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Concentration and Molarity Help (page 2)

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By McGraw-Hill Professional
Updated on Aug 28, 2011

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.

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