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One of the most common ways to approach the preparation of a solution is by providing the desired final concentration in moles per liter of solution (M). The molarity of a solution is always useful as a reference to other solutions in the laboratory.
First, before dissolving the solute to be used in the corresponding solvent, we must know the mass of said solute. Knowing the molarity of the solution will have enough information to know how much solute we need. Molarity is expressed in moles of solute per liter of final solution.
How do we know the mass of solute to use?
In this case we will use sodium chloride (NaCl) and to know the molar mass of this chemical compound we must add the mass of one mole of chlorine (35.45 grams per mole) to the mass of one mole of sodium (22.99 g /mole). The result is 58.44 g/mol. This will be the amount of solute you will need so that the concentration of the solution is one mole of solvent per liter of solution, that is, 1 molar.
How do we prepare the solution correctly?
It is enough to weigh exactly this amount that we previously obtained as a result. Weigh the NaCl on a balance and place it in a container that has a measure corresponding to one liter; then dissolve with water suitable for the laboratory (deionized and distilled) without using 1 liter yet. For now you only need to dissolve the solute. When the compound is dissolved in the water, you can add more water but be careful not to exceed the liter of solution. Remember that you must measure the liters of the complete solution, not the solvent. Therefore, avoid emptying the entire liter of solvent into the container.
If you need a higher or lower molarity you can always multiply the molecular mass of the solute by the corresponding factor. For example, if a quarter of this molarity is needed, we will multiply the molecular mass of the compound being used by 0.25.
It must also be remembered that if it is necessary to alter any characteristic of the solution, it must be done before it is ready. For example, if a pH change is required, something that is quite common, it must be done before adding all the solvent, the water in the case of the example. If we do it with the solution already prepared, we would be altering its volume, and therefore the concentration would no longer be what was sought.
References
True Science. (2019). What is a SOLUTION, SOLUTE AND SOLVENT? Available at: https://youtu.be/XtqzGh-VTt4
Tp Chemical Laboratory. (2021). What is a solution? Available at: https://www.tplaboratorioquimico.com/quimica-general/reacciones-quimicas/que-es-una-solucion.html
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