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Concentrations of Solutions (AQA GCSE Chemistry: Combined Science)
Revision Note
Expressing concentration
- A solid substance that dissolves in a liquid is called a solute, the liquid is called a solvent and the two when mixed together form a solution
- Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent
- Concentration simply refers to the amount of solute there is in a specific volume of the solvent
- The greater the amount of solute in a given volume, the greater the concentration
- A general formula for concentration is thus:
- Concentration can be measured in grams per cubic decimetre
- 1 decimetre cubed (dm3) = 1000 cm3
- 1 decimetre cubed (dm3) is the same as 1 litre
- You may be given data in a question which needs to be converted from cm3 to dm3 or the other way around
- To go from cm3 to dm3 :
- Divide by 1000
- To go from dm3 to cm3 :
- Multiply by 1000
- To go from cm3 to dm3 :
Calculating concentration in mass per volume
- To calculate the concentration of a solution in terms of mass per unit volume, you need to
- Identify the solute and solvent
- Convert the volume units into decimetres cubed
- Divide the mass of the solute by the volume of the solution in decimetres cubed
Worked example
A student dissolved 10 g of sodium hydroxide, NaOH, in 2 dm3 of distilled water.
Calculate the concentration of the solution.
Answer:
- Step 1: Write the relevant information down:
- Mass = 10 g
- Volume = 2 dm3
- Step 2: Write down the relevant equation:
- Concentration =
- Step 3: Substitute the values and calculate the concentration:
- Concentration = = 5 g / dm3
Examiner Tip
Be careful when doing volume unit conversions as it is easy to multiply instead of dividing by 1000 and vice-versa. Always ask yourself - is the result going to be a bigger or smaller number than I started with? Do I get more or fewer cubic decimetres when I convert from cubic centimetres?
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