Aqueous Solutions

This guide will describe the process for preparing aqueous solutions, including how to calculate the appropriate amount of solute for a given volume of solution, how to prepare a complex solution with two or more solutes, and how to dilute a solution—including serial dilutions. 

What is an Aqueous Solution?

An aqueous solution is a mixture in which water acts as the solvent, dissolving a solute to form a homogeneous solution. The solute can be a solid, liquid, or gas, and it is evenly distributed throughout the water, making the solution uniform in composition. Aqueous solutions are common in laboratory and industrial settings due to water's excellent solvent properties, allowing it to dissolve a wide variety of substances. Examples include saltwater (sodium chloride dissolved in water) and sugar water (sucrose dissolved in water).

What You Will Need to Make an Aqueous Solution:

• Personal Protective Equipment (PPE): Necessary for safety, including gloves, lab coat, and eye protection to protect against splashes or exposure.
• Purified Water: Ensures that impurities do not affect the solution's accuracy or reactions.
• Solutes: The substances being dissolved, are critical for creating the desired solution.
• Analytical Balance: Provides precise measurements of solutes to ensure accurate concentrations.
• Weigh Boats: Used for containing solutes during weighing to prevent spillage and contamination.
• Graduated Cylinder: Measures approximate volumes of liquids, useful for less precise volume requirements.
• Volumetric Flask: Ensures precise final solution volumes for accurate dilutions.
• Beaker: Ideal for mixing and dissolving solutes before transferring them to a volumetric flask.
• Magnetic Stir Plate and Stir Bar: Facilitates even mixing of solutes in the solvent.
• pH Meter: Monitors and adjusts the pH of the solution as needed for specific applications.
• Dilute Sodium Hydroxide: Used to adjust the pH upwards if necessary.
• Dilute Hydrochloric Acid: Used to adjust the pH downwards when required.

How to Make an Aqueous Solution 

Step 1: Calculate the Mass of the Compound Needed to Achieve a Specific Molar Concentration and Volume

Determine the desired concentration and final volume of the solution. If concentration will be reported in molarity or normality, the molecular weight or formula weight of the substance is required (this indicates how many grams of the substance are in one mole). This information can usually be found on the container.

This formula is used to calculate the mass of the compound needed to achieve a specific molar concentration and volume:

Mass (g) = concentration (mol/L) x volume (L) x molecular weight (g/mol)

Example: Prepare 1L of a 5M solution of a substance with a molecular weight of 50 g/mol. 

Mass (g) = concentration (mol/L) x volume (L) x molecular weight (g/mol)

Mass (g) = 5 x 1 x 50

Mass (g) = 250

The solution in this example requires 250g of solute.

Do not add 1 L of water to 250g of solute. This will exceed the desired total volume.

Add the solute, then add water to bring the total volume up to 1L (see below).

Step 2: Weigh and Measure the Solute

1. Using a balance and weigh boat, weigh the amount of solute calculated in step 1.
2. Dispense purified water into a graduated cylinder—obtain approximately three-quarters of the final volume of solution. For example, for a solution with a final volume of 1L, obtain approximately 750 ml of water.

Always use purified water when preparing aqueous solutions. Tap water can contaminate the solution and compromise subsequent experiments.

Step 3: Combine the solute and solvent

1. Place a magnetic stir bar into a beaker, and place the beaker on a magnetic stir plate.
2. Add the purified water from the graduated cylinder.
3. Begin stirring the water and add the solute to the beaker.

Step 4: Adjust the pH of the Solution 

1. Once the solute is dissolved, measure the pH using a pH meter, and adjust if necessary.
2. Add dilute sodium hydroxide to increase pH, or dilute hydrochloric acid to decrease pH.
3. Add NaOH or HCL slowly, as pH can change rapidly.

Step 5: Bring the solution to final volume

1. Using a funnel, pour the solution from the beaker into a volumetric flask.
2. Bring the solution to the final volume by topping it up with purified water.
3. To "q.s. the solution" means to bring the solution up to the final volume.

The term "q.s." is the abbreviation of the Latin term quantum satis, meaning "as much as is enough."

How to Make a Complex Solution

A complex solution contains more than one solute. To make a complex solution, repeat Step 1 for each solute. Weigh each solute individually, then complete steps 2-5.

How to Dilute an Aqueous Solution

If the amount of solute required for a solution is too small to accurately weigh, you can make a concentrated stock solution and dilute it for use.

This formula is used to calculate the volume of stock solution and buffer (solvent) required to achieve the desired volume and concentration:

Concentration _(stock) x Volume _(stock) = Concentration _(dilute) x Volume _(dilute)

Example: Using a 5M stock solution, prepare 100 ml of 0.5M solution.

Concentration _(stock) x Volume _(stock) = Concentration _(dilute) x Volume _(dilute)

5M x Volume_(stock) = 0.5M x 100ml  

Volume_(stock) = (0.5Mx100ml)/5M

Volume_(stock) = 50ml/5

Volume_(stock) = 10ml

Add 10ml of 5M stock solution to a container and top up to 100ml with water.

Dilutions for Complex Solutions

Complex solutions may also be made from more than one stock solution. To dilute a complex solution with more than one solute, treat each dilution individually. Using the equation above, calculate the volume of each stock solution individually then add them to the container and top up the container to the final volume. 

What are Serial Dilutions? 

Serial dilutions are stepwise dilutions of substances in a solution. They may be used to reduce the risk of error associated with measuring very small volumes (for example, a serial dilution may be used rather than pipetting 1 µl of the substance in solution). Serial dilutions may also be required to create standard curves or for other experiments. 

How to Perform a Serial Dilution

Example: Obtain 1mM of a 1M compound in a 1 ml solution.

Step 1: Calculate the dilution factor 

You can calculate the dilution factor with the following equation: 

Dilution factor = (Volume_(initial) + Volume_(dilute))/Volume_(initial)

Begin with an initial volume of 100 µl of concentrated solution (1 M) and 900 µl of diluent.

Dilution factor = (Volume_(initial) + Volume_(dilute))/Volume_(initial)

Dilution factor = (100 + 900)/100

Dilution factor = 10

Step 2: Calculate the number of dilution steps

Determine the number of dilutions required to achieve the desired final concentration (x).

(Dilution factor)^x = Concentration_(initial)/ Concentration_(final)

10^x = 1/10^-6   

x = 6

10⁶ is used because Concentration_(stock) is in M and Concentration_(final) is in mM.

1M = 1000000mM

Step 3: Label microtubes

Label 6 microtubes as follows:

C/10                C/10⁴
C/100              C/10⁵
C/10³                   C/10⁶

Step 4: Pipette the diluent and stock solution

1. Pipette 900 µl of diluent into each tube.
2. Pipette 100 µl of 1M stock solution into tube “C/10” and vortex the tube.
3. Pipette 100 µl from tube “C/10” into tube “C/100” then vortex the tube.
4. Repeat for the remaining tubes.

Tube “C/10⁶” now has a final concentration equal to the stock concentration (1 M) divided by 10⁶.

Tips for Success

• Labeling: Always label solutions with concentration, date, and any hazards.
• Mixing: Ensure thorough mixing to avoid concentration gradients.
• Storage: Store solutions in appropriate containers to prevent contamination or degradation.

Conclusion

Mastering the preparation and dilution of aqueous solutions is essential for effective laboratory work. By following these steps, you can ensure accuracy and consistency, enabling reliable experimental results.