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Chemistry (Tim) > Volumetric analysis part 2 > Flashcards

Volumetric analysis part 2 Flashcards

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1
Q

solution=

A

solute + solvent

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2
Q

strength

A

concentration

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3
Q

concentration

A

the quantity of solvent dissolved in a certain amount of solute

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4
Q

how is concentration found?

A

by carrying out a titration

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5
Q

what unit does the titration formula give

A

moles per litre

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6
Q

what does %w/v mean

A

grams/100cm cubed

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7
Q

what does ppm mean

A

mg/litre

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8
Q

how do you find %w/v

A

find the grams in 100cm cubed

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9
Q

how do you find ppm

A

expres g/l in mg/l

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10
Q

g/l =

A

m/l x Mr

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11
Q

what is the Mr

A

all the big numbers added up

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12
Q

standard solution

A

a solution whose concentration is known exactly in m/l, g/l or ppm etc.

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13
Q

only base that can be weighed accurately

A

Na2CO3

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14
Q

what do you do once you put the clock glass on the mass balance

A

tare, adjust to zero

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15
Q

a mass balance should read…

A

… 0.01 grams of accuracy

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16
Q

what size volumetric flask should you use when preparing Na2CO3 solution

A

500 cm cubed

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17
Q

primary standard

A

a solute that can be weighed very accurately and you can add water to make a standard solution

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18
Q

4 characteristics of a primary standard

A
  1. obtainable in 100% form
  2. dissolves completely in water
  3. dry and remains dry during weighing
  4. does not decompose or evaporate during weighing
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19
Q

why can’t HCl be measured accurately?

A

it evaporates when opened

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20
Q

why can’t H2SO4 be measured accurately?

A

absorbs water from the atmosphere

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21
Q

2 ways to obtain a standard solution

A
  • weigh out accurately a primary standard and make up to an exact volume with deionised water
  • carrying out a titration on a solution of unknown concentration
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22
Q

what is a volumetric flask designed to do?

A

to hold a definite amount of liquid

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23
Q

what is a temperature figure on a volumetric flask?

A

liquids expand or contract at different temperatures, if the temperature figure is 20ºC then it can only be read accurately at that temperature

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24
Q

what do you rinse the volumetric flask with?

A

deionised water and the solution that it will contain

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25
Q

how do you fill the volumetric flask

A

first with a funnel and then using a dropper for the last 2 cm cubed s, until the bottom of the meniscus is at the calibration mark

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26
Q

how do you get a homogenous solution in a volumetric flask

A

add a stopper and invert 20 times

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27
Q

why do you invert a volumetric flask

A

the concentration of the solution might be lower in the neck than in the lower bulk of the flask

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28
Q

What is a pipette designed to do?

A

deliver an exact amount of volume of a liquid from one piece of glass apparatus to another

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29
Q

usual volume of a pipette

A

20/25 cm cubed

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30
Q

how is liquid sucked up into a pipette

A

using a pipette filler

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31
Q

how do you fill a pipette

A

to above the mark and let some out into the waste beaker until the bottom of the meniscus is at the mark

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32
Q

after filling the pipette

A

touch the tip of the pipette against the inside of the beaker to remove any drop of liquid sticking to the tip

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33
Q

why do you know blow out the last bit of liquid into the conical flask

A

it is manufactured to give accurate volumes only when when this last amount remains in the pipette

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34
Q

what should you rinse the pipette with before use?

A

deionised water and then the solution it will contain

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35
Q

why rinse with deionised water

A

to remove any impurities or dirt

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36
Q

why rinse with the solution it will contain

A

pushes out deionised water so that the liquid won’t be diluted

37
Q

after use of pipette

A

rinsed with deionised water and will be allowed to empty

38
Q

what is a burette

A

a long narrow tube with a tap at the end and has graduations down it’s full length

39
Q

burette is read…

A

…from the top down

40
Q

the burette reads to

A

0.1cm cubed accuracy

41
Q

before use of a burette

A

check the tap to see if it turns freely, if not, grease it

42
Q

what should you rinse the burette with

A

deionised water and then the solution it will contain

43
Q

how do you fill a burette

A

with a short stems funnel, fill above the graduation mark and let flow until the bottom of the meniscus is at the zero mark, check to see if the area below the tap is full

44
Q

how do you remove any drop of liquid sticking to the nozzle of the burette

A

touch it against the side of a beaker

45
Q

why do you read a burette at eye level?

A

to avoid the error of parallax

46
Q

how do you get a more accurate reading of a burette

A

put a white tile or white paper behind it

47
Q

why is a conical flask, cone-shaped?

A

for swirling without losing some liquid

48
Q

why do you swirl in a conical flask

A

to get a full reaction

49
Q

how do you prevent acid/base sticking to the sides of a conical flask

A

wash them down with deionised water from a wash bottle

50
Q

what do you rinse a conical flask with

A

deionised water but never the solution it is about to contain

51
Q

why do you not rinse a conical flask with the solution it’s about the contain

A

it will change the volume in the conical flask, which matters, but the water doesn’t change the concentration you already measured

52
Q

what can a graduated cylinder measure

A

approximate values only

53
Q

what is the accuracy of a graduated cylinder

A

0.5 cm cubed

54
Q

why is a graduated cylinder not very accurate?

A

it is too wide for accurate measurements

55
Q

which instruments are very accurate

A

narrow instruments such as the pipettes, burettes etc.

56
Q

purpose of a titration

A

to standardise a solution

find the exact concentration in moles per litre

57
Q

why do you not pipette directly from the volumetric flask

A

impurities on the pipette would contaminate the entire solution in the volumetric flask

58
Q

indicator used in Na2CO3 vs HCl titration

A

methyl orange

59
Q

methyl orange in a basic solution

A

yellow

60
Q

methyl orange in an acidic solution

A

red

61
Q

methyl orange in a neutral solution

A

orange/pink

62
Q

what should you not forget to do with the funnel?

A

take it out of the burette

63
Q

what do you do with the 2 white tiles?

A

one underneath the conical flask and one behind the burette reading

64
Q

end products of a titration

A

salt
water
CO2 gas

65
Q

2 reasons not to put a base like NaOH in a burette

A

i. NaOH erodes glass causing taps to stick and leak

ii. NaOH reacts with CO2 in the air forming hardened Na2CO3 causing the tap to be stuck

66
Q

why can a burette and pipette be washed down with the solution that they will contain?

A

the burette and pipette measure exact volumes of exact concentration

67
Q

why can you not wash down a conical flask with the solution it will contain?

A

a conical flask is a container where an exact number of moles of one reactant coming from the burette meet and react with with an exact number of moles of the other reactant coming from a pipette

68
Q

after standardising HCl with primary standard Na2CO3, what is it called?

A

a secondary solution

69
Q

why should you not touch NaOH

A

it can give a severe burn

70
Q

when obtaining NaCl crystals, why do you do another titration?

A

you do another one without the indicator

71
Q

how do you obtain the NaCl crystals?

A

only NaCl and water is left in the conical flask
place on an evaporating dish over a hot plate
white crystals are formed

72
Q

what do you titrate against vinegar (ethanoic acid)

A

NaOH solution

73
Q

why do you dilute the ethanoic acid?

A

because otherwise you would get a very small reading which leads to greater percentage error

74
Q

why would you not just use a higher concentration of NaOH

A

it’s not safe for a school laboratory

75
Q

what indicator is used when standardising vinegar

A

phenolphthalein

76
Q

when is phenolphthalein used

A

with a weak acid and a strong base

77
Q

phenolphthalein in acid

A

colourless

78
Q

phenolphthalein in base

A

pink

79
Q

when do you use the pipette twice?

A

in titrating ethanoic acid (vinegar) because it is a liquid not a solid

80
Q

ratio of vinegar dilution

A

1:5

81
Q

how to find the original concentration of the vinegar

A
  • find in m/l of dilute
  • x5 = m/l of original
  • find in g/l
  • find g/100cm3 %w/v (divide by 10)
  • eg. 7.8%, that’s it
82
Q

what indicator used in finding % water crystallisation in hydrated sodium carbonate

A

methyl orange

83
Q

how do you calculate % of water crystallisation in hydrated sodium carbonate

A
  • find m/l
  • find g of crystals in 1L of H20
  • find g in 250cm3
  • of the 5 g only x was Na2CO3
  • 5-x = y
  • y/5 x100 = percentage
    eg. 39.2%
84
Q

how to find ‘X’ in

Na2CO3–XH2O

A 
100 - 29.2% of Na2CO3
= 60.8% of water in it 
29.2% is 106g
60.8% is y grams
find y by getting 1% and multiply, = 164 grams
1 H2O = 18g
164/18 
X = 9.13 \_\_ 9
85
Q

why do you not get a whole number when finding X?

A

experimental inaccuracy

just round off to the nearest whole number

86
Q

number of moles of solute=

A

volume in cm3 X Molarity
___________________
1000

87
Q

grams =

A

moles x Mr

88
Q

formula used when calculating between different concentrations of the same substance

A

Vol.dil X M.dil =

Vol.conc X M.conc

Chemistry (Tim) (8 decks)

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