Question

Your boss says, "Prepare \(0.5000 \mathrm{~kg}\) of a \(1.00 \%\) solution of hexane \(\left(\mathrm{C}_{6} \mathrm{H}_{14}\right.\), a liquid) in dichloromethane \(\left(\mathrm{CH}_{2} \mathrm{Cl}_{2}\right.\), also a liquid). \({ }^{\prime}\) (a) What is wrong with his request? (b) How would you prepare this solution if he meant a \(1.00 \%\) by mass solution?

Short answer

(a) There is nothing wrong with the request as hexane and dichloromethane are compatible and can form a solution. (b) In order to prepare a 1.00% w/w solution with a total mass of 500g, 5g of hexane is needed. Weigh out 5g hexane and add it to 495g of dichloromethane, making sure to mix thoroughly for a homogeneous mixture.

Step-by-step solution

Analyzing the Initial Request

It's necessary to first identify if the request to make a 1.00% solution of hexane in dichloromethane is valid. Hexane and dichloromethane are both non-polar substances and thus compatible to form a solution, hence the request is needlessly invalid.

Understanding w/w Solutions

Weight-weight solutions are expressed as a percentage of the mass of solute in the total mass of the solution. In a 1.00% w/w solution of hexane in dichloromethane, every 100g of solution contains 1g of hexane. Here, the solution you are asked to prepare must have a total mass of 0.5000 kg or 500g.

Calculation of Mass of Solute and Solvent

In a 1.00 % w/w solution, the mass of hexane (solute) per 100g of solution will be 1g. Therefore, in 500g of solution, the mass of hexane will be \(5 \times 1 = 5g\) (we multiply by 5 as 500g is 5 times 100g). Thus, to prepare this solution, the mass of hexane needed is 5g. Since the total mass of the solution has to be 500g, and we have the amount of hexane to be 5g, we can subtract the mass of hexane from the total mass of the solution to find the mass of dichloromethane (solvent) needed, which is \(500g - 5g = 495g\).

Preparation of Solution

Now, to prepare the solution, weigh out 5g of hexane. Add this to sufficient dichloromethane so that the total mass of the solution equals to 500g (ensure that adding the hexane doesn't cause the mass to exceed 500g). It's important to stir the solution thoroughly to ensure a homogeneous mixture. The result will be a 1.00% w/w solution of hexane in dichloromethane.

Key concepts

Weight-Weight Solutions

When we talk about weight-weight (w/w) solutions, we are referring to the concentration of the solution in terms of the mass of solute in every 100 grams of the total solution. This is a common way to express concentration in chemistry. For example, a 1.00% w/w solution means that there is 1 gram of solute (the substance being dissolved) in every 100 grams of the solution. In practical terms, if you were tasked with making 500 grams of a 1.00% w/w solution of hexane in dichloromethane, it would mean adding 5 grams of hexane to sufficient dichloromethane such that the total mass reaches 500 grams.
This ensures the correct percentage of mass for the solute is maintained.
Understanding this concept is crucial when preparing accurate and precise solutions in the laboratory.

Mass Percentage

Mass percentage is used extensively in chemistry to describe the concentration of a component in a mixture or solution. It is calculated by dividing the mass of the solute by the total mass of the solution, and then multiplying by 100 to get a percentage.
In the exercise, the boss asks for a 1.00% solution of hexane in dichloromethane. This means that hexane should make up 1.00% of the total mass of the solution.
To better understand mass percentage, remember these points:

• It's a simple ratio expressed as a percentage.
• The formula used is: \(\text{Mass Percentage} = \left( \frac{\text{Mass of Solute}}{\text{Total Mass of Solution}} \right) \times 100\% \).
• The solution has to be homogeneous for the percentage to accurately reflect the concentration throughout the solution.

This concept is especially important in solution chemistry for calculations regarding solute and solvent quantities.

Homogeneous Mixture

A homogeneous mixture is a type of mixture where the components are uniformly distributed throughout the solution. This uniform distribution ensures that every portion of the mixture has the same composition.
In the preparation of solutions, achieving a homogeneous mixture is essential to ensure the concentration of the solute remains consistent throughout. If your hexane were to clump or remain undissolved in a clump rather than dispersing throughout the dichloromethane, you wouldn't have a true 1.00% solution.
This can be highlighted by the need to properly stir the solution after combining the hexane and dichloromethane. By doing so, we guarantee that the solute is evenly distributed within the solvent, ensuring that the properties of the solution remain consistent throughout.
Homogeneous mixtures are significant in both industrial applications and academic settings where solution consistency is required.

Non-Polar Substances

Non-polar substances have a balanced distribution of electrical charge, meaning they do not have significant positive or negative sides. This is why such substances dissolve well in non-polar solvents, thanks to the classic 'like dissolves like' principle in chemistry.
Both hexane and dichloromethane are non-polar, which results in hexane dissolving readily in dichloromethane without separation. This compatibility is what allows you to form a stable solution, which is necessary for the task mentioned in the exercise.
Non-polar substances are so named because they lack a dipole moment - their molecules don’t have a charged end. This absence of poles aids in dissolving and mixing processes, creating stable solutions that wouldn’t be feasible with polar solvents. Understanding the nature of non-polar substances aids in predicting solubility and interactions between different chemical substances.