AS Level chemistry

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ghoran
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AS Level chemistry
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2013-11-08 13:15:34
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unit section two amount substance CGP textbook
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  1. what is a mole
    amount of substance is measured using a unit called the mole (mol for short) . one mole is roughly 6 x 〖10〗^23 particles (Avogradro's constant) . It doesn't matter what the particles are . They can be atoms , molecules , electrons or ions
  2. molar mass
    molar mass , M , is the mass of one mole of something . But the main thing to remember is that molar mass is just the same as relative molecular mass M>r (or relative formula mass) . The only difference is you stick a "g mol^-1" for grams per mole on the end
  3. there's a formula that connects the molar mass of a substance to the number of moles of the substance that you have . it looks like this
    • number of moles = mass of substance
    •                            -----------------------
    •                            molar mass 
    • you can also rearrange the formula and use it to work out either the mass of a substance or its relative molecular mass
  4. the concentration of a solution is
    how many moles are dissolved per 1 dm^3 of solution . the units are mole dm^-3 or (M)
  5. here's the formula to find number of moles
    • number of moles = concentration x volume 
    •                                                  (in cm^3)
    •                             -----------------------------
    •                                               1000 
    • or 
    • number of moles = concentration x volume
    •                                                 (in dm^3)
  6. 1 dm^3 is the same as
    1000 cm^3 or 1 litre
  7. note in the exam you may be asked to combine a concentration calculation with a molar mass calculation
    :)
  8. if temperature and pressure stay the same , one mole of gas always has the same
    volume . at room temperature and pressure (r.t.p) this happens to be 24dm^3 (r.t.p. is 298 k (25 degrees centigrade) and 100kPa)
  9. here are the two formulas for working out the number of moles in a volume of gas - don't forget only use them for r.t.p.
    • number of moles = volume in dm^3/24 
    • or 
    • number of moles = volume in cm^3/24 000
  10. in a real world , it's not always room temperature and pressure . the ideal gas equation lets you find the number of moles in a certain volume at any temperature and pressure
    • pV = nRT
    • p = pressure measured in pascals (Pa)
    • V = volume measured in (m^3)
    • n = number of moles 
    • R = gas constant whose value is given in the exam 
    • T = temperature measured in kelvin (K)
  11. you might be given pressure in in kPa (kilopascals) . to convert from kPa to Pa you
    multiply by 1000
  12. you might be given temperature in degrees centigrade . to convert from degrees centigrade to K you
    add 273
  13. you might be given volume in cm^3 . to convert from cm^3 to m^3 you
    multiply by 10^-6
  14. you might be given volume in dm^3 to convert from dm^3 to m^3 you
    multiply by 10^-3
  15. how to balance equations
    balanced equations have the same number of each atom on both sides . You can only add more atoms by adding whole compounds . You do this by putting a number in front of a compound or changing one that's already there . You can't mess with formulas
  16. balancing ionic equations
    in ionic equations only the reacting particles are included . You don't have to worry about the rest of the stuff . First you make sure that both sides have the same number of atoms - just like a normal equation . then you balance the charges by adding extra electrons
  17. calculating masses - you can use a balanced equation for a reaction to work out how much product you will get from a certain mass of reactant . here are the steps to follow
    • 1) write out the balanced equation for the reaction 
    • 2) work out how many moles of the reactant you have 
    • 3) use the molar ratio from the balanced equation to work out the number of moles of product that will be formed from this much of reactant 
    • 4) calculate the mass of that many moles of product
  18. calculating gas volumes . here are the steps to follow
    • 1) write out the balanced equation for the reaction 
    • 2) work out how many moles of the reactant you have
    • 3) use the molar ratio from the balanced equation to work out the number of moles of product that will be formed from this much of reactant 
    • 4) put that number of moles into one of the gas equations
  19. state symbols
    • state symbols are put after each compound in an equation . they tell you what state of matter things are in 
    • s = solid 
    • l = liquid
    • g = gas 
    • aq = aqueous (solution in water)
  20. neutralisation
    when an acid reacts with an alkali you get a salt and water
  21. titrations allow you to find out
    exactly how much acid is needed to neutralise a quantity of alkali
  22. how to carry out a titration
    • measure out some alkali using a pipette and put it in a flask , along with some indicator such as phenolphthalein .
    • add the acid to the alkali using a burette - open the tap to run acid into the alkali a little bit at a time .
    • every time you add some more acid , give the flask a swirl to make sure that the acid and the alkali are properly mixed .
    • first of all do a rough titration to get an idea where the end point is . The end point of the titration is the exact point at which the indicator changes colour - at this point the amount of acid added is just enough to neutralise the alkali .
    • now do an accurate titration . run the acid in to within 2cm^3 of the end point , then add the acid dropwise . If you don't notice exactly when the solution changed colour you've overshot and your result won't be accurate .
    • record the amount of acid used to neutralise the alkali .
    • It's best to repeat this process a few times , making sure you get the same answer each time . this will make sure your results are reliable
  23. the apparatus needed for a titration
  24. note in exam you need to be able to use the results of a titration to calculate the concentration of acids and alkalis
    :)
  25. n the exam you also need to be able to calculate the volume of acid or alkali that you need to neutralise a solution . you'll need to use the formula
    • number of moles = 
    • concentration x volume (in cm^3)                                                   -----------------------------                                                            1000 
    • but this time rearrange it to find volume 
    • volume (cm^3) = number of moles x 1000
    •                          -----------------------------
    •                                       concentration
  26. the empirical formula gives
    just the smallest whole number ratio of atoms in a compound
  27. the molecular formula gives the
    actual numbers of atoms in a molecule . the molecular formula is made up of a whole number of empirical units
  28. if you know the empirical formula and the molecular mass of a compound , you can calculate its molecular formula just follow these steps
    • 1) find the empirical mass - that's just the mass of the empirical formula 
    • 2) divide the molecular mass by the empirical mass . this tells you how many multiples of the empirical formula are in the molecular formula 
    • 3) multiply the empirical formula by that number to find the molecular formula
  29. calculating empirical formulas 
    follow these steps each time
    • 1) [assume you have got 100g of the compound - you can turn the percentages into straight masses] . then you can work out how many moles of each element are in the compound by using number of moles=mass/mr
    • 2) divide each number of moles by the smallest number of moles you found in step 1 . this gives you the ratio of the elements in the compound
    • 3) apply the numbers from the ratio to the formula
  30. monoprotic acids
    acids that only release one H+ from each molecule
  31. theoretical yield
    mass of product that should be formed in a chemical reaction . it assumes no chemicals are lost in the process .
  32. to calculate theoretical yield you can use the masses of reactants and a balanced equation . it's a bit like calculating reacting masses - here are the steps you have to go through
    • 1) work out how many moles of reactant you have 
    • 2) use the equation to work out how many moles of product you would expect that much reactant to make 
    • 3) calculate the mass of that many moles of product - and that's the theoretical yield
  33. for any reaction , the actual mass of the product (the actual yield) will always be ... than the theoretical yield . There are many reasons for this . For example
    • less 
    • sometimes not all the starting chemicals react fully . And some chemicals are always lost , e.g. some solution gets left on the filter paper , or is lost between transfers between containers
  34. once you've found the theoretical yield and the actual yield , you can use work out the percentage yield by using the formula
    • percentage yield = actual yield 
    •                            -------------- x 100
    •                            theoretical yield
  35. atom economy is one way to work out
    how efficient a reaction is .
  36. efficient reactions are better for
    the environment and save the chemical industry money
  37. the efficiency of a reaction is often measured by
    the percentage yield . This tells you how wasteful the process is - it's based on how much of the product is lost because of things like reactions not completing or looses during collection and purification
  38. percentage yield doesn't however measure
    how wasteful the reaction itself is . A reaction that has a 100% yield could still be very wasteful if a lot of the atoms from the reactants wind up the in by-products rather than the desired product .
  39. atom economy is a measure of
    the proportion of reactant atoms that become part of the desired product (rather than by-products) in the balanced chemical equation
  40. atom economy in industry
    chemical companies try to use reactions that have a high atom economy , so they're not producing lots of waste , or spending money making by-products . But reactions with low atom economy may still be used if the waste products can be sold and used for something else (waste products like gases , salts and acids can often be useful reactants for other products)
  41. atom economy is calculated using this formula
    • % atom economy
    •    mass of desired products
    • = -------------------------------- x 100 
    •    total mass of reactants
  42. to calculate the atom economy for a reaction , you just need to
    add up the molecular masses of the reactants , find the molecular mass of the product your'e interested in and put them both into the formual
  43. any reaction where there's only one product has a % atom economy of
    100
  44. when you calculate the masses , you should use
    the number of moles of each compound that is in the balanced equation
  45. you should always calculate % atom economy from a
    balanced equation

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