topic 5 - formulae Flashcards
what is meant by the term molar volume of a gas (1)
volume occupied by one mole of a gas at a specified temperature and pressure ( under standard conditions )
explain how the student would know if the gas syringe had a leak in step 2 and what effect would this have on the molar mass (2)
- plunger does not return to zero/ original position when released
- molar mass will decrease because ‘air’ has a lower mass than the gas
if the temperature had been less than 20 degrees celsius and the pressure remained at one atmosphere, deduce the effect, if any, on the molar mass calculated (2)
- the calculated molar mass would be greater
- at a lower temperature there would be more molecules in the same volume ( greater density )
give a reason why the gas should be dry ( measuring molar mass )
so that the water doesn’t affect the molar mass ( molar mass would decrease )
- gas is now a mixture
give a reason why the student added a small pinch of calcium carbonate to the acid before starting the reaction
- to saturate the solution with CO2, to stop the CO2 formed from dissolving
give a reason why the student repeated the experiment 3 times
to identify anomalies and discard them
give 3 reasons for the difference between your calculated value and the actual volumes of hydrogen gas obtained by the student
1) issue: hydrogen escapes from the apparatus . improvement: use a sealed apparatus with a gas syringe
/use a conical flask / bung with a gas syringe
2) issue: magnesium ribbon covered with oxide
improvement: clean with abrasive before weighing
3) issue: mass of magnesium may be less than 0.04 g (i.e. as
low as 0.035 g)
or
mass of magnesium required is too small to be measured accurately by the balance available improvement: use more precise balance/ use larger mass
(so percentage error is less)
4) issue: large measuring cylinder cannot measure volume accurately (as the graduations are too far apart)
improvement: use a smaller measuring cylinder/burette/ conical flask / bung with a gas syringe
state one way which would ensure a fair test
- same bunsen setting or same distance between flame+tube
state how data obtained in this experiment could be used to make a comparison
- time taken for a given volume of gas to be produced or time taken for limewater to go cloudy
titration: colour change of methyl orange as an indicator
yellow to orange
explain actions the technician might take in the procedure just before the end point titration, to ensure accuracy
- add drop by drop so that too much acid is not added
- swirl to ensure homogenous mixture
- rinse the sides of the flask with distilled water between additions to rinse all reactants into the solution
- use a white tile to clearly see a change of colour
- compare the colour of the solution at the end point with previous tritrations to. ensure consistency of the end point colour
- rinse the end of the jet of the burette with distilled water to ensure all the hydrochloric acid is in the conical flask ( no drip left on burette )
identify three issues with the technicians method and for each issue identify an improvement
issue: mass of (solid) acid not accurately weighed
out (1)
* improvement: weigh mass of acid by
difference/rinse out the weighing bottle/use a
balance reading to 2 d.p./use a more precise
balance (1)
* issue: some acid will be left in the beaker/some acid
will not be transferred to the volumetric flask (1)
* improvement: rinse out the beaker (in which the
solid acid was dissolved) and add the washings to
the volumetric flask (1)
* issue: insufficient mixing of the
solution/concentration of the solution will not be
uniform (1)
* improvement: invert the volumetric flask (several
times) (1)
* issue: burette not rinsed (1)
* improvement: burette should be rinsed with acid
solution before use (1)
