development of practical skills Flashcards
why might a student obtain a smaller titre when compared with another student
- the pipette is rinsed with water before filling with HCl
why might a student obtain a larger titre when compared with another student
- the jet space of the burette is still filled with air before starting the titration
why would rinsing the conical flask with distilled water before completing a titration not affect the titre
- it will not affect the number of moles that react during the titration
when making a standard solution, how can it be ensured that the final concentration of the solution is accurate
- using a dropping pipette to add water up to the graduated mark
what assumptions are made when CALCULATING enthalpy change
- the specific heat capacity of the solution is the same as water
- the density of solution is the same as water
- all reactants have reacted completely
in an experiment to determine ef of magnesium oxide a student tapped the residue with tongs to break it up
why?
how would this affect the ef?
- to ensure all of the magnesium had reacted
- it would be more accurate
what would the effect be when filling the volumetric flask to the top of the flask, not the graduation line
- the titre would be greater than expected
- as the solution is less concentrated
- so more solution is required to reach the end point
what piece of apparatus should be used to measure solutions the most accurately
- volumetric pipette
when finding enthalpy change of neutralisation, how can apparatus be improved to improve accuracy
use a lid/ more insulation around the cup
- therefore, temp change would be greater
- thus, calculated temp would be closer to literature value
what apparatus is the most accurate for collecting gas over water
inverted burette
when heating in a crucible, why should a lid be used
- improves accuracy as prevents loss of solid
- should also be loose fitting to allow gas to escape/ enter
why does a crucible need to be dry before heating
- would cause mass loss to be too large as the water would be lost when heating
what are the potential errors of using a gas syringe
- gas escape before bund inserted
- syringe sticks
- some gases like co2 and so2 are soluble in water so the true amount of gas is not measured
why is a volumetric pipette more accurate than a measuring cylinder
smaller uncertainty
phenolphthalein colour
if ACID is added from the burette the colour change is pink(alkali) to colourless (acid)
methyl orange colours
red in acid
yellow in alkali
orange at end point
when heating in a crucible, why would the tongs be used to lift the crucible lid slightly during heating
to make sure enough o2 is present for the reaction to occur
what would be the effect of filling the volumetric flask to the top and not to the graduated line?
- titre would be greater
- solution less concentrated
- more solution required to reach the end point
what piece of apparatus can be used to collect GAS OVER WATER that would improve accuracy
inverted burette
state two assumptions made when using the q=mc△t equation
- solutions have the same specific heat capacity as water
- density of the solution is the same as water
how to make up a standard solution
- weigh sample bottle containing required mass of solid on a 2dp balance. transfer to a beaker and record the difference in mass
- add 100cm3 of distilled water to the beaker. use a glass rod to dissolve it.
- 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 flask
- make up the graduated line with distilled water using a dropping pipette
- invert flask to ensure uniform solution and concentration
what acid should be used for manganate titrations
dilute sulfuric acid
how to reduce uncertainties in a titration
- replace measuring cylinders with pipettes or burettes
- increase volume and conc of substance in the conical flask
- decrease conc of substance in the burette
