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Quantitative Chemistry

Quantitative chemistry is all about calculations using the mole as a unit. In maths, 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 the GCSE. 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.

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 1023 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.

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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 1023 (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 for GCSE. You then just need to be methodical and start with the correct chemical formulae and balanced equations. Practise, practise, practise 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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  1. Sometimes athletes take drugs to enhance their performance. One such drug is ephedrine which has the formula C10H15NO. What is the molecular mass of ephedrine?
    Total carbon = 10 x 12 = 120. Total hydrogen = 15 x 1 = 15. Total nitrogen = 14 x 1 = 14. Total oxygen = 16 x 1 = 16. Add together = 165 amu
  2. Amphetamine, another drug, has the formula C9H13N. The molecular mass is 135 amu. Calculate the percentage by mass of hydrogen in amphetamine.
    Remember, be methodical. There are 13 hydrogen atoms in the molecule so they contribute 13 x 1 =13 amu to the total. To work this out as a percentage:13 /135 = 0.096 x 100 = 9.6%
  3. A substance used as a cosmetic by the Ancient Egyptians has been analysed. It was found to contain 76% lead, 13% chlorine, 2.2% carbon and 8.8% oxygen. Calculate the empirical formula of this compound.
    Divide the % by RAM for each individual element. Divide those answers by the smallest which will give you your final ratios
  4. Some toothpastes contain sodium fluoride. This compound has the formula NaF. Calculate the relative formula mass of this compound.
    The value of option 2 is also numerically correct, but it is very important to include the units in all your answers
  5. Calculate the % by mass of fluorine in sodium fluoride.
    19/42 x 100 = 45%. You can check this by calculating the percentage by mass of sodium - this gives 55%. 55 + 45 = 100%
  6. A tube of toothpaste contains 1.5 g of sodium fluoride. Calculate the mass of sodium in this tube of toothpaste.
    Again you MUST remember your units. The quick way to this answer is to use the answer to the previous question. 55% of NaF is sodium. 55% of anything is 0.55 so 1.5 x 0.55 gives you the correct answer
  7. Calcium hypochlorite tablets are added to swimming pools to kill microorganisms. The formula of calcium hypochlorite is CaCl2O2. Calculate the relative formula mass of calcium hypochlorite.
    40 + 2 x 35.5 + 2 x 16 = 143 amu
  8. Calculate the percentage by mass of oxygen in calcium hyopchlorite.
    2 x 16 /143 x 100 = 22%
  9. Calculate the mass of oxygen in a 50g tablet of calcium hypochlorite.
    From the previous question - 22% of calcium hypochlorite is oxygen, so 0.22 x 50 gives the correct answer
  10. Car airbags inflate when an impact causes a mixture of chemicals to react. The mixture of chemicals contains sodium azide (NaN3) which undergoes thermal decomposition to produce sodium and nitrogen. Calculate the mass of sodium that would be produced when 170 g of sodium azide decomposes.
    Remember - be methodical and first figure out what calculations would be helpful. RMM of sodium azide is 65 amu therefore by mass, there is 35.4% of Na present. The mass of Na produced from 170 g NaN3 is 170 g x(35.4/100) = 60.18 g which rounds to 60.2 g

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