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Using Molarity in Calculations of Solutions

Molarity is a unit of concentration; it is equal to moles of solute divided by the total volume of the solution in liters.

Learning Objective

  • Translate between molarity, grams of solute in solution, and volume of solution.


Key Points

    • Molar concentration, also called molarity, is the number of moles of solute per liter of solution. Molarity is the most common measurement of solution concentration.
    • Because molarity measurements are mole/L measurements, we often use this unit for stoichiometric calculations to determine the amount of chemical in a given mixture.
    • Do not confuse moles with molarity: molarity is a measure of concentration, while moles are a measure of the amount of substance.

Terms

  • molarity

    The concentration of a substance in solution, expressed as the number moles of solute per liter of solution.

  • solution

    a homogeneous mixture, either liquid, gas, or solid, formed by dissolving one or more substances

  • mole


Full Text

Molarity

In chemistry, molar concentration, or molarity, is defined as moles of solute per total liters of solution. This is an important distinction; the volume in the definition of molarity refers to the volume of the solution, and not the volume of the solvent. The reason for this is because one liter of solution usually contains either slightly more or slightly less than 1 liter of solvent, due to the presence of the solute. The SI unit for molarity is is mol/m3; however, you will almost always encounter molarity with the units of mol/L. A solution of concentration 1 mol/L is also denoted as "1 molar" (1 M). Mol/L can also be written in the following ways (however, mol/L, or simply M, is most common):

1 mol/L = 1 M = 1 mol/dm3 = 1 mol dm−3 = 1000 mol/m3

It is important to distinguish moles from molarity; molarity is a measurement of concentration while moles are a measure of the amount of substance present at a given time.

Molarity

Molarity is a measurement of concentration, with units of mol solute per liter solution.

Using Molarity in Calculations Involving Solutions

Molarity can be used in a various calculations involving solutions. The following formula is very useful, as it relates the molarity of the solution, the total volume of the solution (in liters), and the number of moles solute:

$MV=mol$

Example 1

A student pipettes a 100 mL sample of a 1.5 M solution of potassium bromide. How many moles of potassium bromide are contained in the sample?

$MV=mol$

(1.5 M)(0.100 L) = mol

mol = 0.15 mol KBr

Notice in the example above that volume must be converted to L from mL.

You might notice that the above formula bears some resemblance to our dilution formula:

$M_1V_1=M_2V_2$

Because we now know that MV = mol, we can simplify our the dilution formula to the following:

$mol_1=mol_2$

This shouldn't surprise us. After all, in any dilution, what changes is the amount of solvent, while the number of moles of solute remains constant throughout.

Example 2

What is the molarity of a solution containing 0.32 moles of NaCl in 3.4 liters of solution?

$M= \frac {0.32molNaCl} {3.4 L solution}= {0.094 M NaCl}$ .

Molarity Practice Problems - YouTube

This video demonstrates practice problems with molarity, calculating the moles and liters to find the molar concentration. It also uses conversion factors to convert between grams and moles and between milliliters and liters.

Molarity Practice Problems (Part 2) - YouTube

Use molarity to convert between mass and volume in a solution. This video looks at how to use molarity as a conversion factor. If you know the molarity, you can solve for either the number of moles or the volume of a solution. Also, molarity is a ratio that describes the moles of solute per liter of solution.

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