.0 moles of NaCl per liter of solution. This indicates that the 2.0M solution is more concentrated than the 0.5M solution.

Step 3: Physical Properties

Because the 2.0M solution has a higher concentration of solute than the 0.5M solution, the 2.0M solution will likely have a higher boiling point, a lower freezing point, and a higher osmotic pressure than the 0.5M solution due to colligative properties.

Key Concepts

Concentration

Concentration is a way to express how much of a solute is present in a solution. Imagine you are mixing sugar in water. The more sugar you add to a fixed amount of water, the more concentrated the solution becomes.
In chemistry, concentration is often expressed as molarity, denoted by the symbol M. Molarity is the number of moles of solute divided by the volume of the solution in liters. In our exercise, NaCl is the solute, and water is the solvent.
A 0.5M NaCl solution means you have 0.5 moles of NaCl dissolved in one liter of water, whereas a 2.0M solution contains 2.0 moles of NaCl in the same volume. It's crucial to recognize that higher molarity means more solute in the same amount of solvent, making the solution more concentrated. This difference in concentration affects how the solution behaves in various scenarios.

Colligative properties

Colligative properties are fascinating because they depend on the number of solute particles in a solution, not the identity of those particles. Think of it as the collective impact of all particles present.

• Boiling Point Elevation: Adding solutes increases a solution's boiling point compared to pure solvent. More particles mean there's more hindrance for the solvent molecules trying to escape to the air as vapor.
• Freezing Point Depression: Similarly, adding solutes lowers the freezing point. The presence of solute particles disrupts the formation of a solid structure, so the solution needs to be colder to freeze.
• Osmotic Pressure: This is the pressure needed to prevent the flow of solvent into the solution through a membrane. More particles result in higher osmotic pressure.

The key takeaway here is understanding that a 2.0M NaCl solution will have a higher boiling point, lower freezing point, and greater osmotic pressure compared to a 0.5M NaCl solution due to its higher concentration of particles, not because NaCl behaves differently at higher concentrations.

Aqueous solutions

Aqueous solutions are mixtures where water is the solvent. These are crucial in both natural processes and industrial applications. Water's properties make it an excellent solvent for a wide range of compounds, including salts like NaCl.
In our exercise context, NaCl dissolves in water to form an aqueous solution. Once dissolved, NaCl separates into its ions (Na⁺ and Cl⁻), allowing water molecules to easily interact with them.
The level of interaction depends on the concentration. For instance, in a 2.0M NaCl solution, there's a higher density of ions, enhancing the solution's interaction with water.
This comprehensive mixing at the molecular level is why aqueous solutions are so essential. From biological fluids in living organisms to cleaning agents and food processing in industries, aqueous solutions play a key role due to water's ability to dissolve a wide array of substances.