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Quantitative chemistry is all about calculations using the mole as a unit. In math, you expect to be doing calculations but in chemistry it can come as a bit of a shock to the system! Many students panic at the mention of carrying out chemical calculations but, with a little understanding and knowledge of certain ideas and a few techniques, they can become a lot less worrying. Calculations are a good way of picking up marks in exams. Even when you don't get the answer right, you will always get credit for your working out - so always show how you arrived at your answer.
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The key idea that lies behind quantitative chemistry is the mole. In the same way that you use the word a dozen to mean twelve of something and a couple to mean two of something, the mole simply means 6 x 10^{23} particles of matter. Those particles can be atoms, ions or molecules. This is extremely useful since particles of matter always combine in predictable and definite proportions e.g. one atom of calcium bonds to two atoms of chlorine.
Despite being huge, it is easy to count out this number of particles as it is based on atomic masses; if you weigh out the formula mass of a substance, you have one mole of particles. The formula of sodium is Na so its its formula mass is equal to its atomic mass which is 23 amu so if you have 23 g of sodium, you know that you have 6 x 10^{23} (one mole) of sodium atoms. Weigh out 11.5 g and you have half of a mole; 2.3 g is a tenth of a mole and so on. The mole makes life easier as far as quantitative chemistry is concerned. It works with molecules and giant ionic lattices as well as with elements. Take for example water and salt. Water has the formula mass of 18 amu (two hydrogen atoms with atomic mass of 1 amu plus one oxygen at 16 amu) so 18 g would contain one mole of water molecules; 9 g contains half of a mole and so on. Salt has a formula mass of 58.5 amu so can you work out how many grams would be needed for you to have 0.25 moles?
Quantitative chemistry is therefore about proportions - if you can do basic arithmetic and work out pecentages and proportions, you have the skills needed to do chemical calculations in high school. You then just need to be methodical and start with the correct chemical formulae and balanced equations. Practise, practice, practice and you WILL get the hang of these calculations.
The answer to the salt question: 14.625 g (58.5 x 0.25)
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