Practical skills - Chemistry. Flashcards
Give examples of iritants and the precautions that should be taken.
Dilute acids and alkalis are iritants. Should wear goggles.
Give examples of corrosive substances and the precautions that shoulf be taken.
Stronger acids and alkalis are corrosive. Should wear goggles.
What safety measures should be taken for flamable substances.
Keep away from naked flames.
Give the precautions that shoulf be taken for toxic substances.
Wear gloves and keep away from skin contact - wash hands after use.
For oxidising substances what are the precautions that shoulf be taken.
Keep away from flamable and easily oxidised materials.
What two things can heating in a crucibe be used for.
Measuring mass loss in thermal decomposition reactions, or mass gain when reacting magnesium with oxygen.
Give the method for removing the water of crystalisation as water vapour from calcium sulphate crystals to determin an experimental value for X.
Weigh an empty and dry crucible and lid.
Add 2g of hydrated calcium sulphate to the crucible and weigh again.
Heat strongly with a bunsen.
Allow to cool.
weigh the crucible and contents again.
Heat the crucible again and reweigh untill you reach a constant mass (this will ensure the reaction is complete)
Why should large amounts as hydrated calcium sulphate(e.g 50g) not be used when heating it in a cricible.
Since the thermal decomposition is likely to be incomplete.
Why does the crucible need to be dry for thermal decomposition reactions.
A wet crucible will give innacurate results since it would cause the mass loss to be too high since the water would evaporate on heating.
Why should small amounts of a hyrdayed solid crystal not be used when heating it in a crucible for thermal decomposition. (e.g 0.1 grams)
The percentage uncertainties in weighing the solid will be too high.
The the actual value for X in a thermal decomposition reaction shoud be higher than the experimental value what is likely to have happened.
see test.
Give two possible things a gas syringe could be used for.
Variety of experiments where the volume of gas is measured.
Probably to work out moles of gas or to follow reaction rates.
What should be noted down and why when using a gas syringe.
The Pressure and temperature of the room since the volume of gas wil vary with thease two factors despite the moles staying constant.
Give the equation and the units for each variable in the equation related to a gas syringe.
PV=nRT n=8.31 constant , V=M cubed , pressure in pascals Pa.
What are potential errors that coud occur when using a gas syringe.
The gas could escpae before the bung is inserted
The syringe can stick
Some gassses like carbon dioxide and sulphur dioxide are not soluble in water and so the true amount of gas is not measured.
Giv the method that you would follow to calculate the Mr of propanol.
Extract 0.2cm cubed of propanone into a hyperdermic syringe and then measure the mass of the syringe.
Remove a gas syringe from an oven and note the volume of air already in the barrel.
3. Inject the propanone through the self-seal cap into the barrel. The plunger will move straight away.
4. Put the gas syringe back into the oven.
5. Measure the mass of the empty hypodermic syringe immediately. This change will give you a mass of the volatile liquid in its liquid form as you can’t get this from its gas state.
6. After a few minutes measure the volume of the gas in the gas syringe it will have decreased as the 0.2cm3 of liquid will evaporate due to the heat of the gas syringe as it just came out of the oven, record the temperature of the oven.
shelf and the pressure of the room.
This gives you all the info you need to use ideal gas equation.
Giv the method that you would follow to calculate the Mr of propanol.
Extract 0.2cm cubed of propanone into a hyperdermic syringe and then measure the mass of the syringe.
Remove a gas syringe from an oven and note the volume of air already in the barrel.
Inject the prpanone into the self seal cap into the barrel of the gas syringe. The plunger will move streight away.
Measure the mass of the empty hyperdermic syringe.
After a few mins measure the volume of gas in the syringe and note the temp of the oven and the pressure of the room.
Draw a gas syringe and give the most important parts of the drawing.
There should be measurment marks of the barrel of the gas syringe and no gaps where gas could escape.
Give the method for making up a volumetric solution for a solid.
Weigh the smaple bottle containing the required mass of solid on a 2dp balance.
Transfer this to a beaker.
Reweigh the sample bottle and record the difference in mass.
Add 100cm cubed of distilled water to the beaker.. Use a glass rod to stir to help dissolve the solid.
Sometimes the substance may not dissolve well in cold water so the beaker and its contents could be heated gently until all the solid had dissolved•
Pour solution into a 250cm3 graduated flask via a funnel. Rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask.
Shake the volumetric flask thoroughly to ensure a uniform concentration.
Make up to the mark with distilled water using a dropping pipette for last few drops.
• Invert flask several times to ensure uniform solution.
Remember to fill so the bottom of the meniscus sits on the line on the neck of the flask. With dark liquids like potassium manganate it can be difficult to see the meniscus.
Shake the volumetric flask thoroughly to ensure a uniform concentration.
Give the method for making up a volumetric solution.
Weigh the smaple bottle containing the required mass of solid on a 2dp balance.
Transfer this to a beaker.
Reweigh the sample bottle and record the difference in mass.
Add 100cm cubed of distilled water to the beaker stiring with a glass rod to help the solid dissolve.
Now transfer the solution into a 250cm cubed graduated flask via a funnel.
Rinse both the beaker and funnel and add the washings from the beaker funnel and glass rod.
Using a dropping pipete make up to the mark with distilled water.
Invert flask several times to ensure a uniform solution.
Where should the meniscus sit.
Bottom of the miniscus should sit on the line of the neck of the flask.
What can be done if a substance is not disolving well in cold water.
Heat gently untill it all disolves.
What is a graduated flask. What is it also known as and why should it not be heated or have hot substances put in it.
A graduated flask has one mark on the neck which is the level to fill to get the acurate volume. It is also known as a volumetric flask. Heat would cause the flask to expad and the volume measured would be incorrect.
What problems can occur with measuring out dark luiqids.
With dark luiqids like potasium manganate it can be difficult to see the miniscus.
How can mass be measured out acurately in generlal.
In many experiments the best method is the weigh and re-weigh method in which:
On a 2dp balance measure the mass of a weighing bottle with the required mass inside it.
Empty the mass into the reaction vessel or flask.
Re-weigh the now empty weighing bottle
Subtract the mass of the empty weighing bottle fro the first reading to give the exact mass actually added.
If added streight from the bottle washings should be added too.
Give the general method for diluting a solution.
Pippete 25 centimeters cubed of the original solution into a 250 cm cubed volumetric flask.
Make up to the mark with distiled water using a dripping pipete for the last few drops.
Invert the flask several times for a uniform solution.
Why is a volumetric pippete more acurate than using a measuring cylinder.
It has a smaller uncertainty.The volumetric pipette has one neck Line to fill to whereas the measuring cylinder has loads.
What should be used to make a volumetric flask up to the mark.
A teat pippete should be used to make up to the mark so it doesnt go over the line.
State what titrations are often used for and the two possible reactions you would see during a titration.
Titrations are done often to find out the concentration of one substance by reacting it with another substance of known concentration. They are often done with neutralisation reactions, but can bedone with redox reactions.
Explain generally where the two reactants are placed in a titration.
One substance (generally the one we don’t know the concentration) is put in the conical flask. It is measured using a volumetric pipette.The other substance is placed in the burette
Explain what the standard phrase titrate solution A with solution B means.
It means that A should be in the conical flask and B should be in the burette.
Why , in a titration is a conical flask used in preferene to a beaker.
A conical flask is used in preference to a beaker because it is easier to swirl the mixture in a conical flask without spilling the contents.
Describe in detail how you would use in pipette in the first stage of a titration considering what should and shouldnt be done.
•rinse pipette with substance to go in it (often alkali).•pipette 25 cm3 of solution A into conical flask. The volumetric pipette will have a mark on its neck to show the level to fill to. The bottom of the meniscus should sit on this line.•touch surface of solution with pipette( to ensurecorrect amount is added). A small amount of solution will be left in the pipette at this stage. The calibrationof the pipette will take into account this effect. It should not be forced out.
Describe in detail how you would use a beaurete in the second stage of a titration considering things that should and shouldnt be done.
The burette shouldbe rinsed out with substance that will be put in it.If it is not rinsed out the acid or alkali added may be diluted by residual water in the burette or may react with substances left from a previous titration. This would lead tothe concentration of the substance being lowered and a larger titre being delivered.Don’t leave the funnel in theburette because small drops of liquid may fall from the funnel during the titration leading to a false burette reading (would give a lower titre volume) make sure the jet space in the burette is filled with the solution and air bubbles are removed or it will fill during the titration giving a too great a titre.
What will happen to the titre reading if the jet space in the beurete is not filled.
If the jet space in the burette is not filled properly prior to commencing the titration it will lead to errors if it then fills during the titration, leading to a larger than expected titre reading.
Describe , if given a picture how you would read a beurete and state the accuracy that it should be recorded to in regards to where the miniscus sits.
You should read going down the beurette with the bottom of the miniscus being the value coming downwards that you take.
Even though a burette has marking reading to 0.1cm3, the burette readings should always be given to 2dp either ending in 0.00 or 0.05. 0.05cm3 is the volume of 1 drop of solution delivered from a burette and so this is the smallest difference in readings that can be measured. If the bottom of the meniscus sits on a line it should end with a 0.00 as in the above example 9.00cm3. If the meniscus sits between two lines it should end 0.05. e.g. if the bottom of the meniscus sits between the lines marked 9.1 and 9.2, you should record 9.15
