Concentration of Solutions

Understanding Concentration

SOLUTE is the SOLID SUBSTANCE that dissolves in a liquid.

SOLVENT is the LIQUID it dissolves in.

SOLUTION is the mixture that a SOLUTE and SOLVENT form.

CONCENTRATION is the AMOUNT OF A SUBSTANCE that is in a certain VOLUME of solution, the more SOLUTE there is in a given volume, the MORE CONCENTRATED the solution is.

Measuring Concentration

Volume can be measured in cm³ or dm³.

You can CONVERT between the two by MULTIPLYING AND DIVIDING BY 1000:

Concentration can be measured in GRAMS PER DECIMETER CUBED (g/dm³).

You can calculate it using the formula:

 Concentration can also be measured in MOLES PER DECIMETER CUBED (mol/dm³)

You can calculate it using the formula:

Calculating Mass of Solute

If you know the CONCENTRATION and the VOLUME of a solution, and the concentration in measured in g/dm³, you can calculate the MASS OF SOLUTE using the first formula.

Here's an example:

Calculating Moles of Solute

If you know the CONCENTRATION and the VOLUME of a solution, and the concentration in measured in mol/dm³, you can calculate the MOLES OF SOLUTE using the second formula.

Here's an example:

Reaction of Two Solutions

When two solutions react COMPLETELY, and you know the VOLUME AND CONCENTRATION OF ONE SOLUTION, you can calculate the CONCENTRATION of the other. This is particularly useful in TITRATION EXPERIMENTS where you need to find out the UNKNOWN concentration of a solution.

In order to do this, follow similar steps to normal mole calculations:

First identify the KNOWN and UNKNOWN substances.

The KNOWN substance is the substance you know the concentration AND volume of.

The UNKNOWN substance is the substance you need to find the concentration of.

Step 1:

Find the MOLES of the KNOWN substance by multiplying the concentration by the volume. Remember to convert the units to dm³.

Step 2:

Here you need to look at the BIG numbers in front of the substances in the BALANCED equation.

You need to use the number IN FRONT of the KNOWN and UNKNOWN to write the RATIO of them.

Known : Unknown

Use this ratio to find the MOLES of the UNKNOWN.

Step 3:

Find the concentration of the unknown by dividing the moles by its volume.

Converting mol/dm³ to g/dm³ 

To find concentration in g/dm³ from mol/dm³, you can MULTIPLY BY MOLAR MASS, or you can DIVIDE BY MOLAR MASS to go the other way.