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Physical chemistry (3.1) > Amount of substance (3.1.2) > Flashcards

Amount of substance (3.1.2) Flashcards

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1
Q

what is relative atomic mass (Ar)?

A

the average mass of an element, taking into account its naturally occurring isotopes, compared to 1/12th the relative atomic mass of a carbon-12 atom

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2
Q

what is relative molecular mass (Mr)?

A

the mass of a molecule compared to 1/12th the relative atomic mass of a carbon-12 atom

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3
Q

when should you use the term relative formula mass?

A

when referring to ionic compounds (as they don’t have molecules)

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4
Q

define the Avogadro constant?

A

the number of particles in a mole (and is literally the number of atoms in 12g of 12C)

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5
Q

define a mole?

A

the amount of substance that contains the same number of particles as there are atoms in 12g of 12C (one mole of any substance will contain the same number of particles)

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6
Q

what does MX.nH20 mean?

A

it is the general formula for a hydrated salt - for every 1 mole of MX there are n molecules of water bonded to it as part of the crystal structure (nH20 represents the water of crystallisation)

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7
Q

what happens when a hydrated salt is heated?

A

the water of crystallisation is driven off to leave an anhydrous salt

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8
Q

how do you calculate the yield of a chemical reaction?

A

number of moles of specified product/theoretical maximum number of moles of product X 100
mass of specified product obtained/theoretical maximum mass of product X 100

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9
Q

why might yield not be 100%?

A

the reaction may be reversible, some product could be lost in separation from the reaction mixture, alternative reactions may occur, the reaction may not go to completion

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10
Q

how do you calculate percentage purity?

A

mass of pure substance/ mass of impure substance X 100

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11
Q

how do you calculate percentage of an element in a substance?

A

total amount of element/ total amount of substance X 100
(amount could be mass or relative mass)

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12
Q

how do you calculate the value of n for water of crystallisation (empirical formula method)?

A
  1. work out the mass of the salt and the mass of water
  2. work out the RFM of the salt and the water
  3. work out the number of moles of the salt and the water
  4. divide both these numbers of moles by the smaller number of moles (usually the moles of salt)
  5. this tells you the value of n for every 1 mole of salt
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13
Q

what is an alternative way to calculate the value of n for water of crystallisation?

A
  1. work out how many moles of just the acid/salt you have
    2.find the Mr of the entire crystal by dividing the mass of the crystal by the moles of acid/salt
  2. find the Mr of just the acid/salt
  3. find the Mr of water in that particular crystal by subtracting the Mr of acid/salt from the overall Mr
  4. to find n, divide the Mr of the water by the RFM of water (18)
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14
Q

how do you carry out a simple acid-base titration?

A
  1. rinse a burette with the acid you are using
  2. set the burette up and use a funnel to fill it with the acid
  3. record the initial burette reading in a table
  4. rinse a glass pipette with the alkaline solution
  5. use the pipette to transfer 25cm^3 of alkaline solution to a conical flask
  6. add three of four drops of indicator to the conical flask
  7. add acid from the burette into the conical flask while swirling until the colour change occurs (add dropwise when it nears endpoint)
  8. record the final burette reading
  9. rinse the conical flask with distilled water and repeat the titration 3-5 times
  10. use concordant results to calculate the average titre
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15
Q

what is a back titration?

A

a titration done to analyse a base which doesn’t dissolve

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16
Q

how does a back titration work?

A

the base is treated with an excess of acid of known concentration
the left over acid is titrated with an alkali of known concentration
you can then work backwards to find out about the original base

17
Q

what is the key step when doing back titration calculations?

A

work out the number of moles of acid which reacted with the alkali
work out the number of moles of acid which there were at the start of the reaction
subtract the moles that reacted with the alkali from the initial moles to find out how many moles of acid reacted with the base

18
Q

what is the ideal gas equation?

A

pV=nRT

19
Q

what are the variables and their units in the ideal gas equation?

A

p=pressure (Pa)
v=volume (m^3)
n=moles
R=gas constant
T=temperature (K)

20
Q

how do you convert from degrees celsius to Kelvin?

A

+273
(0 degrees celsius = 273K)

21
Q

how do you convert from dm^3 to m^3?

A

divide by 1000

22
Q

what is the molecular formula of a compound?

A

the actual number of atoms of each element in the compound

23
Q

what are the problems with molecular formula?

A

it doesn’t tell you about the structure of the compound and some compounds have the same molecular formula

24
Q

what is the empirical formula of a compound?

A

the simplest whole number ratio of the atoms of each element in a compound

25
Q

what information do you need to start with to calculate empirical formula?

A

either:
% by mass of each element
actual mass of each element
data that can be used to find the moles of each element

26
Q

how do you use empirical formula to determine molecular formula?

A
  1. find the relative molecular mass of the compound you are looking at
  2. calculate the relative molecular mass of the empirical formula for that compound
  3. divide the Mr of the compound by the Mr of the empirical formula
  4. multiply the empirical formula by this number to get the molecular formula
27
Q

what is the key step in calculating empirical formula?

A

use Ar values to calculate the number of moles of each element and then convert this to the simplest whole number ratio

28
Q

how do you calculate % atom economy?

A

total Mr of useful products/ total Mr of all products x 100
(in this case multiply Mr values by balancing numbers)

29
Q

how can you change or improve atom economy?

A

by finding a use for or selling what was previously a waste product

30
Q

how do you state the error for a given piece of equipment?

A

measured value +- error eg. a 25 cm^3 pipette is 25cm^3 +- 0.06 cm^3

31
Q

how do you calculate percentage error?

A

error value/ value measured x 100
(for pipette it would be 0.06/25 x 100 = 0.24%)

32
Q

what do you need to do when calculating percentage error for pieces of equipment which measure a change or have to be read twice eg. thermometer?

A

multiply the error value by 2

33
Q

how do calculate total % error?

A

add up the individual equipment % errors

34
Q

how do you improve % error?

A

increase the value that you measure with a piece of equipment

Physical chemistry (3.1) (7 decks)

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