To use all functions of this page, please activate cookies in your browser.
With an accout for my.chemeurope.com you can always see everything at a glance – and you can configure your own website and individual newsletter.
- My watch list
- My saved searches
- My saved topics
- My newsletter
The phrase may be prefixed with a number to denote other concentrations. For example, a five molar solution of aqueous hydrochloric acid (written as "5M HCl (aq)") means there are 5 moles of HCl per litre of solution. If the solvent is not mentioned (such as "5M NaOH"), it is safe to assume that the solvent is water or the one most commonly used with that solute.
For work with aqueous solutions, concentrations expressed in terms of molarity are most useful when performing stoichiometric calculations since easily measured volumes correspond directly to moles of chemical substances involved.
Particularly when working with dilute (aqueous) solutions at room temperature this measure of concentration is by far the most common one in use. It should however be noted that it involves the volume of the solution and volumes depend on temperature and pressure. Therefore a solution that is 0.5000 M at room temperature will have a lower molarity at say 80 °C without removing a single molecule from it. For non-dilute 'real' solutions volume even depends on concentration itself. Other concentration measures are available and used in circumstances where this becomes a problem.
Additional recommended knowledge
The most common way to prepare a solution of known molarity is:
If the preparer overshoots the mark, he or she will either properly dispose of solution and start over or find a way to determine the actual concentration they made.
The above procedure only works if the solute (solid or liquid) is available in pure enough form (and you do not overshoot the mark). For some materials that is a problem. For example NaOH is available in pellets but they are hygroscopic. This means that over time they gain mass by taking up water from the atmosphere. This means that if x grams are weighed off this is not all NaOH but in part water. A solution will then end up being slightly lower in concentration than intended. This is why the actual concentration then needs to be determined by standardization. Usually this is done by taking a material that can be weighed off precisely like potassium hydrogen phthalate and performing a number of titrations against this acid.
It is also possible to start from a strong stock solution, e.g. a 50% solution by weight and weighing off twice the weight of the desired NaOH and dilluting this in a volumetric flask. These stock solutions are hygroscopic as well but not as strongly as the pellets.
If lower concentrations than the one available are desired these can be made by dilution. A suitable amount of solution is pipetted into a clean flask and solvent added to the mark.
Warning: always use properly calibrated flasks and pipettes, never graduated cylinders or beakers because their graduations are far too imprecise for analytical work.
Automatic pipetters facilitate working with tiny amounts, but they should be recalibrated regularly. Note that they are typically designed for aqueous work. The plastic tips may dissolve in some other solvents.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Molar_solution". A list of authors is available in Wikipedia.|