# as chemistry amount of substance

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1. how can we find out the formula of a substance such as magnesium oxide through an experiment
• find the mass of a crucible and its lid
• place magnesium in the crucible and reweigh the crucible with the lid
• heat the crucible and when the magnesium starts to glow place the lid over the crucible
• keep lifting the lid to allow air to replace
• continue until there is significantly more glowing
• allow to cool and reweigh
• work out the mass of magnesium by doing the following calculation : mass of magnesium + crucible + lid - mass of crucible + lid
• work out the mass of oxygen by doing the following calculation : mass of magnesium + oxygen + crucible + lid - mass of magnesium + lid + crucible
• calculate the formula from the masses
2. we may have a formula that is different to the actual formula of magnesium oxide this may be because
• in our experiment there was more magnesium than oxygen this may be because : when the lid was lifted magnesium oxide escaped in the form of smoke
• in our experiment there was more oxygen than magnesium this may be because there was a weighing error or perhaps the magnesium was oxidised before the experiment
3. empirical formula
formula showing the simplest ratio of atoms of each element present in a compound
4. molecular formula
formula showing the actual numbers of atoms of each element present in one molecule e
5. examples of compounds with different molecular and empirical formulas
• ethane
• molecular formula C2H6
• empirical formula CH3
6. examples of compounds with the same empirical and molecular formula
water - H20
7. how to calculate the empirical formula
• find the masses of each of these elements present in a compound (by experiment)
• work out the number of moles of atoms of each element use n = m/Mr
• convert the number of moles of each element into a whole number ratio
8. how to calculate molecular formula
• work out empirical formula
• work out molar mass of empirical formula
• find the ratio of empirical formula mass to Mr in terms of 1:y
• x the number of atoms of each element of in the compound by y
9. we want 250cm^3 of sodium chloride solution which has a concentration of 0.1 moldm^3 calculate the mass of sodium chloride to be weighed
• number of moles = mass/Mr
• number of moles = concentration x volume
• number of moles = 0.1 x 250 x 10^-6 = 0.025
• 0.025 = x/58.5
• 0.025 x 58.5 = x = 1.46g
10. concentration (mol dm^-3) =
• moles (mol)
• --------------
• volume (dm^3)
11. 1dm^3 =
1000 cm^3 so when converting from cm^3 to dm^3 divide bt 1000
12. there are actually two different units for concentration
• g dm^-3
• mol dm^-3
13. if given the concentration in g dm^-3 and mol dm^-3
• use moles = mass/Mr to find mass
• and then concentration/mass
14. if given the concentration in mol dm^-3 and want g dm^-3
• use moles = mass/Mr to find mass
• and then mass x concentration
15. it is possible to calculate the limits of accuracy of different measuring instruments in an experiment but you cannot calculate the exact effect of
errors which are not quantifiable
16. you can measure the volume of a liquid with a
measuring cylinder , pipette or burette
17. ....... are least accurate
measuring cylinders
18. percentage error =
• 1/2 garduation (error)
• --------------------------- x 100
19. measuring cylinder error
+ or - 0.5 cm^-3
20. pipette error
+ or - 0.05cm^3
21. burette error
• + or - 0.05 cm^3 x 2 because you take readings at the start and the end so total error = + or -0.10 cm^3
• if you are using a burette to do a titration there may be another error of one or two drops which is due to your judgement of when the indicator changes colour . This means that in a titration you may have an error of + or - 0.2 cm^3
22. thermometer error
+ or - 0.5 degrees centigrade

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 Author: ghoran ID: 243135 Filename: as chemistry amount of substance Updated: 2013-10-27 18:39:27 Tags: unit Folders: Description: revision Show Answers:

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