Chemistry 3b

1. A mole is 6.023 x 10
   1. When you get exactly that amount of atoms of carbon -12, it weighs exactly 12g.
      1. So, get that number of atoms or molecules of any element or compound (they weigh the same number of grams as the relative atomic mass, Ar or Mr of the element or compound)
         1. You can use moles as a unit of measurement when talking about an amount of a substance.

1. The concentration of a solution can be measured in moles per dm 3 (ie. moles per litre).
   1. 1 mole of the solution in 1 dm 3 of solution has a concentration of 1 mol/dm 3.
      1. Concentration can also be measured in grams per dm 3. So 56 grams of stuff dissolved in 1 dm 3 of solution has a concentration of 56 g/dm 3.
2. The more solute you dissolve in a given volume, the more crowded the solute molecules are and the more concentrated the solution.

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1. Titrations allow allow you to find out exactly how much acid is needed to neutralise a quantity of alkali (or vice versa).
2. 1. You put some alkali in a flask, along with some indicator - phenolphthalein or methyl orange. You don't use universal indicator because it changes colour gradually and you want a definite colour change.
   1. 2. Add the acid, a bit at a time to the alkali using a burette - giving the flask a regular swirl. Go especially slowly when you think the acid is nearly neutralised.
      1. 3. The indicator changes colour when all of the alkali has been neutralised - phenolphthalein is pink in alkalis but colourless in acids and methyl orange is yellow in alkalis but red in acids.
         1. 4. Record the amount of acid used to neutralise the alkali. It's best to repeat the process a few times making sure you get a similar result each time. You can then take the mean of your results.

1. To find the concentration in MOLES per dm3
   1. Say you start off with 25cm 3 of sodium hydroxide in your flask and you know that its concentration is 0.1 moles per dm 3.