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General Practical Flashcards

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1
Q

What is the dp or sf for pipette?

A

1dp

eg. 25.0cm³

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2
Q

What is the dp or sf for burette readings?

A

2dp to nearest 0.05cm³

eg. 27.05cm³

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3
Q

What is the dp or sf for measuring cylinder?

A

same dp as half the smallest division

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4
Q

What is the dp or sf for thermometer?

A

same dp as half the smallest division

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5
Q

What is the dp or sf for mass reading?

A

Follow dp of mass balance

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6
Q

What is the dp or sf for stopwatch?

A

Depends on task instructions

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7
Q

What is random error?

A

Causes experimental data to be scattered more or less symmetrically around a mean value

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8
Q

What is systematic error?

A

Causes experimental error to be consistently higher or lower than the accepted value

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9
Q

What is gross error?

A

occasional error in experimental data, usually large and is often a product of personal errors

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10
Q

What is the uncertainty of burette?

A
  • For each burette reading: ±0.05cm³
  • For each volume measured: ±0.10cm³

(because like ruler the inital reading and final reading both have uncertainty)

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11
Q

What is the uncertainty of 50.0cm³ measuring cylinder?

A

±0.5cm³

(measuring cylinder max V ÷ 100)

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12
Q

What is the uncertainty of 25.0cm³ measuring cylinder?

A

±0.25cm³

(measuring cylinder max V ÷ 100)

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13
Q

What is the uncertainty of 10.0cm³ measuring cylinder?

A

±0.1cm³

(measuring cylinder max V ÷ 100)

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14
Q

For weighing of solids, the reading should be within how much of the required mass?

A

0.05g

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15
Q

How to present weighing of solids using tare method?

A

Mass of solid FA1 used = 1.280g (TARE)

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16
Q

How to present experimental results for weighing by subtraction method?

A
  • mass of empty weighing bottle/g
  • mass of weighing bottle and solid FA1/g
  • mass of weighing bottle and residual solid FA1/g
  • mass of solid FA1 used/g
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17
Q

What is a standard solution?

A

Solution whose concentration is accurately known

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18
Q

What is a titrant?

A

Solution that is added from the burette

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19
Q

What is aliquot?

A

The fixed volume of solution in the conical flask

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20
Q

What is range for consistent titres?

A

within 0.10cm³ difference

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21
Q

How to present titration results?

A

Titration 1 2 3
Initial burette reading/cm³
Final burette reading/cm³
Volume of FA2 used/cm³
[ticks]

average volume of FA2 used= (X1+ X2)/2
(sometimes is next part)

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22
Q

What are carbonates and hydrogencarbonates?

A

Weak bases

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23
Q

How does acid-carbonate and acid-hydrogencarbonate titrations work?

A

CO₃²⁻, a diprotic base undergoes acid-base titration as the acid is added from the burette

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24
Q

What is the equation for acid-hydrogencarbonate titration?

A

HCO₃⁻ (aq) + H⁺ (aq) → CO₂ (g) + H₂O (l)

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24
Q

What is the equation for acid-carbonate titration?

A
  • Step 1: CO₃²⁻ (aq) + H⁺ (aq) → HCO₃⁻ (aq)
  • Step 2: HCO₃⁻ (aq) + H⁺ (aq) → CO₂ (g) + H₂O (l)
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25
Q

How does manganate titrations work?

A

MnO₄⁻ is a powerful oxidising agent and can be used to titrate against reducing agents like Fe²⁺, C₂O₄²⁻, H₂O₂

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26
Q

What is the reduction equation of MnO₄⁻ being reduced and what’s the colour change?

A

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Purple MnO₄⁻ to colourless (in dilute solutions) or pale pink (in concentration solution) Mn²⁺

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26
Q

What acid is used for the medium and why?

A
  • H₂SO₄ (aq) used
  • HNO₃ not used because it is an oxidising agent itself
  • HCl not used because it can be oxidised by MnO₄⁻
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27
Q

What happens if too little acid is added to maintain an acidic medium?

A

MnO₄⁻ will be reduced to MnO₂ which is brown ppt., the presence of brown ppt. will hinder the accurate detection of end-point

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28
Q

What is the colour change for the titration of MnO₄⁻ with C₂O₄²⁻ and H₂O₂?

A
  • C₂O₄²⁻ and H₂O₂ are colourless and their products are colourless
  • End-point is reached when one excess drop of KMnO₄ turns the solution in the conical flask from colourless to pale pink

(In conical flask: colourless
In burette: purple KMnO₄
When all reacted, One drop of purple KMnO4 in large amounts of colourless will have pale pink)

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29
Q

What is the equation and the colour change for the titration of MnO₄⁻ with Fe²⁺?

A
  • Fe²⁺ (pale green) → Fe³⁺ (yellow) + e⁻
  • End-point is reached when one excess drop of KMnO₄ turns solution from yellow to orange/pink

(large amounts of yellow + one drop of purple KMnO₄ = pink/orange)

30
Q

How to determine end-point colour change?

A
  • Look at the colour of the products of the reaction (that is in large quantities)
  • Then one drop of a colour will be added
  • What colour will it likely be? - product colour will be dominant with minor influence of that one drop
31
Q

What is the equation and colour change of iodine-thiosulfate reaction?

A
  • I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻

I₂ + 2S₂O₃²⁻: brown
2I⁻ + S₄O₆²⁻: colourless

32
Q

What is the colour change at the end-point in the absence of an indicator for iodine-thiosulfate titrations?

A

pale-yellow to colourless

(pale yellow: large qty of colourless + brown then one drop turns everything colourless)

33
Q

What is added for iodine thiosulfate titrations and how will this affect the end-point?

A
  • Starch indicator is added near end-point of titration when solution is pale yellow, unreacted iodine forms a blue-black complex with starch
  • End-point is reached when thiosulfate solution reacts with all the remaining iodine to give a distinct colour change from blue-black to colourless
34
Q

What happens if starch is added too early during the titration?

A
  • blue-black colour could persist even after end-point
  • because more iodine molecules will be trapped in the starch-iodine complex, resulting in more time required for all the iodine molecules to be freed for titration
35
Q

What are the variants of iodine-thiosulfate titrations?

A
  • iodimetric titration: direct titration of molecular iodine present
  • iodometric titration: titration of iodine that is liberated from an iodine-containing analyte (eg. IO₃⁻) with thiosulfate
36
Q

What are the equations for iodometric titration with IO₃⁻?

A
  1. Use excess KI (aq) to liberate I₂
    IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O
  2. Titration of I₂ with S₂O₃²⁻
    I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
37
Q

How can iodine thiosulfate titrations be related to kinetics?

A

samples of reaction mixture drawn at regular time intervals can be titrated to determine the amount of I₂ produced or left

38
Q

When is back titration used?

A
  • If the sample to be quantitatively analysed does not react with standard reagents available
  • or is insoluble in water (i.e. cannot dissolve in water to form aq solution for titration)
39
Q

What is the main idea of back titration?

A
  • If you have an impure solid that cannot be titrated, then use a known volume of X to react with the solid
  • then titrate X with something else to determine the X that is left then you can find X that is reacted
  • From that you can find stuff about solid
40
Q

What is gravimetric method?

A

determine the amount of analyte (sample to be analysed) through measurement of masses

41
Q

What chemical reactions can gravimetry be used?

A
  • precipitation (measure mass of ppt.)
  • acid-carbonate reaction (mass of CO2 evolved = different in mass of reaction mixture)
  • thermal decomposition (mass of CO2 evolved = different in mass of reaction mixture)
42
Q

What kind of acid-carbonate reactions do gravimetry not work with?

A
  • reactions that produce gas with low density (eg. H₂)
  • do not give rise to significant change in mass
43
Q

What is volatilisation gravimetric method?

A
  • heat solid analyte to complete thermal decomposition
  • complete reaction is indicated by consistent mass of residue obtained after repeated heat-cool-weigh cycles
  • difference in mass before and after gives mass of gaseous product lost
44
Q

How many times are you supposed to repeat the heat-cooling-weigh cycle?

A

until residue within 0.05g of difference

45
Q

How to present experimental results for volatilisation gravimetric method?

A
  • mass of empty crucible with lid/g
  • mass of crucible and FA1 before heating/g
  • mass of solid FA1 used/g
  • mass of crucible and residue after 1st heating/g
  • mass of crucible and residue after 2nd heating/g
  • mass of residue after 3rd heating/g
46
Q

What is the safety precaution for heating with crucible?

A
  • use a pair of tongs to handle hot crucible and lid
  • turn of gas tap when bunsen burner is not in use
47
Q

How to set up the reaction flask for solid and solution?

A
  • weighing bottle with solid that is not in contact with the solution
  • All this in a conical flask with a stopper
48
Q

How to set up the reaction flask for reaction between two liquids?

A
  • use a dropping funnel
  • dropping funnel introduces liquid in the conical flask which displaces the air in the conical flask
  • assume that the volume of air displaced is equal to volume of solution introduced
  • V(gas produced) = V(gas collected) - V(air displaced by FA2) [which is equal to volume of solution]
49
Q

How to present experimental results for dropping funnel?

A
  • Volume of gas collected/ cm³ x
  • Volume of FA2 added from dropping funnel/cm³ y
  • Volume of gas evolved/cm³ x-y
50
Q

Can gas collection method be used for thermal decomposition of solid?

A
  • Yes
  • But gas expands upon heating so time is required for cooling the gas required to rtp in a gas syringe
51
Q

Can hot gas be collected through water?

A
  • No, gives rise to experimental error
  • air originally present in the set up before heating will expand and get collected over water
52
Q

What additional precaution needs to be taken to prevent suck back for hot gas over water?

A
  • cool water may be suck back into the hot reaction flask
  • sudden cooling of reaction flask may cause it to crack
  • to prevent suck back: remove rubber tube first before heat source
53
Q

What gases can you downward displacement of water?

A

H₂, CO₂, O₂

54
Q

What safety precaution should be taken for downward displacement of water?

A

set-up must be appropriately clamped to prevent toppling and breakages

55
Q

How to present experimental results for downward displacement of water?

A
  • initial reading/cm³
  • Final reading/cm³
  • Volume of gas collected/cm³
56
Q

How to present experimental results for well-greased gas syringe?

A
  • Initial syringe reading/cm³
  • Final syringe reading/cm³
  • Volume of gas collected/cm³
57
Q

What is the absolute uncertainty of 50cm³ gas syringe?

A

±1cm³

58
Q

What are acidic gases?

A

CO₂, SO₂, Cl₂, HCl, NO₂

59
Q

What drying agent should be used for acidic gases?

Nature of drying agent

A
  • nature of drying agent: acidic
  • conc. H₂SO₄
60
Q

What gases are alkaline?

A

NH₃

61
Q

What drying agent should be used for alkaline gases?

Nature of drying agent

A
  • nature of drying agent: alkaline
  • anhydrous CaO
  • anhydrous Ca(OH)₂
62
Q

What is the nature and the drying agent that can be used for all gases?

A
  • nature of drying agent: neutral
  • anhydrous CaCl₂
63
Q

How to minimise heat exchange with surroundings?

A
  • carry out experiment in draught-free environment to minimise heat loss to surroundings
  • use polystyrene cup to carry out the reaction
64
Q

What are the safety hazards for calorimetric experiment?

A
  • calorimeter made of polystyrene cup can be easily topped, use a beaker to stabilize it
  • mercury is a neurotoxin, breaking of thermometer is a health hazard
65
Q

How to present experimental results of endothermic reaction?

A
  • initial temperature/°C
  • minimum temperature reached/°C
  • temperature drop/°C
66
Q

How to present experimental results of exothermic reaction?

A
  • Initial temperature of FA1/°C
  • Initial temperature of FA2/°C
  • Average temperature of FA1 and FA2/°C
  • Maximum temperature reached/°C
  • Temperature rise/°C
67
Q

What if the formula of weighted average of initial temperatures?

A

weighted average = [Vₛₐ₁/Vₜₒₜₐₗ × (Initial temp. of SA1)] + [Vₛₐ₂/Vₜₒₜₐₗ × (Initial temp. of SA2)]

68
Q

What is the solubility of group 1 salts and NH₄⁺ in water?

A

soluble

69
Q

What is the solubility of nitrates in water?

A

All soluble

70
Q

What is the solubility of halides in water?

A

Most soluble except Ag⁺, Pb²⁺, Cu⁺

71
Q

What is the solubility of sulfates?

A

Most soluble except Pb²⁺, Ca²⁺, Sr²⁺, Ba²⁺

72
Q

What is the solubility of oxides?

A

Most insoluble except oxides of group 1, NH₄⁺ and Ba²⁺

73
Q

What is the solubility of hydroxides?

A

Most hydroxides insoluble except Group I and Ba²⁺

74
Q

What is the solubility of carbonates and phosphates?

A

Most insoluble except Group 1 and NH₄⁺

Chemistry Practical (4 decks)

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