Titrations
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2015 Mock
(a) a stronger concentration of vinegar would require a greater quantity of NaOH to reach the end point of titration ✓
vinegar too concentrated and would require a very concentrated NaOH solution
(b) a pipette is rinsed with deionised water and a sample of vinegar
25cm³ of vinegar is pipetted using a pipette filler until the 25cm³ mark reads from the bottom of the meniscus at eye-level
25cm³ of vinegar is transferred to a 500cm³ volumetric flask
the solution is made up to 500cm³ with deionised water until the mark reads from the bottom of meniscus at eye-level.
the flask is stoppered and inverted 20 times ✓
c) phenolphthalein ✓
strong base-weak acid ✓
pink → colourless ✓
(d) (i) 0.0935M ✓
(ii) 5.61g per litre ✓
112.23g per litre ✓
11.22% ✓
(e) benzoic acid ✓
2016 Mock
(a) Eriochrome Black T ✓
wine-red → blue ✓
(b) rinse the pipette with deionised water and a sample of the solution it will contain
fill pipette using pipette filler until the mark reads below the meniscus at eye-level
drain under gravity and allow drainage time when transferring solution to conical flask
touch the pipette against the wall of the conical flask
c) pH buffer ✓
to keep the pH of the solution at pH 10 as the indicator works best in alkaline conditions
ensure sharp end-point
(d) gently swirl the conical flask wash down sides of conical flask with deionised water
the flask has been swirled, mixing the solution throughout no effect on titration as the water contains no ions
(e) (i) 7.8 ×10^-4 moles per litre ✓
(ii) 0.078g ✓
(iii) 78 p.p.m ✓
(f) smaller titre value ✓
the temporary hardness in the water would be removed by boiling, reducing the number of Ca2+ and Mg2+ ions in the water ✓
2018
(a) CaSO4 ✓
(b) pipette is rinsed with deionised water and then a sample of the hard water ✓
pipette is filled using a pipette filler until 25cm³ mark reads from bottom of meniscus at eye-level
hard water transferred from pipette into a clean, dry conical flask.
allow hard water to drain under gravity and allow drainage time.
touch the pipette against the inside wall of the conical flask ✓
repeat the procedure for another 25cm³ to fill the conical flask to 50cm³ ✓
c) (i) eriochrome black T ✓
(ii) wine-red ✓
(iii) blue ✓
(d) (i) pH 10 buffer ✓
(ii) to ensure a sharp end-point of titration ✓
(e) (i) 9.3 × 10^-5 moles ✓
(ii) 9.3 × 10^-5 moles ✓
(iii) 0.186g ✓
(iv) 186 p.p.m ✓
(f) the water sample is boiled before titrating ✓
if the titration figure is smaller than the titration figure of the unboiled sample, then the water contained temporary hardness which was removed when boiled ✓
2019
(a) (i) manganese (II) sulfate ✓
(ii) to ensure reaction was carried out to completion X ensures all O2 is detected
(iii) the brown precipitate dissolved leaving a reddish-brown coloured solution of the I2 ✓
(b) (i) conical flask was cleaned and dried. ✓
a pipette was rinsed with deionised water and a sample of the solution it would contain
the pipette was filled using a pipette filler until the mark read from below the meniscus at eye-level
the solution was transferred from the pipette to the conical flask under gravity, allowing drainage time.
pipette was touched against the inside wall of the conical flask.
conical flask was swirled as sodium thiosulfate was added. ✓
when solution in conical flask was a straw-yellow colour starch indicator was added
sodium thiosulfate continued to be added until the blue-black colour of indicator changed to colourless.
conical flask was rinsed with deionised water and dried
conical flask was swirled to mix contents
inside walls of conical flask were washed down with deionised water
the conical flask was placed on a white tile to view colour change
(ii) the burette was held vertically upright in a retort stand
bend until eye at level of the liquid
c) (i) reddish-brown → orange → straw-yellow ✓
(ii) blue-black → colourless ✓
(d) (i) 7.5 × 10^-5 moles ✓
(ii) 1.875 × 10^-5 moles ✓
(iii) 9.375 × 10^-5 moles per litre ✓
(iv) 3 × 10^-3 g
= 3 p.p.m
3 p.p.m < 5 p.p.m, therefore the dissolved oxygen levels are too low and are a threat to the fish survival in this water system ✓
2019 Mock
(a) a primary standard is a substance that is
available in a highly pure state
easily dissolved in water
stable in air
has a high relative molecular mass ✓
anhydrous
(b) 1.06g of salt weighed on clock glass is transferred to a clean, dry beaker and rinsings are added to beaker
salt is dissolved in deionised water and stirred with a glass rod
solution is poured from beaker into volumetric flask through funnel
rinsings from beaker and funnel are added to volumetric flask
flask is made up to mark with deionised water, adding last drops drop by drop, until mark reads from bottom of meniscus at eye-level
stopper and invert flask 20 times ✓
c) methyl orange ✓
orange → pink ✓
(d) resulting titration would decrease as the concentration of the sodium carbonate has already been reduced in the hydrated sodium carbonate salt ✓
(e) (i) 1.25 × 10^-3 moles per 25cm³ ✓
(ii) 1.21 × 10^-4 moles per cm³ ✓
(iii) 0.121 M ✓
(f) sodium carbonate to standardise a solution of HCl ✓
standardised HCl to standardise the NaOH solution ✓
sodium carbonate titrated with hydrochloric acid
hydrochloric acid titrated with NaOH
2020
(a) anhydrous sodium carbonate
(b) transfer washing soda from weighing boat into a clean, dry beaker by rinsing with wash bottle of deionised water ✓
add rinsings to beaker
stir with glass rod to dissolve
transfer from beaker to volumetric flask through a funnel ✓
add rinsings from beaker, funnel and glass rod to flask✓
make the solution up to the 250cm³ mark until mark reads from bottom of meniscus at eye-level ✓
stopper and invert the flask 20 times ✓
c) (i) methyl orange ✓
(ii) strong acid-weak base titration ✓
(iii) yellow → red ✓
(d) (i) 2.58 × 10^-3 moles ✓
(ii) 1.29 × 10^-3 moles ✓
(iii) 0.0129 moles in 250cm³ ✓
(iv) 1.3674g in 250cm³ ✓
(v) 2.0926g of water in 250cm³ ✓
= 0.116 moles of water ✓
(vi) x = 9 ✓
2020 Mock
(a) hydrated salt previously weighed on clock glass is transferred to a clean, dry beaker using a wash bottle of deionised water
rinsings from the clock glass are added to beaker
solution stirred with a glass rod to dissolve
solution poured through funnel from beaker into clean, dry volumetric flask
rinsings from beaker, glass rod and funnel added to vol. flask
solution made up to the 250cm³ mark using deionised water until mark reads from below meniscus at eye-level
stopper and invert the flask 20 times. ✓
(b) methyl orange ✓
yellow → red ✓
c) (i) 2.622 × 10^-3 moles ✓
(ii) 1.311 × 10^-3 moles ✓
(iii) (a) 0.0656 M ✓
(b) 6.95g per litre ✓
(iv) 2.953g of water ✓
= 62.9% ✓
x = 10 ✓
2021
(a) rinse a pipette with deionised water and a sample of the bleach
using a pipette filler, fill pipette to the 25cm³ mark until mark reads from bottom of meniscus at eye-level
transfer bleach from pipette to clean, dry volumetric flask under gravity
allow drainage time and touch the pipette against the inside walls of the volumetric flask
make the solution up to 500cm³ mark with deionised water until mark reads from bottom of meniscus at eye-level
stopper and invert flask 20 times
(b) to ensure reaction is carried out to completion
c) colourless → reddish-brown
(d) (i) starch ✓
(ii) blue-black → colourless ✓
(e) (i) 1.926 × 10^-3 moles ✓
(ii) 9.63 × 10^-4 moles ✓
(iii) 0.77M ✓
(iv) 4% ✓
2021 Mock
(a) 4 iron tablets are weighed on a clock glass using an electronic balance
tablets are crushed using a pestle and mortar
tablets washed into a beaker using a wash bottle of solution of deionised water and dilute sulfuric acid
stirred with a glass rod to dissolve
solution is transferred to a clean, dry volumetric flask through a funnel
add rinsings from the beaker, glass rod and funnel
make solution up to 250cm³ mark using solution of deionsied water and dilute sulfuric acid until mark reads from bottom of meniscus at eye-level
stopper and invert flask 20 times
(b) to ensure Mn7+ ion is reduced to Mn2+ ion and not Mn4+ ion
c) to ensure Fe2+ ion was not oxidised by air to Fe3+ ion
(d) solution changed from colourless → permanent pale pink ✓
(e) nitric acid is strong oxidising agent
(f) (i) 1.41 × 10^-4 moles ✓
(ii) 7.05 × 10^-4 moles ✓
(iii) 0.0353M ✓
(iv) (a) 0.1236g per tablet ✓
(b) 36.35% ✓
2022
(a) (i) Ca2+ and Mg2+ ✓
(ii) permanent hardness + temporary hardness = total hardness ✓
(b) (i) Eriochrome Black T ✓
(ii) wine-red → blue ✓
c) (i) pH10 ✓
(ii) to ensure a sharp end-point of titration ✓
(d) burette is clamped vertically upright in a retort stand, hanging over a laboratory bench so that the 0cm³ mark is at eye-level
edta solution poured through funnel into burette until approaching the 0cm³
final drops are added dropwise using a dropper until 0cm³ mark reads from bottom of meniscus at eye-level ✓
(e) (i) 9.2 × 10^-5 moles ✓
(ii) 9.2 × 10^-5 moles ✓
(iii) 3.68 × 10^-3 moles ✓
(iv) 0.368g ✓
368 p.p.m ✓
2022 def.
(a) using a funnel the solution was transferred from the beaker to the flask
rinse beaker number of times with deionised water
rinsings from the beaker and the funnel were then added to the volumetric flask ✓
(b) burette is rinsed with deionised water and a sample of KMnO4
burette is clamped vertically upright in a retort stand
fill burette using a funnel until approaching 0cm³ mark
remove funnel
add final drops dropwise using a dropper until 0cm³ mark reads from top of meniscus at eye-level ✓
c) to ensure Fe2+ was not oxidised to Fe3+ by the air
ensure Mn7+ is oxidised to Mn2+ and not Mn4+
(d) solution in conical flask changed from colourless → permanent pale pink ✓
(e) (i) 0.12M ✓
(ii) 1.68g in 250cm³ ✓
(iii) 14.29% ✓
2023 def.
(a) (i) manganese (II) sulfate ✓
(ii) to ensure detection of all dissolved O2 in water sample ✓
(iii) brown ✓
(b) (i) a solution of accurately known concentration ✓
(ii) pipette is rinsed with deionised water and sample of the iodine solution ✓
pipette filled using a pipette filler until 25cm³ mark reads from bottom of meniscus at eye-level ✓
solution transferred from pipette to conical flask under gravity
allow drainage time and touch the pipette against the inside walls of the conical flask
repeat this procedure another 5 times to fill the conical flask to 150cm³ of solution ✓
(iii) starch ✓
(iv) blue-black → colourless ✓
c) (i) 1.14 × 10^-4 moles ✓
(ii) 5.7 × 10^-5 moles ✓
(iii) 1.9 × 10^-4 moles per litre ✓
(iv) 0.00608 ✓
= 6.08 p.p.m ✓