practical techniques questions

Why are crystals rinsed with a small amount of ice cold water?

• Rinsed to remove impurities that remain in solution

• Ice cold - prevents crystals redissolving

Why are the crystals put in a warm oven

To ensure water of crystallisation is not removed

2 possible reasons why yield is less than 100%

• solid lost when heated in basin

• Remains in solution

• Crystals remain on filter paper

• Transfer losses from reaction flask

• Water of crystallisation lost during drying process

How could not using a lid affect the calculated value of n

• some of the salt y could be lost from crucible during heating

• Mass lost is greater n is greater than expected

Explain how decreasing amount of time heating could affect the value of n

• mean not all water has been removed

• N is less than expected

Why is sodium carbonate used in iodine clock reaction?

Quench the reaction neutralising remaining H2SO4

Explain why filter paper and funnel are warmed in an oven

• make sure the solution doesn’t cool down

• Prevent crystallisation

• Would reduce the yield of product

How does allowing the filtrate to cool and recrystallise and filter the crystals under a reduced pressure remove impurities from crystalline product

Step 4: product less soluble in cooler solvent soluble impurities stay in solution

Step 5: filtering under reduced pressure removed more of the soluble impurities

Devise an experiment to determine Ka for a solution of ethanoic acid of unknown concentration

• Measure pH at regular intervals

• Plot pH against volume

• Use graph to find pH at half equivalence point

• At half neutralisation pH = pKa Ka = 10-pH

Some of the crystals jumped out of crucible while it was being heated

Crystals jumped out of crucible

• value of z increases

• Bc more mass than expected

• Prevented by placing a lid on crucible or heating to a constant mass

Not all water of crystallisation lost

• less mass

• Prevented by heating to a constant mass

• This means value of z decreases

Why do you cool the mixture before adding the conc H2SO4 drop by drop

• reaction is v exo

Why do you add a few anti bumping granules

• smooth bubbling

• Surface for bubbles to form

Why do you heat under reflux

• Prevent the loss of volatile substances

Identify 3 errors and what the effect is

• there is a gap between condenser and flask + products will escape

• Water is flowing the wrong way doesn’t fill with water

• There is a stopper on condenser there will be a build of pressure

Explain why brown vapour forms

Bromide ions oxidised

State the position of the aqueous layer in the separating funnel

Aqueous layer on top

Why do you add sodium hydrogencarbonate

To react with HCl

Why do you invert the funnel and open the tap

To avoid build up of pressure

Why do you add anhydrous sodium sulfate and swirl the flask until the liquid becomes clear

Drying agent

Give a suitable range over which to collect the pure 1-bromobutane in recrystallisation step

• 99-101

Why would you have a yield less than 100%

• recation is incomplete

• Not all crystallised

Why do you have a higher yield than expected

• impurities in crystals

• Crystals were not dry

Give 2 reasons why the wire is made of nichrome

• Unreactive

• Uncoloured

Why is fresh conc HCl used in second stage of flame test

• contaminated with residue of previous tests

Why is HCl used in 2nd stage of the flame test

• acid is contaminated

Describe how you would ensure that all the sodium hydroxide was transferred to volumetric flask

• rinse beaker

• Transfer washings to volumetric flask

State why the procedure has to be restarted rather than using a test pipette to remove excess water

Remove some of dissolved NaOH

Give 2 steps needed before the student takes the initial burette reading

• eyes are level with bottom of meniscus

• Remove the funnel

Give the effect of any on the value of the first titre

• titre large

• NaOH decreases

Why was the aqueous layer was the lower of the 2 layers

More dense

Why was the ether layer washed with deionised water

• some of sodium benzoate dissolves in water

Explain why the addition of HCl results in precipitation of benzoin acid

• benzoate ion is protonated by HCl

• Benzoic acid is less soluble than Na salt

Explain why the front windows of the fume cupboard must be below the safety line even with the exhaust fan switched on

• safety line exhaust system is not strong enough to draw in the fumes

• Toxic fumes will escape

Outline the procedure that the student could use, including a diagram and the measurements needed.

• add HCL and immediately stopper the flask

• Record the volume of gas

• Collect gas at regular intervals

Describe what happens to the melting temperature of the paracetamol is not pure

• melting temperature lower

• Melting temperature over a range of temperatures

Give a reason why using a burette than a measuring cylinder will not improve the accuracy of experiment

Describe changes to the method and how the data is used that would improve the accuracy of the determination of the temperature of the temperature change in experiment 2. Your description should involve the use of a clock and plotting a graph

• measure temp of HCl

• Every 30 seconds 2 ½ mins

• Add Na2SO3 at 3 mins

• Stir and measure temp every 30 secs for another 5 mins

• Plot graph of temperature against the join 2 sets of points

• With 2 best fit straight lines

• Determine max temp change

Identify the 2 changes that must be made to be apparatus before heating the pear-shaped flask giving a reason for each change

• Replace a thermometer at the top to act as a seal prevent vapour from escaping

• Water in and out to keep condenser full better cooling

Give 3 reasons for difference in combustion values

• energy lost to the surroundings

• Incomplete combustion

• Evaporation of methanol/water

Value for the enthalpy change of combustion of methanol from this experiment would differ a

.

Student planned to obtain any dissolved magnesium carbonate by evaporating the filtrate and then weighing the residue

MgCO3 may decompose

Residue would contain XS Na2CO3

Explain how if at all very slight solubility of magnesium carbonate I’m water would affect the value of ax

• X would increase

• Moles of MgCO3 would decrease

Explain how the student could ensure that the hydrated salt was fully decomposed

• heat to a constant mass

Identify 2 modifications to the method that would enable the student to lower the percentage uncertainty in measurement of the mass of solid residue

• increase the mass

• Use a balance with more decimal places

Explain one procedural error which could have resulted in student 2 obtaining a molar mass greater

• bung not replaced quickly

• Some of CO2 escaped

Give one reason why the mass of the carbonate measured by student 3 has a greater uncertainty than that mesured of student 1

• Used a smaller mass

• Larger percentage error

student B’s mistake would affect the titre

• number of NaOH as diluted

• Moles decrease

• Titre would decrease

Explain how if at all student c’s use of a wet conical flask affects the value of titre

• titre would be unchanged

• Number of moles of NaOH would be the same