New Volumetric Analysis - Dublin Academy · 2016. 2. 11. · Volumetric Analysis No part of this...
Transcript of New Volumetric Analysis - Dublin Academy · 2016. 2. 11. · Volumetric Analysis No part of this...
5th Year Chemistry
Higher Level Sinéad Nolan
Volumetric Analysis
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Ref: 6/che/h/sn/volumetric analysis
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@Dublin School of Grinds Page 2 Sinéad Nolan
Table of Contents
Volumetric Analysis Procedures .................................................................................................. 3
Determination of the amount of water of crystallisation in hydrated Na2CO3 .................. 7
Determination of the concentration of ethanoic acid in vinegar ......................................... 9
Estimation of iron in an iron tablet ........................................................................................... 11
Iodine – thiosulphate titration ................................................................................................... 13
Determination of the percentage of hypochlorite in bleach ............................................... 15
Estimation of the total hardness of a water sample .............................................................. 17
Estimation of dissolved oxygen by redox titration ................................................................ 19
Past Exam Question 1’s ............................................................................................................... 21
Past Exam Solutions to Question 1’s ......................................................................................... 49
@Dublin School of Grinds Page 3 Sinéad Nolan
Volumetric Analysis Procedures
Primary standard
pure / stable / anhydrous (not hydrated) / no water loss (no efflorescence) / not
deliquescent / not hygroscopic /does not sublime / high molecular (molar) mass (Mr)
from which solutions of known concentration (molarity) can be made / no need to
standardise by titration / water soluble
NB Primary standard for acid-base reactions = anhydrous sodium carbonate (Na2CO
3)
[Allow (3) for sodium carbonate.]
[OTHER POSSIBILITY: sodium tetraborate (disodium tetraborate]
Primary standard for redox reactions = ammonium iron(II) sulphate
A standard solution is a solution whose concentration (molarity) is known
A standardised solution is a solution whose concentration (molarity) known (found, got,
etc.) by another titration (colorimetry, u.v. spectroscopy)
Making up a solution starting with a solid solute
rinse from clock glass into beaker containing deionised water //
stir // dissolve //
pour (add) through funnel into volumetric flask //
add rinsings of beaker //
add deionised water until bottom of meniscus on (level with) mark /
read at eye level //
stopper and invert* several times
[* Do not allow “shake” for “invert”]
@Dublin School of Grinds Page 4 Sinéad Nolan
Making up a solution with iron tablets
tablets crushed with mortar and pestle //
washed into beaker //
stirred to dissolve //
transferred into flask using funnel / glass rod //
rinsings of beaker added to flask //
flask on level surface / mark at eye-level //
add drop-by-drop / add using dropper (pipette, wash bottle) / top up carefully //
until bottom of meniscus level with mark //
invert / mix / shake [“swirl” not acceptable]
Diluting a solution of bleach or vinegar
pipette (burette) vinegar/bleach (3) can be shown on diagram
into volumetric flask (3) can be shown with diagram provided line on neck present
add deionised water (3)
when near mark, add dropwise (using dropper/pipette/wash bottle) / until bottom of
meniscus on (at) mark / read bottom of meniscus
Procedure for preparing the burette
rinse with deionised water
rinse with reagent (solution)
clamp vertically
use funnel when adding reagent / remove funnel after filling
open tap to fill below tap (tip, jet, nozzle) / remove air bubbles [Allow ‘tap is full’]
set bottom of meniscus on mark / read bottom of meniscus
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Importance of filling the part below the tap
air will be displaced by the solution (reagent) / some of measured volume replaces air /
some of measured volume not delivered / some of measured volume goes to fill space /
causes (gives) wrong (inaccurate, too high, too low) reading (result, titre) / air will be
displaced (removed, got rid of) during the titration / will be filled during the titration /
affects result / burette only works properly when it (part below tap) is full / burette
designed to work properly when it (part below tap) is full / distorts result (reading)
Description of how the level of the liquid in the burette was adjusted to the zero mark
fill above mark and adjust with tap / fill to below mark and add dropwise
Procedure for preparing the pipette
rinse with water followed by the solution it is going to contain //
fill pipette using a pipette filler to above the mark (graduation line) //
adjust to have bottom of meniscus on mark / read at eye level (vertically)
remove droplets adhering to outside //
drain under gravity into titration flask //
touch tip of pipette against side of flask to add droplet adhering to outside tip // do
not blow out drop inside pipette
Importance of using a pipette filler
safety / avoid solution getting into mouth / hygiene
Explanation of operations involving the conical flask and its contents during the titration
swirl to mix //
allow time after addition from burette for reaction //
On white surface //
Wash down sides with deionised water (NB this does not affect number of mols of
reactants in the flask)
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Importance of placing the conical flask on a white tile
so that colour-change (end-point) clearer (more easily seen)
Precautions (and explanation of how this precaution would have contributed to the
accuracy of the titration result) that should have been taken as the end point of the
titration was approached.
Precaution Explanation
add drop by drop (slowly) add dropwise so that end point will be
precisely (accurately) detected (correct
end point not passed) / one drop of
solution would change colour near
end point
wash down inner sides of conical flask wash sides so that all reagent(s) (acid)
in the reaction mixture
swirl (shake) flask contents swirl to ensure thorough mixing of
reactants
@Dublin School of Grinds Page 7 Sinéad Nolan
Determination of the amount of water of crystallisation in hydrated Na2CO3 (2006)
1Na2CO3 = 2HCl
1
25 x M =
2
26.05 x 0.11
M = 25 x 2
1 x 26.05 x 1 0.1
(i) M = 0.05731 mol L–1 (molarity of the Na2CO3 solution)
x 106 (Mr of Na2CO3)
(ii) = 6.075 g L–1 (concentration of the Na2CO3 solution in g L–1)
2 (solution only made up to 500 cm3)
= 3.0375 g of Na2CO3 in 500 cm3
Mass of water of crystallization in the crystals = mass of crystals – mass of Na2CO3 in 500
cm3
= 8.20 – 3.0375
= 5.1625 g
% by mass of water of crystallisation = crystals theof mass total
solution theof cm 500in water of mass 3
= 8.2
5.1625x
1
100
= 62.9 % (% by mass of water of crystallisation in
crystalline Na2CO3)
106
3.0375:
18
5.1625(divide by Mr)
0.0287 : 0.287
0.0287
0.0287:0.0287
0.287(divide by smallest number)
1 : 10
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Notes
Indicator: methyl orange
Colour change at the end-point: yellow (base) to pink (acid)
Example of a strong acid – weak base titration
Methyl orange suitable as an indicator as the pH at the end point passes through the
indicator range
Not more than 1 – 2 drops of indicator should be used as indicators themselves are
weak acids or weak bases and may affect titration results if added in larger quantities
@Dublin School of Grinds Page 9 Sinéad Nolan
Determination of the concentration of ethanoic acid in vinegar (2008)
1CH3COOH = 1NaOH
1
22.65 x M =
1
25 x 0.1
M = 22.65 x 1
1 x 25 x 0.1
M = 0.11 mol L–1 (molarity of the diluted vinegar)
x 60 (Mr of CH3COOH)
= 6.6 g L–1 (concentration of the diluted vinegar in g L–1)
x 10 (dilution factor 25 cm3 to 250 cm3)
= 66 g L–1 (concentration of the original vinegar in g L–1)
x 10 (g L–1 to g 100 cm–3 = % (w/v))
= 6.6 % (w/v) (concentration of original vinegar in % (w/v))
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Notes
The vinegar is diluted because it is too concentrated and there would have been a
very large volume of NaOH needed to get a reasonable titration and also it would
have required a very concentrated solution of NaOH for neutralisation
Indicator: phenolphthalein
Colour change at the end-point: pink (base) to colourless (acid)
Example of a weak acid – strong base titration
Phenolphthalein suitable as an indicator as the pH at the end point passes through
the indicator range
Ethanol in white wine is oxidised to ethanoic acid in vinegar
Methanoic acid is the acid which occurs in nettles and stinging ants
Benzoic acid and it’s salts are used as food preservatives, disinfectants, antiseptics,
biocides and fungicides
Not more than 1 – 2 drops of indicator should be used as indicators themselves are
weak acids or weak bases and may affect titration results if added in larger quantities
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Estimation of iron in an iron tablet (2009)
5Fe2+ = 1MnO4-
5
25 x M =
1
18.75 x 0.1
M = 25 x 1
5 x 18.75 x 0.1
(i) M = 0.0375 mol L–1 (molarity of the Fe2+ solution)
4 (solution only made up to 250 cm3)
= 0.009375 mol 250 cm–3
x 56 (Ar of Fe)
(ii) = 0.525 g 250 cm–3 (total mass of iron in 250 cm3 of the solution)
5 x 0.325 = 1.625 g (mass of the 5 tablets)
% by mass of iron in the tablets = tablets theof mass total
solution theof cm 250in iron of mass 3
= 1.625
0.525x
1
100
(iii) = 32% (% by mass of iron in the tablets)
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Notes
KMnO4 is a secondary standard as it decomposes in strong sunlight and so must be
standardised immediately prior to use.
KMnO4 is an oxidising agent and is easily reduced. It does not dissolve easily in water.
The titre reading is taken from the top of the meniscus when KMnO4 is in the burette
as the bottom of the meniscus is difficult to see due to the opaque nature of the
KMnO4
Iron tablets are prescribed to prevent anaemia
H2SO4 added when making up tablet solution to prevent Fe2+ being oxidised to Fe3+
by oxygen in the air
H2SO4 added before each titration to ensure complete conversion of MnO4- to Mn2+
and to prevent formation of Mn4+, a dirty brown precipitate
Neither HCl or HNO3 can be used to acidify the solution in the conical flask as the Cl-
ions in the HCl would be oxidised to produce Cl2 which is poisonous, and HNO3 is a
strong oxidising agent itself and will take part in the reaction
Self indicated: no indicator required
Colour change at the end-point: colourless to first permanent pink colour
Reaction is autocatalysed, i.e. Mn2+ which is a product of the reaction catalyses
reaction. Rate of reaction increases as reaction proceeds
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Iodine – thiosulphate titration (2007)
2S2O3 = 1I2
2
20 x M =
1
25x 0.05
M = 20 x 1
2x 25x 0.05
M = 0.125 mol L–1 (molarity of the Na2S2O3.5H2O solution)
x 248 (Mr of Na2S2O3.5H2O)
= 31 g L–1 (concentration of crystalline Na2S2O3.5H2O in g L–1)
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Notes
KI is added to bring/keep the I2 in solution; pure I2 is insoluble in water, as water is
polar and I2 is non-polar and hence cannot form hydrogen bonds with water
molecules
Indicator: starch
Colour change at the end-point: blue-black to colourless
Colour changes from the start of the titration:
KMnO4 + KI /
H2SO4 Na2S2O3 + starch + Na2S2O3
purple reddy/
brown
straw
yellow
blue-
black colourless
Starch is only added once the solution turns straw yellow. If the starch was added
before this stage then the blue-black complex that is formed is too concentrated and
too stable to decompose fast enough to give an accurate end-point
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Determination of the percentage of hypochlorite in bleach (2011)
1ClO- = 1I2
1I2 = 2S2O3
1ClO- = 2S2O3
1
25 x M =
2
16.1 x 0.1
M = 25 x 2
1 x 16.1 x 0.1
(i) M = 0.0322 mol L–1 (molarity of the diluted bleach)
x 20 (dilution factor 25 cm3 to 500 cm3)
(ii) = 0.644 mol L–1 (molarity of the original bleach)
x 74.5 (Mr of NaClO)
(i) = 47.978 g L–1 (concentration of NaClO in g L–1)
10 (g L–1 to g 100 cm–3 = % (w/v))
(i) = 4.7978 % (w/v) (concentration of NaClO in % (w/v))
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Notes
The bleach is diluted because it is too concentrated and there would have been a very
large volume of Na2S2O3 needed to get a reasonable titration
KI added for two reasons (i) to ensure all the bleach reacted and that the maximum
amount of iodine was liberated and (ii) to bring/keep the I2 in solution; pure I2 is
insoluble in water, as water is polar and I2 is non-polar and hence cannot form
hydrogen bonds with water molecules
Indicator: starch
Colour change at the end-point: blue-black to colourless
Colour changes from the start of the titration:
bleach + KI /
H2SO4 Na2S2O3 + starch + Na2S2O3
colourless reddy/
brown
straw
yellow
blue-
black colourless
Starch is only added once the solution turns straw yellow. If the starch was added
before this stage then the blue-black complex that is formed is too concentrated and
too stable to decompose fast enough to give an accurate end-point
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Estimation of the total hardness of a water sample (2010)
1M2+ = H2Y2-
1
50 x M =
1
9.20 x 0.01
M = 50 x 1
1 x 9.20 x 0.1
(i) M = 0.00184 mol L–1 (moles per litre of calcium and magnesium ions (M2+))
x 100 (Mr of CaCO3)
(ii) = 0.184 g L–1 (grams per litre expressed in terms of CaCO3)
x 1000 (g L–1 mg L–1 ppm)
(iii) = 184 ppm (ppm in terms of CaCO3)
@Dublin School of Grinds Page 18 Sinéad Nolan
Notes
EDTA = ethylene diamine tetraacetic acid
Indicator: Eriochrome Black T or Solochrome Black T, a grey-blue solid
Colour change: wine-red to blue
Complexiometric titration – EDTA forms a complex with the Ca2+ and Mg2+ ions that
are present in the hard water
Indicator complexes with Ca2+ and Mg2+ ions at the start of the experiment = wine-red
colour
Add the EDTA, Ca2+ and Mg2+ ions will form a complex with the EDTA
Indicator without the Ca2+ and Mg2+ ions = blue colour
Buffer pH = 10 is added as the indicator only works accurately at pHs greater than 9
Not using a buffer may lead to a poor endpoint, incomplete complexing or EDTA
complexing with other ions
The general purpose of buffers is to resist changes in pH or to stabilize the pH
EDTA is stored in a plastic bottle as it may extract metal ions from glass if left in a glass
container for long periods of time
@Dublin School of Grinds Page 19 Sinéad Nolan
Estimation of dissolved oxygen by redox titration
1O2 = 4Mn(OH)3
4Mn(OH)3 = 2I2
2I2 = 4S2O3
1O 2 = 4S2O3
1
100 x M =
4
5.7 x 0.02
M = 100 x 4
1x 5.7x 0.02
M = 0.000285 mol L–1 (molarity of the dissolved oxygen in the water)
x 32 (Mr of O2)
= 0.00912 g L–1 (concentration of the dissolved oxygen in the water in g L–1)
x 1000 (g L–1 mg L–1 ppm)
= 9.12 ppm (concentration of the dissolved oxygen in the water in ppm)
NB 1:4 ratio if asked to calculate O2 concentration directly; 1:2 ratio if asked to calculate
I2 concentration first
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Notes
Indicator: starch
Colour change at the end-point: blue-black to colourless
Colour changes from the start of the titration:
white ppt brown ppt reddy/brown straw yellow blue-black colourless
Starch is only added once the solution turns straw yellow. If the starch was added
before this stage then the blue-black complex that is formed is too concentrated and
too stable to decompose fast enough to give an accurate end-point
The following precautions should be observed when collecting water for B.O.D.
analysis (i) fill the bottle under the surface of the water to prevent atmospheric
oxygen being trapped and, thus, giving an artificially high oxygen level (ii) fill the
bottle completely to ensure that no air is trapped between the top of the water and
the stopper of the bottle and (iii) the second bottle is stored in the dark for 5 days at
20 oC to prevent photosynthesis leading to an increased oxygen content or to prevent
respiration which will lead to a decrease in the oxygen content
Concentrated solutions of MnSO4 and alkaline KI are used to minimise the amount of
the water sample that is displaced, to minimise the change in the oxygen dissolved in
the sample and to ensure that a small volume of solution supplies an excess
The additions were made well under the level of the water, using a dropper, making
sure not to bubble air into the water in the process
Water samples that may have high B.O.D.s should be diluted by a fixed amount with
well-oxygenated water in order to ensure that dissolved oxygen will be present
during the 5 day period and that a measurable amount of oxygen will be present at
the end of the test period
It can be concluded that no dissolved oxygen is present had a white precipitate been
observed instead of the brown precipitate after the first two additions of reagents to
the bottle filled with river water?
Kits, designed for use in the field, allow the dissolved oxygen concentration to be
measured immediately on collection of the sample. The immediate determination of
dissolved oxygen is considered best practice as biochemical (biological) reactions
(photosynthesis, respiration, metabolism) may occur which will alter the dissolved
oxygen concentration
@Dublin School of Grinds Page 21 Sinéad Nolan
Past Exam Question 1’s
LC 2015 – Question 1
Solution
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Solution (cont’d)
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LC 2014 – Question 1
Solution
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Solution (cont’d)
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LC 2013 – Question 1
Solution
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Solution (cont’d)
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LC 2012 – Question 1
Solution
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Solution (cont’d)
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LC 2011 – Question 1
Solution
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Solution (cont’d)
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LC 2010 – Question 1
Solution
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Solution (cont’d)
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LC 2009 – Question 1
Solution
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Solution (cont’d)
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LC 2008 – Question 1
Solution
@Dublin School of Grinds Page 36 Sinéad Nolan
Solution (cont’d)
@Dublin School of Grinds Page 37 Sinéad Nolan
LC 2007 – Question 1
Solution
@Dublin School of Grinds Page 38 Sinéad Nolan
Solution (cont’d)
@Dublin School of Grinds Page 39 Sinéad Nolan
LC 2006 – Question 1
Solution
@Dublin School of Grinds Page 40 Sinéad Nolan
Solution (cont’d)
@Dublin School of Grinds Page 41 Sinéad Nolan
LC 2005 – Question 1
Solution
@Dublin School of Grinds Page 42 Sinéad Nolan
Solution (cont’d)
@Dublin School of Grinds Page 43 Sinéad Nolan
LC 2004 – Question 1
Solution
@Dublin School of Grinds Page 44 Sinéad Nolan
Solution (cont’d)
@Dublin School of Grinds Page 45 Sinéad Nolan
LC 2003 – Question 1
Solution
@Dublin School of Grinds Page 46 Sinéad Nolan
Solution (cont’d)
@Dublin School of Grinds Page 47 Sinéad Nolan
LC 2002 – Question 1
Solution
@Dublin School of Grinds Page 48 Sinéad Nolan
Solution (cont’d)
@Dublin School of Grinds Page 49 Sinéad Nolan
Past Exam Solutions to Question 1’s
LC 2015 – Question 1 Solution
@Dublin School of Grinds Page 50 Sinéad Nolan
LC 2014 – Question 1 Solution
@Dublin School of Grinds Page 51 Sinéad Nolan
LC 2013 – Question 1 Solution
@Dublin School of Grinds Page 52 Sinéad Nolan
LC 2012 – Question 1 Solution
@Dublin School of Grinds Page 53 Sinéad Nolan
LC 2011 – Question 1 Solution
@Dublin School of Grinds Page 54 Sinéad Nolan
LC 2010 – Question 1 Solution
@Dublin School of Grinds Page 55 Sinéad Nolan
LC 2009 – Question 1 Solution
@Dublin School of Grinds Page 56 Sinéad Nolan
LC 2008 – Question 1 Solution
@Dublin School of Grinds Page 57 Sinéad Nolan
LC 2007 – Question 1 Solution
@Dublin School of Grinds Page 58 Sinéad Nolan
LC 2006 – Question 1 Solution
@Dublin School of Grinds Page 59 Sinéad Nolan
LC 2005 – Question 1 Solution
@Dublin School of Grinds Page 60 Sinéad Nolan
LC 2004 – Question 1 Solution
@Dublin School of Grinds Page 61 Sinéad Nolan
LC 2003 – Question 1 Solution
@Dublin School of Grinds Page 62 Sinéad Nolan
LC 2002 – Question 1 Solution