Introduction to Concentration and Molarity
In the laboratory, chemists must make use of what they have on hand. It would be a waste of time to run out and buy 20 different concentrations of hydrochloric acid, for example, when they could dilute down the concentrated hydrochloric acid on the shelf and get whatever lower concentration was needed.
Solutions
A solution is a homogeneous mixture of a solute , the element or compound that dissolves, in a solvent , the solution that disperses the solute. Solutes and solvents can be elements or compounds. Commonly, the larger sample is called the solvent and the smaller sample is called the solute. When salt is dissolved in water, the salt is the solute and water (known as the universal solvent ) is the solvent.
A solute is an element or compound dissolved to form a solution. A solvent is a liquid to which an element or compound has been added to form a solution.
To understand how atoms, elements, and molecules interact in solutions, it is important to understand concentration. You have seen how a high concentration of an element or solution can be much different from a low concentration. Consider salt water. Different concentrations can have very different effects. A physiological saline, 0.85% NaCl solution, given intravenously and used to stabilize patients’ fluid levels in hospitals, is much different from the highly concentrated salt brine (30−50% NaCl) used to preserve codfish. In fact, confusing the two could be deadly.
When two or more liquids are able to form a solution, they are called miscible . Alcohol is miscible in water. In fact, there is an entire area of study in physics called fluid dynamics that tests the miscibility of fluids and the way they flow at different concentrations.
When two liquids don’t mix, like oil and water, they are immiscible . Picture a lava lamp. There are generally large round globules of “lava,” made of a specially compounded and colored wax, floating lazily around in a specially formulated liquid (most likely water). This mixture is heated with a light bulb in the base of the lamp that causes the lava to heat and expand until it becomes less dense than the contained liquid above. When this happens, the lava begins to rise to the top of the container where it slowly cools. When the lava is cool enough to sink to the bottom, it is heated again and the process is repeated. This is truly an example of chemistry as entertainment.
A colloid is like a homogeneous solution, but it is made up of larger particles of one solution mixed and spread all through another solution; think of chocolate chip cookie dough. There are two parts to a colloidal mixture, the dispersing medium and the dispersed phase . The dispersing medium is the substance in a colloidal mixture that is in the greater amount, like the cookie dough. The dispersed phase is the substance in the colloidal mixture that is in the smaller amount, like the chocolate chips.
Colloidal mixtures are not true solutions. Remember, in solutions the solute dissolves in the solvent. In colloidal solutions, the components don’t dissolve, they just mix. When the parts of a compound mix in this way, they are said to have colloidal properties.
Percent (%) By Mass
It is often important to figure out how much of a mixture or solute has been added to the original solution or solvent . You can calculate this simply by finding the percent by mass of the solute.
To find the % of a specific compound in a solution, the formula is
Examples
Using the saltwater example above, let’s figure out the % by mass of sodium chloride (NaCl), if 2.35 g of NaCl is dissolved in 7.45 g of water. First, what is the total mass of the solution? What is the % of NaCl? The total mass of 2.35 g NaCl (solute) and 7.45 g water (solvent) = 9.80 g solution. Then if you divide the 2.35 g NaCl solute by the 9.80 g solvent and multiply by 100%, you will get 23.9795 or 24% NaCl by mass.
Parts Per Million
Sometimes we hear of a chemical compound mistakenly released into the air. To decide whether or not to evacuate the surrounding area, scientists and officials first need to find out the type of compound released and its properties. For example, a gas may be fine when contained, but upon release interacts with oxygen in the air and produces a nasty brew.
In and around cities, there is the extra problem of industrial releases combining with already present environment pollutants (think smog) such as carbon dioxide (CO 2 ), sulfur dioxide (SO 2 ), sulfuric acid (H 2 SO 4 ), nitrogen oxide (NO 2 ), and nitric acid (HNO 3 ).
After finding out what the release is made of, the chemical dose that someone passing by on the street might be exposed to must be determined. The higher the dose, the greater the effect and sometimes the greater the risk to life and limb. Though many industrial releases are minimal, some chemicals are highly toxic at even the very lowest levels.
Most environmental releases are measured in the parts per million (ppm) or parts per billion (ppb) range. Such low levels make it hard to know if there is a problem. Chemicals released into the air or water may show up weeks or years later to be dangerous even when diluted.
A pinch of chilli pepper in 10 tons of beans would roughly be the equivalent of one part per billion. Such small levels seem insignificant and practically undetectable, but a chemical’s reactivity must also be considered in order to know for sure if a release of a hundred parts per billion presents a problem.
Examples
Parts per million can be found by multiplying the ratio of the mass of solute to mass of solution by 10 6 ppm instead of 100%.
In the above example, the chemical release would probably be reported in ppm.
Table 7.1 lists some ppm levels of common chemical contaminants.
Table 7.1 Pollutants are most often measured in parts per million in the air and water.
Molarity
Many times the exact amount of a solute is required for a specific volume of solution. Researchers aren’t able to repeat their own experiments, let alone someone else’s, unless exact quantities and measurements are made. In order to improve accuracy, the concentration of a solution is given in molarity .
Molarity (M) equals concentration. M equals the number of moles of solute (n) per volume in liters (V) of solution.
A mole in chemical terms is not a small furry animal that lives in dark underground burrows, but an SI unit amount of a sample. One mole is defined as being the amount of sample having as many atoms (or molecules or ions or electrons) as carbon atoms in 12 grams of carbon.
Concentration
Sometimes an experiment requires a weaker acid solution than what a chemist has on the shelf. In order to do the experiment, the solution must be diluted. This is done by figuring out the molarity and volume of the solution.
n d = the number of moles in the dilute solution
v d = the volume of the solution
M d = the molarity of the solution
To figure out the number of moles of solute in the dilute solution, use
M d × v d = n d
To figure out the number of moles of solute in the concentrated solution, use
M c × v c = n c
The trick is that the number of moles does not change.
Moles of solute = n d = n c
So the equation looks like M c × v c = M d × v d
M d = M c × v c / v d
As you begin working in a chemistry lab, you will use this formula more. For now, just an understanding of the relationship between solute and solvent is enough.
Dilution
Solutions can be found in their concentrated forms in many laboratories. They have a variety of uses, but, in general, it is easier to keep one concentrated solution on hand than five dilutions of the same solution. There is just not that much space on the shelves. It is important to learn how to make a less concentrated solution from a concentrated one. Look at the following example.
Examples
Example 1
What would you do if an experimental procedure called for 1 M of hydrochloric acid (HCl) and all you had in the lab was 12 M HCl? Could you use what you had on hand? Sure! Just prepare the 1 M HCl by measuring a volume 1/12 or 82 milliliters of the concentrated solution into 1 liter of distilled water. The final concentration is equal to 1 M HCl.
Example 2
How about nitric acid (HNO 3 )? What if you needed 1 M of nitric acid for an experiment and only had concentrated nitric acid (16 M) on hand? You would measure out 63 ml of the concentrated HNO 3 , then add enough distilled water to equal 1 liter. The resulting solution would be equal to 1 M HNO 3 . If you needed a 3 M solution, multiply the 63 ml by 3 to get 189 ml to add to water to bring it to 1 liter. The resulting solution would be a 3 M solution.
To make diluted solutions, chemists use volumetric flasks or beakers for accurate measuring. With a bit of practice, making dilutions of concentrated solutions right off the shelf will be a snap.
Some people compare laboratory chemistry with cooking in the kitchen. Sample preparation, dilution, and mixing are all done with measured care to produce a final product with specific characteristics and qualities. In other words, it might be a masterpiece or a mess, depending on how you follow the directions.
Practice problems for these concepts can be found at – Concentration and Molarity Practice Test
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