AS-Level Amounts of Substance

  • The number of moles of a solute in a solution can be calculated using the formula:

    • number of moles = concentration (mol dm ) x volume (dm ) / 1000

  • 1 dm is the same as 1000 cm

  • To convert dm to cm x 1000

  • To convert cm to dm / 1000 

  • Concentration refers to how many moles of solute are in 1dm  of solution (units are mol dm )

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QUICK NOTES

Calculating Moles in a Solution

Remember there are two main ways of measuring how much of a substance you have - mass and volume.

 

When you know the mass and formula of a substance, it is straightforward to calculate the number of moles in a sample. However, it is not as possible to calculate number of moles of compound in a solution using the same process.

 

It is important to point out here that we are talking about substances dissolved in liquids. Think about a beaker of water with salt dissolved in it: the more salt you dissolve, the greater the number of moles of salt you will have mixed in with the water.

 

If we take two beakers, both the same size, in one beaker (A) we dissolve 10 grams of salt and in the next beaker (B) we dissolve 20 grams of salt. There will be more salt in beaker B than beaker A. As the amount of water in each beaker is the same, we can say that the solution in beaker B is more concentrated than the solution in beaker A.

Concentration is just a way of describing how much of a substance we have in a given volume of solution. In chemistry, the standard definition for concentration is moles per decimetre cubed (moles per dm ). Sometimes you will see concentration written as ‘M’ for molarity – this is the same thing when dealing with solutions. This just describes how many moles of a substance we have in 1      of solution (1 dm = 1000     ).

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dm

If a solution of hydrochloric acid has a concentration of 2 mol dm , then 1 dm of solution would have 2 moles’ worth of hydrochloric acid (HCl) molecules in it. Equally, if a solution of sodium hydroxide has a concentration of 4 mol dm, then 1dm of solution would have 4 moles’ worth of sodium hydroxide (NaOH).

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If the volume and the concentration of a solution is known, it is possible to calculate how many moles of a substance there is in that volume.

 

The equation used is straightforward, but you need to able to use it well.

number of moles = concentration (moles per dm ) x volume (dm )

 

 

Concentration is always given as moles per dm, but volumes can be stated as either dm or cm. If a volume is given in cm, in order to find the moles, it needs to be converted from cm to dm. To do this, it is divided by 1000.

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number of moles = concentration (moles per dm ) x volume (cm ) / 1000

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