Moles and Empirical Formulae
- Mole is a measure. One mole of atoms or molecules, of any substance, will have a mass in grams equal to that of the relative formula mass (Mr) for that substance
- Number of moles=Mass(g)/Mr
- Mr is the average mass of an atom in relation to an atom of C-12, as atoms are too small to weigh- duh!
- Substances always react in the same ratios
- Empirical Formula are the simplest ratio in a compound, e.g. the empirical formula of glucose (C6H1206) would be CH20
- To work out the Empirical formula from two reacting masses, you need to divide the experimental masses by the Ar and convert it into its simplest ratio.
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What is the empirical formula of the product produced when 15.9kg of copper oxide reacts with 8.8kg of carbon dioxide?
- Reacting masses of CuO is 15.9 and CO2 is 8.8 kg
- Mr of CuO is 63.5+16=79.5
- Mr of CO2 is 12+ (16*2) = 44
- So if you divide the reacting masses by the Mr you get for CuO- (15.9/79.5)=0.2, and for the CO2 is (8.8/44)=0.2
- So they are both in the ratio of 1:1
- 1CuO+1CO2 -> CuCO3
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- Concentration is the number of particles, in a certain space.
- It is measured in mol/dm3 or g/dm3
- 1 litre= 1000 cm3 = 1dm3
- number of moles=concentration(in mol/dm3) *volume( in dm3)
- mass(grams)=number of moles* Mr
- Things like medicine, baby milk, weedkillers, gravy granules, and sqash can bought in concentrated form
- In order to work out how to dilute something with a given concentration, you need to work out the ratio of the two concentrations and multiply this with the volume of substance, which concentration you want. Then work out the volume of water you need (by subracting it from the total volume)
- Confused? Turn over.
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How would you produce 500cm3 of 0.1 mol/dm3 HCl from a 2 mol/dm3 concentration of acid?
- 0.1 mol/dm3 / 2 mol/dm3 = 1/20 (The ratio is 1:20)
- (1/20)*500=25cm3 (multiply by the final volume to work out thr amount original acid needed)
- 500-25=475 (The volume of water you need)
- So the answer is: dilute 25cm3 of 2M HCl with475cm3 of water
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