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experimental chemistry Flashcards

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1
Q

advantages of using a burette compared to a measuring cylinder

A

burette (measures a range of volumes to the nearest 0.05 cm^3) is more accurate in measurement than the measuring cylinder (measures a range of volumes to the nearest 0.5 cm^3)

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2
Q

what apparatus should you use to measure 600 cm^3 of liquid

A

volumetric flask. it measures accurate fixed large volumes

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3
Q

how can parallax error be avoided when reading volume from measuring cylinder

A

position the eye at the level of the liquid, and read the scale at the bottom of the meniscus

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4
Q

meniscus definition

A

the upward or downward curve at the surface of a liquid/solution in a container

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5
Q

what apparatus should you use to measure 5 cm^3 of gas?

A

gas syringe

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6
Q

two properties of gas that determines gas collection method

A
  1. solubility: how easily the gas dissolves in water
  2. density: how dense the gas is as compared to the surrounding air
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7
Q

deduce the solubility of a gas that can be collected through the displacement of water

A

insoluble in water OR slightly soluble in water

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8
Q

explain why a gas with Mr lower than that of air can be collected through upward delivery

A

the gas is less dense than air and will rise into the gas jar for collection

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9
Q

similarity between gases collected through upward delivery and downward delivery

A

the gases can be insoluble or soluble

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10
Q

how does the density of gas affect the displacement of water

A

it does not affect the displacement of water

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11
Q

properties of gases that can be collected through downward delivery

A
  • denser than air
  • soluble/insoluble in water
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12
Q

limitation of concentrated sulfuric acid as a drying agent

A

alkaline gases that react with sulfuric acid cannot be dried with this method (eg. ammonia)

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13
Q

property of gases that enables it to be dried with quicklime

A

they are alkaline gas (ONLY AMMONIA)
- turn damp red litmus paper blue

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14
Q

why is fused calcium chloride the best way to dry gases

A

it is neutral –> can dry both alkaline and acidic gases EXCEPT AMMONIA

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15
Q

why must fused calcium chloride be freshly heated before use

A

it to remove all traces of water –> readily absorbs moisture from surrounding air

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16
Q

how to determine if a substance is pure

A

if a substance is pure, it will have a fixed melting and boiling point.
if not, it will melt and boil over a range of temperatures

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17
Q

effect on impurities on melting and boiling points

A

with impurities,
1. a substance melts below its melting point over a range of temperatures
2. a substance boils higher than its melting point over a range of temperatures

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18
Q

solid-solid physical separation techniques

A
  1. magnetic attraction –> magnetic properties
  2. sieving –> particle size
  3. using suitable solvents –> solubility
  4. sublimation –> ability to sublime
19
Q

solid-liquid physical separation techniques

A
  1. filtration –> solubility
  2. crystallisation –> solubility
  3. evaporation to dryness –> melting/boiling point
  4. simple distillation –> difference in melting/boiling points
20
Q

liquid-liquid physical separation techniques

A
  1. chromatography –> solubility
  2. separating funnel –> immiscibility
  3. fractional distillation –> boiling point
21
Q

what is ability to sublime mean

A

the ability to change into a gas directly without melting –> with the presence of heat

21
Q

steps for a filtration experiment (eg. NaCl, sand)

A
  1. add water to dissolve NaCl
  2. filter out the mixture to remove the excess sand
  3. evaporate NaCl solution to dryness
  4. solid NaCl crystals will be formed
22
Q

steps for a crystallisation experiment (eg. sugar, sand)

A
  1. add water to dissolve sugar in the solid-solid mixture of sand and sugar
  2. filter out the mixture to remove excess insoluble solid sand
  3. heat the solution gently until saturation
  4. allow the saturated solution to cool. sugar crystals will form
  5. filter out crystals
  6. cool distilled water –> sheets of filter paper
23
Q

during simple distillation, when does collection of the liquid with lower boiling point stop

A

when temperature starts to rise above the boiling point of the liquid

24
Q

purpose of boiling chips in simple distillation

A

to ensure smooth and uniform boiling by reducing bumping of liquid

25
Q

purpose of water in/water out position

A

cool water enters the condenser through lower tube and leaves by the upper tube, opposite to the direction that vapour moves in the condenser to ensure efficient cooling and complete condensation of vapour, by ensuring the cool water will stay in the condenser long enough to cool down any vapours into liquid droplets from its permanent cool surface

26
Q

purpose of thermometer bulb position

A

the bulb is placed near the opening to measure the temperature of the vapour accurately, which helps to determine the identity and purity of the distillate

27
Q

purpose of the type of receiving flask

A

conical flask is used because its narrow neck can reduce loss of distillate through splashing

28
Q

what can be determined from how far a substance travels in a chromatogram

A

solubility of substance in a solvent
- move further from start line –> more soluble
- closer to base of the chromatogram –> less soluble

29
Q

why must the starting line be drawn with pencil

A

graphite pencil is insoluble in e solvent and will not be carried by e solvent to interfere with e results.
ink is made up of different dyes that can dissolve in e solvent and separate into different components as they move up the chromatogram along with the dyes, interfering with e results

30
Q

why must e start line be above solvent level

A

this prevents sample spots from dissolving in e solvent

31
Q

why must the sample spot be as small as possible

A

to prevent smudging and tailing of e different components, helping us to identify e respective spots

32
Q

why must the solvent front be as high as possible

A

so that all e components will be separated

33
Q

why must e beaker be covered with a lid

A

to prevent evaporation of solvent

34
Q

how to locate colourless spots

A

chromatogram is sprayed with locating agents (ninhydrin) that can react with the substances to produce a coloured product, or placed under UV light

35
Q

improvement for chromatogram

A

use a longer piece of filter paper to allow components to move a longer distance

36
Q

Rf value formula

A

distance from start line to spot/distance from start line to solvent front
- expressed as a decimal in 3 s.f.
- higher, more soluble

37
Q

why may a substance not be pure based on paper chromatography test when compared to known substances

A

there is [no. of unknown spots] of unknown substances, as there are [no. of spots] spots that do not correspond with pure [substance]

38
Q

purpose of glass beads in tall fractionating column

A

they increase e surface area for both evaporation and condensation of substances with different boiling points to take place –> more efficient

39
Q

how does the tall fractionating column help to separate more efficiently

A
  1. part of column where temp is lower than boiling point: vapour condenses and drips back to distillation flask
  2. part of column where temp is higher than boiling point: vapour will continue to flow into condenser and condense –> distillate
  3. substance with a lower boiling point will be distilled first
  4. closer boiling points, taller fractionating column
40
Q

how to avoid contamination of different collected substances in a fractional distillation experiment

A

once temp starts to increase above boiling pt, indicates that e first distillate has collected completely –> receiver containing first distillate must be removed before second substance distills over

41
Q

define immiscible

A

it refers to two or more liquids that do not mix completely together and form a heterogenous mixture (separate into different layers known as phases)

42
Q

how does a separating funnel separate immiscible liquids

A
  1. mixture forms two layers in funnel. liquid with higher density = lower layer, liquid with lower density = upper layer
  2. tap is opened, lower layer is drained off into a beaker first
  3. tap is closed once lower layer has been drained off completely

chemistry (4 decks)

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