12 core practicals from an A* student, which recieved a pass on the practical endorsement requirement of the OCR a A level Chemistry course. Each core practical is detailed and comprehensive including: (i) apparatus table with purpose and safety of equipment listed, (ii) method (iii) results (iv) c...
1.1 Empirical formula of magnesium TTTTTTTTT
Piece of Purpose of piece of Comment on how piece of equipment
apparatus apparatus was used accurately/appropriately/safely
Crucible/lid Can contain the reaction The crucible can be heated until it is red hot. While
between magnesium and the crucible can be heated to such an extreme
oxygen whilst temperature; it crack or break quite easily. - handle
withstanding high temps gently and with caution
from Bunsen burner.
Bunsen The reaction between Always hold the burner by its base when moving it.
burner magnesium and oxygen is Keep long hair tied to prevent burning and roll up
a combustion one so it long sleeves. Keep the bunsen burner on blue flame.
requires heat.
Ring stand + Holds and supports the Be sure not to knock either apparatus down.
clay triangle crucible.
Tongs Used to open crucible lid Prevents direct contact with the hot crucible lid.
Spatula/ Scrape the magnesium
sandpaper ribbon
Scale Measure the mass of the Zero the scale to ensure an accurate reading.
crucible + lid before and
after
METHOD
● Scrape the surface oxidation from both sides of the magnesium ribbon to expose the
shiny metal
● Coil the magnesium ribbon into a tight spiral
● Measure and record the mass of the crucible
● Set up the apparatus as shown below
● Turn the Bunsen burner on and place the magnesium ribbon in the crucible
● Use your tongs to leave the crucible lid ajar every so often [do not look directly at the
burning magnesium]
● Once the crucible and lid have cooled to room temperature, lift them from the clay
triangle and record their combined mass
Variables:
Independent - The mass of the magnesium ribbon
Dependent - The presence of oxygen
Control - The same crucible must be used throughout
,Results
Object Mass (g)
Crucible + lid 58.45
Crucible + lid + magnesium ribbon 58.85
Magnesium 0.40
Crucible + lid + magnesium oxide 58.97
Magnesium oxide 0.52
Oxygen 0.12
ANALYSIS Steps to determine the empirical formula of MgO:
1. Calculate moles of magnesium and oxygen n=mxmr
2. Determine molar ratio
3. Compare practical results to theoretical results
4. Identify likely sources of error.
2Mg(s) + O2(g) 2MgO(s)
No of moles in Mg
= 0.40/24
= 0.0166666667
No of moles in oxygen
= 0.12/32
= 0.00375
Ratio
= 0.0166666667 : 0.00375
= 1 : 4.444444453
Therefore the empirical formula is MgO4.
EVALUATION
The results obtained from the experiment were not equal to the actual ratio of magnesium oxide
because several things went wrong during the experiment. The oxidation layer on the
magnesium ribbon was not completely scraped at the start. Some product was also lost as
,some “white smoke” escaped the crucible when ajar. The crucible that was used had chemical
residue and this may have added more mass to the crucible, which would affect the results.
CONCLUSION
The hypothesis was not supported. The practical ratio did not equal the theoretical ratio.
, 2.2 Titration TTTTTTTTTTTTTTTTTTTTTTTTTTTT
Piece of Purpose of piece Comment on how piece of equipment was
apparatus of apparatus used accurately/appropriately/safely
Burette Used to dispense Get readings from the line of the meniscus to
the analyte prevent parallax error as liquid seems to bend to
the naked eye when seen from above.
White tile Used to observe The colour change may not be clearly visible
clear colour change during the titration; place white tile underneath the
conical flask.
Pipette + filler Used to measure
known solution into
flask
Clamp stand Used to hold burette Ensure burette is properly hooked on to prevent it
in place over conical falling/breaking as it is delicate glassware.
flask
Filter funnel Used to transfer
solution
Glass rod Used to stir Stir constantly to ensure solutions are completely
mixed/react together.
Conical flask x2 Used to contain Swirl the conical flask when adding the acid to
known solution- ensure that the reactants are well mixed.
where neutralisation
occurs
Measuring cylinder Used to measure the Measure from meniscus line to prevent parallax
volume of solutions error.
Glass beaker Used in transferring
250cm^3 and stages
100cm^3
Weighing boat Used to weigh solids Some of the weighted NaHCO3 may not be
transferred from the weighing boat; use the
weighing by difference technique.
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