PREPARATION OF SOLUTIONS: HOW TO DO IT, EXAMPLES, EXERCISES - CHEMISTRY - 2025

The preparation of solutions is one of the most developed activities within and outside of experimental science, especially with regard to chemistry, biology, bioanalysis, medicine and pharmacy. In the commercial sphere, many of the products we buy, whether they are food or for bathrooms, consist of aqueous solutions.

A solution in simple terms is a homogeneous mixture made up of a solvent, usually liquid, and a solute. This is associated with a concentration, the units of which vary depending on the purposes for which it has been prepared, as well as the accuracy with which its concentration is expressed.

Solutions are one of the most common and important elements in chemistry and have a number of rules to prepare them. Source: Pxhere.

The principle of all solution preparations is essentially the same: dissolve the solute in an appropriate solvent, or starting from a concentrated (stock) solution, take aliquots to prepare more dilute ones. The ultimate goal is that there is high homogeneity and that the solution has the desired characteristics.

In daily life solutions are prepared following the criterion of taste, that is, how intense the taste of a drink should be. Laboratories or industries, however, require a less subjective parameter: a predetermined concentration, which is met by following a series of rules and usually simple mathematical calculations.

How do you prepare a solution?

Previous steps

Before preparing a solution, it is necessary to determine what concentration its solute will have, or each one of them, and what will be the solvent that will be used. Do you want it to be 1% m / v? Or 30% m / v? Will it be prepared at a concentration of 0.2M or 0.006M? Likewise, it must be known for what purposes it will be used: analytical, reagents, media, indicators, etc.

This last point determines whether or not the use of volumetric flasks or balloons will be necessary. If the answer is negative, the solutions can be prepared directly in a beaker, and therefore the preparation will be simpler and less meticulous.

Solute dissolution

Regardless of the concentration, or if you want it to be as exact as possible, the first step in preparing a solution is to weigh the solute and dissolve it in the appropriate solvent. Sometimes even when the solute is soluble in the chosen solvent, it is necessary to heat it on a plate or have a magnetic stirrer.

In fact, the solute is the factor that makes a marked difference in the method in which different solutions are prepared. On the other hand, if the solvent is a volatile liquid, the solution will be prepared inside a fume hood.

The entire solute dissolution process is carried out in a beaker. Once dissolved, and with the help of a support and funnel, its contents are transferred to the volumetric flask or balloon of the desired volume.

If an agitator was used, it must be properly washed to ensure that no traces of solute remain adhering to its surface; and one must also be cautious when transferring, otherwise the shaker will fall into the volumetric balloon. For this it is convenient and very useful to help yourself with a magnet. On the other hand, a glass rod can also be used instead of the stirrer.

Fill the volumetric flask or balloon

By dissolving the solute in this way, we ensure that there are no suspended solids in the volumetric flask, which will then be difficult to dissolve and will affect the analytical quality of the final solution.

Once this is done, the volume of the balloon is made to the level or completed with the solvent, until the surface of the liquid coincides with the mark indicated on the glassware.

Finally, the closed balloon or flask with its respective caps is shaken a couple of times, leaving the solution ready.

Examples of solutions

In a laboratory it is common to prepare solutions of acids or bases. These must first be added to a considerable volume of solvent; for example, water. It should never be done the other way around: add water to acids or bases, but rather to a volume of water. The reason is because their hydrations are very exothermic, and there is even the risk of the beaker exploding.

Sulfuric acid

Suppose you want to prepare a dilute solution of sulfuric acid. Having clear what will be the aliquot to be taken from the stock or concentrated solution, this will be transferred to the volumetric balloon, which will already have a volume of water.

Even so, heat will be released, and it should be made to the mark with water very slowly, waiting for the balloon to cool down or not get too hot.

Sodium hydroxide

On the other hand, a sodium hydroxide solution is prepared by weighing the NaOH dragees in a beaker with water. Once the NaOH has been dissolved, with or without a magnetic stirrer, the alkaline water is transferred to the respective volumetric flask and made up to the mark with water or ethanol.

Exercises

Exercise 1

You want to prepare a liter of a 35% m / v solution of sodium chloride in water. How much salt should be weighed and how do you proceed?

The 35% m / v concentration means that we have 35 g of NaCl for every 100 mL of water. As they ask us for a liter of solution, ten times that amount, we will weigh 350 g of salt that we will try to dissolve in a volume of one liter.

Thus, in a large beaker the 350 g of NaCl are weighed. Subsequently, a sufficient quantity of water (less than one liter) is added to dissolve the salt using a glass rod. Because salt is very soluble in water, the use of a magnetic stirrer is not mandatory.

Dissolved these 350 g of NaCl, the salt water is transferred to a one-liter volumetric flask and made up to the mark with water; or simply fill the liter of water in the same beaker and continue stirring to ensure the homogeneity of the salt. The latter applies when the solution does not need to have an exact concentration but an approximate one.

Exercise 2

You want to prepare 250 mL of vinegar (5% v / v acetic acid) starting from a bottle of glacial acetic acid (100% pure). What volume of this bottle should be taken?

No matter what volume of glacial acetic acid is measured, it will have a concentration of 100%; one drop, 2 mL, 10 mL, etc. If we divide 100/5, we will have 20, which indicates our dilution factor; that is, the volume that we measure from the bottle we are going to dilute 20 times. Therefore, 250 mL of vinegar should correspond to this volume 20 times larger.

So 250/20 gives us 12.5, which means that from the glacial acetic acid bottle we will take 12.5 mL and dilute it in 237.5 mL of water (250-12.5).

For this, a graduated and sterilized pipette will be used, or a small volume of glacial acetic acid will be transferred into a clean beaker from which the 12.5 mL aliquot will be taken, and it will be added to a 250 mL volumetric balloon with a previous and sufficient quantity of water. Thus, we will add the acid to the water, and not water to the acid.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Joshua Halpern, Scott Sinex & Scott Johnson. (June 05, 2019). Preparing Solutions. Chemistry LibreTexts. Recovered from: chem.libretexts.org
3. Helmenstine, Anne Marie, Ph.D. (September 16, 2019). How to Prepare a Solution. Recovered from: thoughtco.com
4. ChemPages Netorials. (sf). Stoichiometry Module: Solutions. Recovered from: chem.wisc.edu
5. The Science Company. (2020). Preparing Chemical Solutions. Recovered from: sciencecompany.com