General Practical Flashcards
(76 cards)
1
Q
What is the dp or sf for pipette?
A
1dp
eg. 25.0cm³
2
Q
What is the dp or sf for burette readings?
A
2dp to nearest 0.05cm³
eg. 27.05cm³
3
Q
What is the dp or sf for measuring cylinder?
A
same dp as half the smallest division
4
Q
What is the dp or sf for thermometer?
A
same dp as half the smallest division
5
Q
What is the dp or sf for mass reading?
A
Follow dp of mass balance
6
Q
What is the dp or sf for stopwatch?
A
Depends on task instructions
7
Q
What is random error?
A
Causes experimental data to be scattered more or less symmetrically around a mean value
8
Q
What is systematic error?
A
Causes experimental error to be consistently higher or lower than the accepted value
9
Q
What is gross error?
A
occasional error in experimental data, usually large and is often a product of personal errors
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)
11
Q
What is the uncertainty of 50.0cm³ measuring cylinder?
A
±0.5cm³
(measuring cylinder max V ÷ 100)
12
Q
What is the uncertainty of 25.0cm³ measuring cylinder?
A
±0.25cm³
(measuring cylinder max V ÷ 100)
13
Q
What is the uncertainty of 10.0cm³ measuring cylinder?
A
±0.1cm³
(measuring cylinder max V ÷ 100)
14
Q
For weighing of solids, the reading should be within how much of the required mass?
A
0.05g
15
Q
How to present weighing of solids using tare method?
A
Mass of solid FA1 used = 1.280g (TARE)
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
17
Q
What is a standard solution?
A
Solution whose concentration is accurately known
18
Q
What is a titrant?
A
Solution that is added from the burette
19
Q
What is aliquot?
A
The fixed volume of solution in the conical flask
20
Q
What is range for consistent titres?
A
within 0.10cm³ difference
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)
22
Q
What are carbonates and hydrogencarbonates?
A
Weak bases
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
24
Q
What is the equation for acid-hydrogencarbonate titration?
A
HCO₃⁻ (aq) + H⁺ (aq) → CO₂ (g) + H₂O (l)
24
What is the equation for acid-carbonate titration?
* Step 1: CO₃²⁻ (aq) + H⁺ (aq) → HCO₃⁻ (aq)
* Step 2: HCO₃⁻ (aq) + H⁺ (aq) → CO₂ (g) + H₂O (l)
25
How does manganate titrations work?
MnO₄⁻ is a powerful oxidising agent and can be used to titrate against reducing agents like Fe²⁺, C₂O₄²⁻, H₂O₂
26
What is the reduction equation of MnO₄⁻ being reduced and what's the colour change?
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Purple MnO₄⁻ to colourless (in dilute solutions) or pale pink (in concentration solution) Mn²⁺
26
What acid is used for the medium and why?
* H₂SO₄ (aq) used
* HNO₃ not used because it is an oxidising agent itself
* HCl not used because it can be oxidised by MnO₄⁻
27
What happens if too little acid is added to maintain an acidic medium?
MnO₄⁻ will be reduced to MnO₂ which is brown ppt., the presence of brown ppt. will hinder the accurate detection of end-point
28
What is the colour change for the titration of MnO₄⁻ with C₂O₄²⁻ and H₂O₂?
* 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)
29
What is the equation and the colour change for the titration of MnO₄⁻ with Fe²⁺?
* 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
How to determine end-point colour change?
* 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
What is the equation and colour change of iodine-thiosulfate reaction?
* I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
I₂ + 2S₂O₃²⁻: brown
2I⁻ + S₄O₆²⁻: colourless
32
What is the colour change at the end-point in the absence of an indicator for iodine-thiosulfate titrations?
pale-yellow to colourless
(pale yellow: large qty of colourless + brown then one drop turns everything colourless)
33
What is added for iodine thiosulfate titrations and how will this affect the end-point?
* 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
What happens if starch is added too early during the titration?
* 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
What are the variants of iodine-thiosulfate titrations?
* 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
What are the equations for iodometric titration with IO₃⁻?
1. Use excess KI (aq) to liberate I₂
IO₃⁻ + 5I⁻ + 6H⁺ → 3I₂ + 3H₂O
2. Titration of I₂ with S₂O₃²⁻
I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
37
How can iodine thiosulfate titrations be related to kinetics?
samples of reaction mixture drawn at regular time intervals can be titrated to determine the amount of I₂ produced or left
38
When is back titration used?
* 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
What is the main idea of back titration?
* 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
What is gravimetric method?
determine the amount of analyte (sample to be analysed) through measurement of masses
41
What chemical reactions can gravimetry be used?
* 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
What kind of acid-carbonate reactions do gravimetry not work with?
* reactions that produce gas with low density (eg. H₂)
* do not give rise to significant change in mass
43
What is volatilisation gravimetric method?
* 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
How many times are you supposed to repeat the heat-cooling-weigh cycle?
until residue within 0.05g of difference
45
How to present experimental results for volatilisation gravimetric method?
* 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
What is the safety precaution for heating with crucible?
- use a pair of tongs to handle hot crucible and lid
- turn of gas tap when bunsen burner is not in use
47
How to set up the reaction flask for solid and solution?
* weighing bottle with solid that is not in contact with the solution
* All this in a conical flask with a stopper
48
How to set up the reaction flask for reaction between two liquids?
* 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
How to present experimental results for dropping funnel?
* Volume of gas collected/ cm³ x
* Volume of FA2 added from dropping funnel/cm³ y
* Volume of gas evolved/cm³ x-y
50
Can gas collection method be used for thermal decomposition of solid?
* Yes
* But gas expands upon heating so time is required for cooling the gas required to rtp in a gas syringe
51
Can hot gas be collected through water?
* No, gives rise to experimental error
* air originally present in the set up before heating will expand and get collected over water
52
What additional precaution needs to be taken to prevent suck back for hot gas over water?
* 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
What gases can you downward displacement of water?
H₂, CO₂, O₂
54
What safety precaution should be taken for downward displacement of water?
set-up must be appropriately clamped to prevent toppling and breakages
55
How to present experimental results for downward displacement of water?
* initial reading/cm³
* Final reading/cm³
* Volume of gas collected/cm³
56
How to present experimental results for well-greased gas syringe?
* Initial syringe reading/cm³
* Final syringe reading/cm³
* Volume of gas collected/cm³
57
What is the absolute uncertainty of 50cm³ gas syringe?
±1cm³
58
What are acidic gases?
CO₂, SO₂, Cl₂, HCl, NO₂
59
What drying agent should be used for acidic gases?
| Nature of drying agent
* nature of drying agent: acidic
* conc. H₂SO₄
60
What gases are alkaline?
NH₃
61
What drying agent should be used for alkaline gases?
| Nature of drying agent
* nature of drying agent: alkaline
* anhydrous CaO
* anhydrous Ca(OH)₂
62
What is the nature and the drying agent that can be used for all gases?
* nature of drying agent: neutral
* anhydrous CaCl₂
63
How to minimise heat exchange with surroundings?
* carry out experiment in draught-free environment to minimise heat loss to surroundings
* use polystyrene cup to carry out the reaction
64
What are the safety hazards for calorimetric experiment?
* 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
How to present experimental results of endothermic reaction?
* initial temperature/°C
* minimum temperature reached/°C
* temperature drop/°C
66
How to present experimental results of exothermic reaction?
* 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
What if the formula of weighted average of initial temperatures?
weighted average = [Vₛₐ₁/Vₜₒₜₐₗ × (Initial temp. of SA1)] + [Vₛₐ₂/Vₜₒₜₐₗ × (Initial temp. of SA2)]
68
What is the solubility of group 1 salts and NH₄⁺ in water?
soluble
69
What is the solubility of nitrates in water?
All soluble
70
What is the solubility of halides in water?
Most soluble except Ag⁺, Pb²⁺, Cu⁺
71
What is the solubility of sulfates?
Most soluble except Pb²⁺, Ca²⁺, Sr²⁺, Ba²⁺
72
What is the solubility of oxides?
Most insoluble except oxides of group 1, NH₄⁺ and Ba²⁺
73
What is the solubility of hydroxides?
Most hydroxides insoluble except Group I and Ba²⁺
74
What is the solubility of carbonates and phosphates?
Most insoluble except Group 1 and NH₄⁺
