Techniques

Question 1

What is gravimetric analysis used to do?

Answer 1

to determine the mass of an element or compound in a substance

Question 2

In gravimetric analysis, how is the mass of the analyte present in a substance determined?

Answer 2

The substance is converted into another solid substance (with an appropriate reagent) of known chemical composition which can be readily isolated, purified and weighed.
The conversion can either occur through precipitation or volatilisation.

Question 3

In gravimetric analysis, what is heating to a constant mads/steps of gravimetric analysis

Answer 3

Used to dry final product completely

heating the substance->allowing it to cool in a dessicator (dry atmosphere so no water is absorbed)->reweighing it all in a crucible

This is repeated until a constant mass is obtained (shows no water)

Question 4

In gravimetric analysis, when heating to a constant mass, what are the conditions to heating with a bunsen burner?

Answer 4

the lid should be left partially off to allow water to escape

A blue flame should be used to avoid a built up of soot on the outside of the crucible (could affect mass)

Heating should be started off gently and then more strongly

Question 5

What does volumetric analysis involve?

Answer 5

using a solution of known concentration (standard solution) in a quantitative reaction to determine the concentration of the other solution

Question 6

In volumetric analysis, what procedure is used?

Answer 6

titration in the form: standard, complexometric or back titrations

Question 7

What do titrations involve?

Answer 7

measuring one solution quantitatively into a conical flask using a pipette . The other solution is added to a burette until a permanent colour change of an indicator is seen in the conical flask.

A rough titration is carried out first followed by more accurate ones until concordant titre values are achieved (+-0.1ml). The average is used in calculations.

Question 8

What can a standard solution be directly prepared from?

Answer 8

a primary standard

Question 9

What characteristics must a primary standard have?

Answer 9

a high state of purity (>99.9%)
stability in air and water
solubility (usually in water)
reasonably high formula mass (reduces errors)

Question 10

standard solution preparation steps

Answer 10

-calculate mass of primary standard
-weigh out primary standard as accurately as possible
-dissolve primary standard in small volume of deionised water in beaker
-transfer solution and all rinsings into standard flask
-make the solution up to mark with deionised water
-i vert the stoppered flask several times to mix

Question 11

substances used as a primary standard

Answer 11

oxalic acid
sodium carbonate
potassium hydrogen phthalate
potassium iodate
potassium dichromate

substances like sodium hydroxide cannot be used ad they readily absorb h2o and co2 from atmosphere

Question 12

What does the use of controls in chemical reactions do?

Answer 12

validates a technique and may consist of carrying out a determination on a solution of known concentration to give a referencing point on which to base all other results from impure samples e.g. in the determination og vitamin c content in fruit juice, a sample of absorbic acid (pure vitamin c)woulf be analysed

Question 13

what is a back titration used for?

Answer 13

to find the number of moles of a substance by reacting it with an excess volume of reactant of known concentration. This resulting mixture is then titrated to work out the number of moles of reactant in excess.

Useful when trying to work out quantity of substance in an i solublesolif

Question 14

What relationship does colorimetry use?

Answer 14

between colour intensity of a solution and the concentration of the coloured species present to determine concentration

Question 15

What is distillation used for?

Answer 15

to purify and identify a compound by separating it from a non-volatile or less-volatile material by boiling points

-stirrer/heat plate
-anti-bumping granules
-round-bottom flask
-condenser with water out (top) and water in (bottom)
-cooling bath

-thermometer

Question 16

What is refluxing?

Answer 16

a technique used to apply heat energy to a chemical reaction mixture over an extended period of time

-round-bottom flask
-anti-bumping granules
-condenser

-dropping funnel
-thermometer

Question 17

Why is vacuum filtration used instead of normal filtration?

Answer 17

faster

Question 18

What can vacuum filtration be carried out using?

Answer 18

buchner, hirsch or sintered glass funnel

Question 19

What is recrystallisation used for?

Answer 19

To purify solids based upon solubility

Question 20

What is thin-layer chromatiography used for?

Answer 20

To assess product purity

Question 21

weighing accurately approximately

Answer 21

weigh out as close as possible to a certain mass but ensure that you record the actual mass given on the balance

Question 22

weighing by difference

Answer 22

a way to weigh materials accurately

the weight of the material is the difference between the two weights of the ‘vial’ before and after you transfer the material from the vial

Question 23

why is deionised water used to make a standard solution

Answer 23

tap water contains dissolved salts that may react with our sample and affect the concentration of the solution

Question 24

What is standardisation?

Answer 24

the process of exact determination of concentration of a solution

You use primary standard A to determine the concentration of a solution B (not a primary standard). This is done by titrating B (in a conical flask) with A (in burette)

Once you have found the concentration of B, you can use B to titrate other substances

cv stock=cv diluted

Question 25

benefits of refluxing

Answer 25

prevents vapours escaping allows extended heading more vigorous heating to dissolve/extract maximum more efficient

Question 26

Colorimetry-how is a calibration graph produced?

Answer 26

1-standard solutions prepared 2- a suitable filter used 3 - mention of a blank / solvent only Measurement 4- absorbance of each is plotted

Question 27

colorimetry-how to find concentration of unknown

Answer 27

the absorbance of unknown is measured and a mention of using the calibration graph

Question 28

What is a primary standard?

Answer 28

Primary standards are materials containing a known concentration of a substance. They provide a reference to determining unknown concentrations or to calibrate analytical instruments

Question 29

recrystallisation steps

Answer 29

-dissolve an impure solid gently i. a minimum volume of hot solvent -hot filtration of the resulting mixture to remove any insoluble impurities -cooling the filtrate slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent -filtering, washing and drying the pure crystals

Question 30

How is the solvent for crystallisation chosen?

Answer 30

so that the compound being purified is completely soluble at high temperatures and only sparingly soluble at lower temperatures

Question 31

How do you make a solution from a solution?

Answer 31

c1v1=c2v2 then e.g. ‘Measure 50cm3 of stock solution with a pipette and transfer to a 250cm3 standard flask. Add deionised water up to the mark, stopper and invert

Question 32

Simple explanation of -TLC -Heating under reflux -Recrystallisation -Distillation

Answer 32

-TLC separates chemicals with different polarities/size -Heating under reflux allows volatile chemicals without reactant escape -Recrystallisation allows chemical to be extracted from impurities due to solubility in a second solvent -Distillation separates chemicals with different boiling points

Question 33

Explain whether gravimetric or volumetric analysis is more suitable for measuring substances at low concentrations.

Answer 33

Volumetric analysis is more suitable for low concentrations because it can accurately measure small volumes of solution using precise apparatus like burettes and pipettes. In contrast, gravimetric analysis relies on forming and weighing a solid precipitate, but at low concentrations, too little solid may form to be weighed reliably. This makes gravimetric analysis less accurate and less practical for low concentrations.

Question 34

What properties do appropriate gravimetric analysis reagents have?

Answer 34

the precipitate must be of LOW SOLUBILITY high purity and known composition

Question 35

name a step in gravimetric analysis that you forget

Answer 35

test filtrate to ensure no more precipitate forms

Question 36

What happens in gravimetric analysis

Answer 36

a precipitate is formed SO (aq) + (aq) -> (s)

Question 37

What is best to measure 1. Cl- concentration in sea water and 2. Ca2+ concentration in milk

Answer 37

1. volumetric analysis 2. complexometric analysis titration (volumetric)

Question 38

recrystallisation steps

Answer 38

1-minimum/small volume 2-hot solvent 3-solvent cool

Question 39

what properties must a solvent have for crystallisation?

Answer 39

-doesn't react with solute -solute more soluble in hot than cold solvent -impurities to be soluble in hot solvents -boiling point -polarity

Question 40

What must the filter be for colorimetry?

Answer 40

opposite colour to transmitted wavelength (so the absorbed wavelength)

Question 41

how would you remove separated samples from a furnace

Answer 41

gas-›collected at top and cooled in condenser liquid->collected at bottom

Question 42

does distance solvent travels affect the Rf

Answer 42

No as solvent and solute are proportional

Question 43

what affects rf?

Answer 43

solvent used

Question 44

How would make filtration faster?

Answer 44

Use vacuum filtration Fluted shape filter paper

Question 45

What does drops of silver nitrate on the filtrate do?

Answer 45

check reaction is complete (e.g. all Cl- ions have reacted)

Question 46

How would you reduce the uncertainty in a titre value?

Answer 46

titrate larger samples dilute sample less use class A glassware

Question 47

What do you need to mention when describing how a standard solution is made in the transferring to standard flask part and the made up to mark part

Answer 47

SOLUTION is transferred to the standard flask Made up to the mark WITH DEIONISED WATER

Question 48

Gravimetric-What happens in precipitation conversion?

Answer 48

-The substance undergoes a precipitation reaction -The precipitate is separated from the filtrate -The filtrate is tested to ensure the reaction had gone to completion -The precipitate is washed, dried to a constant mass and then weighed

Question 49

Gravimetric-What happens in volatilisation conversion?

Answer 49

The substance is heated and any volatile products (often water) are evaporated. The substance is heated to a constant mass and the final mass is recorded.

Question 50

Why may a sample for colorimetry be diluted

Answer 50

because the absorbance was outwith the calibration range

Question 51

Why may experimentally determined concentration be higher than the actual value?

Answer 51

other ingredients/impurities may be reacting with the reactant

Question 52

control for an experiment to find concentration

Answer 52

pure substance

Question 53

steps of solvent extraction

Answer 53

-add 2 solvents to separating funnel -shake to mix -leave to separate into layers -run off the lower layer -evaporate solvent to get solute

Question 54

how could you improve solvent extraction to extract more?

Answer 54

higher number of extractions with smaller volume of solvent

Question 55

what must the solvent be like for solvent extraction?

Answer 55

-immiscible with water -dissolve the target compound well -volatile

Question 56

What is solvent extraction?

Answer 56

A technique used to separate a compound based on its solubility in 2 immiscible liquid (normally organic solvent and water)