Chemistry practical questions

Question 1

Why is the funnel removed after it has been used for filling the burette (titration)

Answer 1

a droplet from the funnel could enter the burette, affecting the volume and readings = affects titre

Question 2

Name the sources of error when using the burette to carry out a titration

Answer 2

• leaving the funnel in
• air bubble in jet space
• not reading at eye level and recording where the bottom of the miniscus touches

Question 3

During the titration, the inside of the conical flask is rinsed with distilled water.
Suggest why rinsing improves the accuracy of the titre

Answer 3

Ensures all reagents are able to react

Question 4

Explain why adding extra water when rinsing the conical flask during titration, does not change the volume of acid solution that is required in the titration.

Answer 4

Because it doesn’t change the number of moles for the reagents

Question 5

State why it is imporant to fill the space below the tap in the burette with solution before beginning an accurate titration

Answer 5

prevent air bubbles = to fill the space during titration, which prevents the volumes being added from being too high

Question 6

Suggest why 250cm³ conical flask is preferred to a 250cm3 beaker for titration

Answer 6

Less chance of losing liquid when swirling

the liquid doesn’t splash

Question 7

Why does repeating a titration make the value of the average titre more reliable

Answer 7

a single titration could be anomalous

in order to obtain concordant results

Question 8

How do you reduce percentage uncertainty when measuring volume?

Answer 8

Increase the amount of the solution used. Remember, % error = (uncertainty in equipment / reading) x 100

Uncertainty in the equipment remains constant, but if you have a larger reading (more solution) then you will have a smaller % error.

Decrease the concentration of the substance in the burette.

As a larger volume has to be delivered to add the same amount of moles to the solution.

Question 9

A student heated a solid sample of Na₂CO₃. xH₂O for 1 minute to remove water and determine a value for x

The diagram shows the apparatus used.

a) the experimental value for x was much lower than the correct value of x. Suggest a reason for this
b) suggest how the procedure could be improved, using the same apparatus to give a more accurate value of x and justify your answer

Answer 9

a) Failure to drive off all the water/ Failure to heat for long enough/ Not heated to constant mass
b) Heat to constant mass / heat for longer / use a smaller mass

You can be sure all / more of the water has been driven off

Question 10

How do you make up a standard solution?

Answer 10

Using a weighbing boat, weigh out accurately the mass of salt

Transfer the solid to a 250cm^3

3)Re-weigh the weighing boat and record the mass. Calculate the difference in mass

Add about 100cm3 of deionised water. Stir the mixture to dissole the solid, adding more water if necessary to aid the dissolving process

Transfer the solution to a 250cm^3 volumetric flask through a filter funnel. Rinse the beaker, glass rod and runnel and transfer all washings into the volumetric flask

add distilled water to the volmetric flask until the bottom of the meniscus sits on the graduation mark on the neck of the flask

stopper the flask and invert it 20 times so the solution is homogeneous solution

Question 11

What kind of chemical should be used in a standard solution?

Answer 11

• Does not absorb or lose moisture to/from the environment.
• Has accurately known relative formula mass so number of moles can be calculated
• Is very pure
• Has high relative formula mass to minimise weighing errors.

Question 12

Why is a volumetric pipette more accurate than a measuring cylinder?

Answer 12

because it has a smaller uncertainty

Question 13

Why only use small drops of indicator?

Answer 13

indicators tend to be weak acids - so adding too much will affect the result of the titration.

Question 14

How do you investigate the rate of a reaction changes with temperature?

Answer 14

Measuring cylinders - measure 50cm3 of 0.05 mol of sodium thiosulphate. In another- measure 5cm3 of HCl - place them I seperate boiling tubes.

measure the initial temperature of the 2 solutions before you add one to another in a conical flask.

Add the sodium thiosulphate to conical flask first and then the HCl. The conical flask should be placed over a white tile with a black cross, start the stopwatch and measure the time it takes for the cross to disappear due to the cloudy suffer being produced.

Question 15

If equipment has uncertainty, it can still be used in an experiment and it is still adequate. What would make it adequate?

Answer 15

If the reading is bigger than the uncertainty. (For example, for a thermometer, if the temperature change is bigger than the uncertainty it is adequate)

Question 16

How do you convert m3 to dm3 and dm3 to cm3

Answer 16

x 1000

Question 17

Why is it important to note down the temperature and pressure of the room when using a gas syringe?

Answer 17

because the volume of a gas depends on the pressure and temperature

Question 18

What are the potential errors when using a gas syringe (3)

Answer 18

• gas escapes before bung inserted
• syringe sticks
• some gases like co2 or so2 are soluble in water so the true amount of gas is not measured

Question 19

Why is a burette more suitable than a pipette for titrations

Answer 19

because it can measure variable volume

Question 20

Why should we never seal the top of a condenser?

Answer 20

the build up of gas pressure could cause the apparatus to explode

Question 21

Why do we extrapolate the temperature curve back to the time the reactants were added together?

Answer 21

to obtain an accurate temperature rise

when the reaction is slow it is hard to calculate without extrapolating it

Question 22

How do you increase accuracy when heating in a crucible?

Answer 22

• make sure the crucible is dry or a wet crucible wold give inaccurate result (the mass loss would be too large as the water would be lost when heating)
• small amounts of the solid being heated should not be used as the percentage uncertainties in weighing would be too high
• lid would prevent loss of solid from the crucible (should still be a bit loose to allow gas to escape)

Question 23

What are the errors usually made when measuring enthalpy change?

Answer 23

• energy transfer from surroundings
• assuming all solutions have the same heat capacity of water
• ignoring energy absorbed by the apparatus
• reaction may be incomplete
• density of the solution is taken to be the same as water.

Question 24

How do you stop reactions and why does this method work?

Answer 24

stop reaction by dilution

the reactant will become less concentrated = less successful collisions = slow down the reaction

Question 25

Describe the iodine clock reaction

Answer 25

1. Add dilute sulfuric acid and starch solution to beaker
2. Add sodium thiosulfate to reaction mixture
   
   1. Add potassium iodide solution
   2. Add hydrogen peroxide solution
3. Sodium thiosulfate reacts with iodine being formed
4. Once all sodium thiosulfate is used up = any more iodine formed remains in solution
5. Turns starch indicator blue-black
6. Varying conc. of iodide or hydrogen peroxide while keeping everything constant = different times for colour change ◦ Used to work out reaction order

*

Question 26

How do you measure the EMF of the cell?

Answer 26

Clean a piece of copper and zinc using fine grade sandpaper.

Degrease the metal using cotton wool and propanone.

Place the copper into a 100cm3 beaker with 50cm3 of 1moldm-3 of copper sulphate

Place a strip of Zinc into a 100cm3 beaker with 50cm3 of 1moldm-3 of Zinc Sulphate.

use a strip of filter paper soaked in potassium nitrate for the salt bridge

connect the 2 half cells by connecting the metals to crocodile clips attached to leads joined to the voltmeter.

Question 27

How do you investigate pH changes?

Answer 27

Transfer 25cm3 of acid to a conical flask using a volumetric pipette.

measure the initial pH of the acid with a pH meter

Add the alkali from the burette in small amounts e.g. 2cm3 at a time. Noting the volume added.

Stir mixture to equalise the pH

measure and record the pH to one d.p.

when approaching end point add alkali in smaller volumes, drop by drop until the alkali is in excess.

Question 28

How and why do you calibrate the pH meter?

Answer 28

Calibrate pH meter by measuring the pH of a known pH buffer solution// distilled water.

Because pH meter can lose accurate over time when in storage.

Question 29

Apart from calibrating the pH meter, how else can you improve the accuracy of measuring pH

Answer 29

maintain a constant temperature

Question 30

How do you carry our recrystallization?

Answer 30

dissolve the crude product in the minimum amount of hot solvent

filter the solution whilst hot to remove insoluble impurities

cool in ice bath to recrystallise/ let it cool to room tempertaure

filter with a Buchner flask under pressure

wash with a cold solvent and dry

Question 31

Why is the minimum quantity of hot water used in recrystallization

Answer 31

• to dissolve the desired product rather than the impurities as well
• to allow for a saturated solution so the solution isn’t too dilute for cyrstals to form

Question 32

Why is the flask cooled to room temperature before the crystals were filtered off

Answer 32

so large crystals can form = allows for a slower cooling process which decreases the chances of trapping impurities in the developing lattice

Question 33

Why are solutions filtered whilst hot after they are dissolved in a hot solvent

Answer 33

to remove insoluble impurities

prevent crystallisation before separation of impurities

Question 34

Why are the crystals compressed in a Buchner funnel

Answer 34

Air passes through the sample not just around it

Question 35

Why is a little cold water poured through the crystals?

Answer 35

to wash away impurities off the crystals surface without dissolving the crystals

Question 36

How is yield loss during recrystallization?

Answer 36

• crystals lost when filtering or washing
• some product stays in solution after recrystallisation
• other side reactions ocurring

Question 37

Why do crystals sometimes have a percentage yield over 100%

Answer 37

because they are not dried properly

Question 38

Suggest why an electric heater is used rather than a Bunsen burner in the hydrolysis of ethyl benzoate with sodium hydroxide

Answer 38

the ester and the ethanol are both flammable

Question 39

How do you make aspirin?

Answer 39

add to a pear-shaped flask salicylic acid (2-hydroxybenzoic acid) and ethanoic anhydride. To this mixture add drops of phosphoric(v) acid (which acts as an acid catalyst) and swirl to mix

Fit the flask with a reflux condenser and heat the mixture on a boiling water bath for about 5 minutes. Without cooling the mixture, carefully add 2 cm3of water in one portion down the condenser.

When the vigorous reaction has ended, pour the mixture into cold water in a 100 cm3 beaker, stir and rub the sides of the beaker with a stirring rod necessary to induce crystallisation and, finally, allow the mixture to stand in ice bath to complete crystallisation. Collect the product by suction filtration and wash it with a little water.

recrystallise

Question 40

How do you carry out nitration of methyl Benzoate

Answer 40

add methyl benzoate into a small conical flask then dissolve with concentrated sulfuric acid. When the liquid has dissolved,cool the mixture in ice

prepare the nitrating mixture by adding drop by drop of sulfuric acid to concentrated nitric acid. Cool this mixture in ice as well. [This acids react together to make NO2+]

add the nitrating mixture drop y drop to the methyl benzoate, stir the mixture with a thermometer

keep the temperature below 10 °C. [This is to prevent multiple substitution of nitrogroups on the benzene ring]

When the addition is complete, allow the mixture to stand at room temperature for another 15 minutes. After this time, pour the reaction mixture on to crushed ice and stir until all the ice has melted and crystalline methyl 3-nitrobenzoate has formed.

recrystallise

Question 41

How do you purify an organic liquid?

Answer 41

• Put the distillate of impure product into a separating funnel
• wash product by adding
  
  • sodium hydrogen-carbonate solution to remove unreacted acid/acid catalyst : make sure to shake and release the pressure from CO2 produce
  • Saturated sodium chloride solution: this allows the aqueous and organic layers to separate and draws water from the organic layer into the aqueous layer
• Allow the layers to separate in the funnel, and then run and discard the aqueous layer.
• Run the organic layer into a clean, dry conical flask and add a few spatulas of drying agent (e.g. anhydrous sodium sulphate, calcium chloride) to dry the organic liquid aka remove any traces of water. When dry the organic liquid should appear clear.
• Carefully decant the liquid into the distillation flask
• Distil to collect pure product

Question 42

How would you determine an accurate value for the melting point of aspirin

Answer 42

Heat melting point tube in an oil bath slowly near the melting point

Question 43

Why do you add sodium hydrogen carbonate or sodium chloride to the impure product when purifying an organic liquid?

Answer 43

Sodium hydrogencarbonate will neutralise any remaining reactant acid. Sodium chloride will help separate the organic layer from the aqueous layer

Question 44

What features should drying agents have when purifying an organic liquid

Answer 44

• be insoluble in the organic liquid
• not react with the organic liquid

Question 45

How do you prove you have made a pure compound

Answer 45

measure the melting point by heating the compound in an oil bath (the compound is in a melting point tube) slowly.

compare to data book

Question 46

Give a detailed method for the partial oxidation of propan-1-ol

Answer 46

• Place about 10 cm3 of dilute sulphuric acid in a flask and add about 3g of potassium dichromate(VI) and 2 or 3 anti-bumping granules.
• Shake the contents of the flask until solution is complete (do not warm).
• Add 1.5 cm3 of propan-1-ol in drops from a dropping pipette, shaking the flask so as to mix the contents,and then assemble distillation apparatus
• Gently heat and slowly distil

Question 47

Why does water go in the bottom of the condenser?

Answer 47

to go against gravity which prevents back flow of water and allows more efficient cooling

Because water enters from the bottom, it means the coldest water will be at the bottom of the condenser. Therefore the distil will be at the coldest point as it leaves the apparatus

Question 48

Give a detailed method for the FULL oxidation of propan-1-ol

Answer 48

• Measure 5 cm3 of water into a boiling tube. Add 6 g of sodium dichromate(VI), shake and set aside to dissolve.
• Put about 1.5 cm3 propan-1-ol into a 50 cm3 round bottomed flask and add about 5 cm3 of water and two or three anti-bumping granules. Put a condenser on the flask for reflux, as shown in figure below.
• Add 2 cm3 of concentrated sulphuric acid down the condenser in drops from a dropping pipette. While the mixture is still warm, start to add your sodium dichromate(VI) solution down the condenser in drops from a dropping pipette. The energy released from the reaction should make the mixture boil. Add the solution a drop at a time so that the mixture continues to boil without any external heating.
• When all the sodium dichromate(VI) solution has been added, use a low Bunsen burner flame to keep the mixture boiling for 10 minutes, not allowing any vapour to escape.
• At the end of that time remove the Bunsen burner and arrange the apparatus for distillation. Gently distil 2-3 cm3 of liquid into a test tube.

Question 49

How do you carry out fractional distillation in the lab

Answer 49

• Heat the flask, with a Bunsen burner or electric mantle . This causes vapours of all the components in the mixture to be produced.
• Vapours pass up the fractionating column.
• The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
• The thermometer should be at or below the boiling point of the most volatile substance.
• The vapours with higher boiling points condense back into the flask.
• Only the most volatile vapour passes into the condenser.
• The condenser cools the vapours and condenses to a liquid and is collected.

Question 50

In detail how do you test for halogenalkenes?

Answer 50

• Arrange three test tubes in a row and add three drops of halogenoalkane in the sequence 1-chlorobutane, 1-bromobutane, 1-iodobutane.
• Add 4 cm° of 0.02 M silver nitrate to each halogenoalkane.
• Without delay, put all three test tubes simultaneously in a hot water bath.
• Note the order in which precipitates appear

Question 51

What does the rate of precipitate formation tell u about the haloalkenes in the test for haloakenanes with silver nitrate

Answer 51

• quicker precipitate is formed = formed the substitution reaction = more reactive haloalkane
• the weaker the C- halogen bond, the easier it is to break and the faster the reaction
• AgI precipitate forms the fastest as it has the weakest bond

Question 52

Why is the flask in recyrstallisation sometimes cooled in ice instead of room temperature?

Answer 52

because ice increases the yield of the cyrstals