Concentrations of Solutions Simplified Revision Notes for GCSE AQA Chemistry

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3.2.5 Concentrations of Solutions

What is Concentration?

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The concentration tells us how much solute (the substance that's dissolved) is in a certain volume of solvent (the liquid it's dissolved in). A solution with a high concentration has more solute in it than a solution with a low concentration.

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Concentration is usually measured in grams per cubic decimetre (g/dm³). 1 dm³ is equal to 1000 cm³, so it's a larger volume.

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Calculating Concentration:

To calculate the concentration of a solution, use the formula:

Concentration (g/dm^3)= \frac{Mass of Solute (g)}{Volume of Solvent (dm^3)}

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Example: If you dissolve 25 grams of sodium chloride ( $NaCl$ ) in 1000 cm³ of water (which is 1 dm³), the concentration is:

\frac{25 g}{1 dm^3} = 25 g/dm^3

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Using Moles to Calculate Concentration:

Concentration (g/dm^3)= \frac{Mass of Solute (g)}{Volume of Solvent (dm^3)}

You can also calculate concentration using moles instead of grams:

Concentration (mol/dm3)= \frac{Moles of Solute}{Volume of Solvent (dm^3)}

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Example: If you dissolve 0.5 moles of $NaCl$ in 2 dm³ of water, the concentration is:

\frac{0.5 mol}{2 dm^3} = 0.25 mol/dm^3

Summary:

Grams per dm³: Used when the mass of the solute is known.
Moles per dm³: Used when the amount of solute is measured in moles.

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