Practical Questions Flashcards
How to remove soluble impurities
Wash with or add deionised water or wash with a small amount of cold water
Explain why different observations are made when aq barium chloride is added seperately to aq magnesium sulfate and to aq magnesium nitrate
As baso4 is insoluble and no reaction occurs so it remains in a solution so the reacs differ
You are given a sample of saturated calcium hydroxide solution, outline the practical steps that you would take to determine the solubility of calcium hydroxide in this sol
Take a known vol of the saturated solution,
Evaporate the filter to dryness
Weigh the residue
The student used a wash bottle containing deionised water when approaching the end point to rinse the inside of the conical flask, explain why this improved the accuracy of the titration
Ensures the acid wasnt left behind to react and give invalid results
What does concordant titres mean
They are within 0.1cm cubed of each other
The mass of mgo obtained in this exp is slightly less than expected from the mass of mgno3 used suggest one practical reason why
As some of the solid may have been lost for example blown away with the gas
Suggest a reason other than apparatus inaccuracy for why the mr is smaller when experimental
As the vol of gas may be greater than the true value as some air may have leaked into the syringe causing an increase in the mass so mr decreases
Safety precaution needed for toxic gases
Carry out the experiments in a fume cupboard to avoid toxic vapour
The student rinsed the burette before filling it w sodium hydroxide. Explain why the student should use the sodium hydroxide sol rather than the water
As water would dilute the sodium hydroxide
If we make hcl in a reac or smth and it asks how the company could profit
Just say by selling the hcl
strontium carbonate reacts with nitric acid to produce a solution of strontium nitrate.
Strontium sulfate does not react w nitric acid describe briefly how you could obtain strontium sulfate from a mix of strontium carbonate and strontium sulfate
Add nitric acid to the mixture until in excess
And filter to isolate strontium sulfate
How to sep and purify smth
Filter off remaining ppt, wash to remove soluble impurities, dry to remove water
Pure mg reacts completely w an excess of dilute h2so4 and is v rapid initially. This reac slows down and stops before all the calcium has reacted. Use your knowledge of gp 2 sulfates to explain why these reacs w h2so4 are so diff
Bc magnesium sulfate is soluble and calcium sulfate is insoluble. The calcium is coated with sulfate which prevents further reac w the acid
Why do anti bumping granules prevent bumping
As they form smaller bubbles preventing larger ones to be formed
give two ways of maximising the yield of propanal obtained by distillation of the reac mixture
Keep the temp of the reac mixture bellow the bp of propan-1-ol
Cool the distillate
explain why a mixture could be filtered before the addition of something
to remove excess solid
why would something be washed before it was dried
to remove excess acid`
how would an average titre change if a few cm rather than a few drops of the indicator was used by mistake
decrease, bc less acid would be needed
give one reason why a 250cm cubed conical flask is preferred to a beaker
bc less chance of losing liquid when swirling
suggest one reason why calcium hydroxide sol is not used in the titration of a 0.200 mol dm-3 solution of an acid
as dilution of the acid is needed
describe a method for increasing accuracy calorimetry
insulate beaker to reduce heat loss, stirring, record temp for a suitable amount of time to have an accurate initial temperature, plot the results on a graph, get the temp change at the 4th minute extroplate cooling
the mass of x obtained in the experimetn is slightly less than expected, suggest one practical reason for this
solid lost in weighing
the rates of hydrolysis of two chloroalkanes can be investigated aqueous silver nitrate to the chloroalkanes. outline a method to compare the rate of hydrolysis of 1-chlorobutane with 2-chlorobutane. state how it would involve a fair test
add the same volume of agno3 in each experiment, use the same temp and record the time to measure a sensible observation of the amount of agcl precipitate formed. and the rate is the amount formed/ time taken. less time taken and more product means a faster rate
before adding the solution from the burette in the rough titration there was an air bubble below the tap. at the end of this titration it wasnt there. explain why this air bubble increases the final burette reading of the rough titration
as the air bubble would take up volume that would have been filled by the solution
during the titration the student washed the inside of the conical flask with some distilled water. suggest why this washing does not give an incorrect result
as it does not react with the alkali
the student identified the use of the burette as the largest source of uncertainity in the experiment. using the same apparatus suggest how the procedure could be improved to reduce the percentage uncertainity in using the burette
use a larger mass of the solid so a larger titre would be needed
another student is required to make 250cmcubed of an aq solution that contains a known mass of mhco3. the student is provided with a sample bottle containing the mhco3. describe practical they should use to prepare the sol (6 marks)
weigh sample bottle containing the solid on a balance, transfer to beaker and reweigh sample bottle record the differences in mass
add distilled water stir with a glass rod to ensure that the solid dissolves
transfer to graduated flask, with washings,make up 250cm cubed and shake it or invert it.
a solution of sodium chlorate was added to a colourless solution of potassium iodide. suggest what is observed
goes brown due to iodine because iodine is oxidised
suggest one reason other than incomplete combustion or heat transfer to the atmosphere why the students value for the enthalpy of combustion of methanol is different from that in a data book- 3 points
experiment not done under standard conditions
evaporation of alcohol
thermometer not being taken into account
suggest one reason other than use of mean bond enthalpies why a value for the enthalpy of combustion of a liquid alkane is different from the value obtained using the expression (liquid alkane vs gaseous)
as the alkane is not gaseous
the student used a wash bottle containing deionised water when approaching the end-point to rinse the inside of the conical flask. explain why this improved the accuracy of the titration
ensures that all the acid remained in the mixture
how does the contamination occur whereby ethanol turned into ethanoic acid
oxidation
the student failed to fill the burette correctly so that the gap between the tip of the burette still contained air. suggest what effect this would have on the measured vol of bromine water in this trial
measured volume higher, level in burette falls as tap is filled before the liquid is delivered.
how could the student improve the titration w drops
choose a larger vol of oil
dissolve it in organic solvent
transfer this to a flask and make up to 250cm cubed with solvent
titrate 25cm cubed samples from the flask
suggest a reason for the diff between the exp value for x and the correct value
failute to heat to a constant mass or failure to evaporate all the water
suggest how the procedure could be improved, using the same apparatus to give a more accurate value for x
heat for longer, ensure all the water has been driven off
some of the liquid injected did not evaporate because it dripped into the gas synringe nozzle outside the oven. explain how this would affect the value of the mr of y
lower volume, therefore mr would be greater
suggest one possible danger when a metal carbonate is reacted with an acid in a sealed flask
the pressure may build up from gases leading to an explosion
if one reactant goes to make products, why could the yield be less than expected
decomposition could not be complete, or loss of a gas
suggest one reaosn why it is hard to obtain a pure sample of no2 from a reac producing no2 pbo and o2
bc it may be hard to seperate the no2 from the o2
how to measure rate in an experiment
the change in ph of a reaction. (ph meter can be used to measure this at regular intervals)
amount of heat lost (measure mass lost as gas lost, moles can then be used to work out gas lost and reactants left)
volume of gas produced (Using gas syringe, ideal gas equation to wrok out moles of gas produced= molar ratio for reactant concs)
colour change then using a colorimeter or cross which is subjective
colorimeter
measure of absorbance of light
more conc a sample is, the darker the colour and hence more light is absorbed.
have to plot a calibration curve, then we can use that and absorbance to find out the conc of iodine at that point
The student purified the crude solid product, N−phenylethanamide, by
recrystallisation.
(i) Outline the method that the student should use for this recrystallisation
Dissolve the product in the minimum volume of water / solvent
(in a boiling tube / beaker)
If dissolving is not mentioned, CE = 0 / 4
1
Hot water / solvent
Steps must be in a logical order to score all 4 marks
1
Allow the solution to cool and allow crystals to form.
1
Filter off the pure product under reduced pressure / using a
Buchner funnel and side arm flask
Outline how you would carry out a simple laboratory process to show that the
recrystallised product is a pure sample of N−phenylethanamide.
Measure the melting point
1
Use of melting point apparatus or oil bath
1
Sharp melting point / melting point matches data source value
Assume that the reaction goes to completion.
Suggest two practical reasons why the percentage yield for this reaction may
not be 100%
Product left in the beaker or glassware
Sample was still wet
Sample lost during recrystallisation
You are provided with a small sample of pure aspirin in a melting point tube.
Describe briefly how you would determine an accurate value for the melting point of
aspirin
heat melting tube in an oil bath
slowly near the melting point
State two observations, during this melting point determination, that would indicate
that the sample is not pure.
wide melting range
below the true mp
Suggest why a pure sample of aspirin may sometimes appear to melt at a temperature different from 135 °C.
Temperature on thermometer not the same as the sample
Step 1: The sample is dissolved in a minimum volume of hot water.
Step 2: The solution is filtered hot.
Step 3: The filtrate is cooled in ice to form crystals.
Step 4: The crystals are collected by filtration, washed with cold water and left to dry.
Explain the purpose of each underlined point.
Minimum volume ………………………………………………………………………………………..
…………………………………………………………………………………………………………………
Hot water ………………………………………………………………………………………………….
…………………………………………………………………………………………………………………
Filtered hot ………………………………………………………………………………………………..
…………………………………………………………………………………………………………………
Cooled in ice ……………………………………………………………………………………………..
…………………………………………………………………………………………………………………
Washed with cold water
Minimum volume and hot water: Any two from: to obtain saturated solution to increase yield / reduce amount left in solution enable crystallisation (on cooling)
Filtered hot: to remove insoluble impurities
Cooled in ice: to increase amount of crystals that are formed
Washed with cold water: to remove soluble impurities
Give one reason why using a water bath is better than direct heating with a Bunsen
burner.
alcohol is flammable
