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# Concentration Units for AP Chemistry (page 2)

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By McGraw-Hill Professional
Updated on Feb 9, 2011

### Molarity

Percentage concentration is common in everyday life (3% hydrogen peroxide, 5% acetic acid, commonly called vinegar, etc.). The concentration unit most commonly used by chemists is molarity. Molarity (M) is the number of moles of solute per liter of solution.

M = moles solute/liter solution

In preparing a molar solution, the correct number of moles of solute (commonly converted to grams using the molar mass) is dissolved and diluted to the required volume.

Determine the molarity of sodium sulfate in a solution produced by dissolving 15.2 g of Na2SO4 in sufficient water to produce 750.0 mL of solution.

The most common error is not being careful with the units. Grams must be converted to moles, and milliliters must be converted to liters.

Another way to prepare a molar solution is by dilution of a more concentrated solution to a more dilute one by adding solvent. The following equation can be used:

(Mbefore)(Vbefore) = (Mafter)(Vafter)

In the preceding equation, before refers to before dilution and after refers to after dilution.

Let's see how to apply this relationship. Determine the final concentration when 500.0 mL of water is added to 400.0 mL of a 0.1111 M solution of HC1. Assume the volumes are additive.

The most common error is forgetting to add the two volumes.

### Molality

Sometimes the varying volumes of a solution's liquid component(s) due to changes in temperature present a problem. Many times volumes are not additive, but mass is additive. The chemist then resorts to defining concentration in terms of the molality. Molality (m) is defined as the moles of solute per kilogram of solvent.

m = moles solute/kilograms solvent

Notice that this equation uses kilograms of solvent, not solution. The other concentration units use mass or volume of the entire solution. Molal solutions use only the mass of the solvent. For dilute aqueous solutions, the molarity and the molality will be close to the same numerical value.

For example, ethylene glycol (C2H6O2) is used in antifreeze. Determine the molality of ethylene glycol in a solution prepared by adding 62.1 g of ethylene glycol to 100.0 g of water.

The most common error is to use the total grams in the denominator instead of just the grams of solvent.

Practice problems for these concepts can be found at:

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