Concentration and Dilution

Measuring concentration

Concentration is an important property in chemistry.  We most often talk about substances dissolved in a solution, for example the amount of salt dissolved in a volume of water. A common measure is percent by weight. Seawater is about 3.5% salt by mass. This means one kilogram (one liter) of water has 35 gms of salt.

When increase the concentration of solute, you also decrease the concentration of water. Notice in the diagrams below that when there are fewer solute molecules (shown in blue) there are more water molecules shown in yellow.

low concentration


high concentration













Low concentration of solute                                                                     High concentration of solute
High concentration of water                                                                      Low concentration of water


Explore the following visualization to see what solutions look like at the molecular level. You can change the concentration of solute, or add the solute as a solid to see it dissolve. You can also change the temperature of the solution to see the effect on particle motion. The simulation uses random movement of particles to simulate the effect of the kinetic energy from heat. It also shows how diffusion causes solutions to eventually become uniform in concentration. The green and red particles represent water. The blue dots represent the solute.

Solutions

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Regardless of the concentration what is important is the total amount of dissolved substance that is taken in, and the mass of your body. An elephant would not feel the effects of a caffeinated drink because the a relatively small  amount of caffeine is diluted into large elephant body. A small child would have a much higher concentration in his or her body after taking the same drink. That is why you have be very careful with giving medications to babies.

Dilutions

When we test chemicals on organisms we usually apply a series of solutions with different concentrations, or dilutions. It is possible that a low concentration of a drug is beneficial to an organism while a high concentration may be deadly. Homeopathic medicines use plant extracts at extremely low concentrations by dilution.  In microbiology we dilute a solution containing bacteria so that when the solution is spread on an agar plate separate colonies will grow from individual cells.

To prepare a dilution series use the formula:

Concentration x Volume  = Concentration(after) x volume (after)

Here is a simple example. If you start with one liter of a 10% solution, and add enough pure water water to bring the solution up to 2 liters total volume, then the new solutions must be a 5% concentration.

10% x 1 liter = 5% x 2 liters

Dilution Calculator

A Serial Dilution is made repeatedly adding a small amount of a soluiton to fresh water, mixing it, and then taking some of this dilution an diluting it again. Soon you will have an extremely dilute solution.

How to make a serial dilution

Solutions in chemistry

In chemistry we usually measure concentration by the number of atoms per volume. Since there are very many atoms in any measurable volume we count them with a very large number, the mole or  Avogadros number which is

6.022 ×1023

that's 602,216,900,000,000,000,000,000 atoms.

This leads to the standard concentration of moles per liter, or molarity which is shown with the symbol M.

To make a solution of a certain molarity you can't count the atoms. Instead you have to measure out a precise mass of the substance after you know its molar mass. This can be looked up on using the masses of the elements on the periodic table. The molar mass of salt, sodium chloride is 58.44 gm/mol. So adding 58.44 grams of salt to one liter of water gives a 1 Molar solution, or a 5.844% solution.

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Shaun Neill Taylor,
Dec 1, 2013, 11:24 AM
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