Advanced level Chemistry: Some notes on practical exercises and calculations
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Doc Brown's Advanced A Level Chemistry Revision Notes
Some Advanced A Level Practical Exercises and Calculations involving volumetric or gravimetric analysis
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(a) How to determine the concentration of ethanoic acid in vinegar
You can find the concentration of ethanoic acid in vinegar by titrating vinegar with a standard solution of sodium hydroxide.
Since ethanoic acid is a weak acid and sodium hydroxide a strong base you would use phenolphthalein indicator.
All you need to know/do is covered on ....
(b) How to determine the mass of calcium carbonate in an indigestion tablet
You can find the mass of calcium carbonate in an indigestion tablet by dissolving the tablet in excess acid - provided from a precise volume of known concentration e.g. 0.1 mol/dm3 hydrochloric acid.
You would then have to do a back-titration to determine the quantity of unreacted acid.
You then have to subtract the unreacted acid from the total acid at the start to get to moles of acid that reacted with the carbonate.
From the moles of reacted acid you calculate the moles of calcium carbonate in the tablet.
(c) How to determine the formula mass of an MHCO3 hydrogencarbonate
You can find the formula mass of a hydrogen carbonate of a group 1 metal by titrating a known mass with standard hydrochloric solution.
An appropriate quantity of the carbonate is dissolved in deionised water and made up to 250 cm3 in a standard volumetric flask.
25 cm3 aliquots are then titrated with 0.1 molar hydrochloric acid using methyl orange indicator.
(d) How to determine the molecular mass of a dibasic acid eg succinic acid
You can find the molecular mass of a dibasic acid by titration with standard sodium hydroxide solution
(e) How to determine the mass of aspirin in an aspirin tablet
You can find the mass of aspirin in an aspirin tablet by titration with standardised sodium hydroxide solution.
You can titrate the crushed and accurately weighed powder directly, but dissolve in a little alcohol before adding more water and phenolphthalein indicator.
(f) How to determine the yield for the conversion of magnesium to magnesium oxide
You can find the yield of magnesium oxide from heating magnesium in air in a crucible.
2Mg(s) + O2(g) ==> 2MgO(s)
Puzzled on this one. You readily convert magnesium to magnesium oxide by heating a known mass of magnesium ribbon in a crucible (plus lid, and everything accurately weighed).
You have to heat, cool and reweigh the crucible + lid several times until the total mass is constant.
There should be no unreacted magnesium, but you do get a little magnesium nitride (Mg3N2) formed (especially with the lid on which restricts oxygen, but stops MgO 'smoke' escaping from the crucible!).
From the weighings you can compare the actual yield of MgO and the theoretical amount of MgO, as long as nothing else is formed!
This experiment is fraught with error in my experience!
See Mg ===> MgO reacting mass calculation but I'll work in moles!
Example of calculation
Relative masses: Mg = 24, O = 16, MgO = 40
initial Mg + O2 ==> MgO formed + Mg unreacted
I'll just ignore the weighings and get to the heart of the calculation!
Suppose 1.92g of magnesium ribbon yields 3.10g of white residue - MgO
The mass of oxygen gained by Mg = 3.10 - 1.92 = 1.18g
therefore mol oxygen = 1.18/16 = 0.07375
and from the equation ...
mol O atoms = mol MgO = 0.07375, so mol MgO formed = 0.07375
initial mol Mg atoms = maximum mol MgO = 1.92/24 = 0.080
percentage conversion Mg to Mg O = 100 x 0.07375 / 0.080 = 92.2%
BUT this calculation assumes some Mg in the residue.
This is an example of gravimetric analysis - analysis from 'weighings'!
(g) How to determine the formula mass of a hydrated salt (eg magnesium sulfate) by heating to constant mass
You can find the formula and formula mass of some hydrated salts by heating a given quantity of it to constant mass.
(h) How to determine the percentage conversion of a Group 2 carbonate to its oxide by heat
You can find the percentage conversion of a group 2 carbonate to its oxide by heating an accurately known mass of the carbonate in a crucible (+ lid).
You need accurate mass weighings of (m1) crucible + lid, (m2) crucible + lid + carbonate, (m3) final mass of crucible + lid + oxide residue
(m2) - (m1) gives mass of carbonate, from this calculate theoretical yield of oxide by reacting mass calculation
(m3) - (m1) gives mass of oxide, compare with calculated mass above (maybe!?, see final comment)
Example of method results and calculation
Example calculation - m = mass 1-6 in grams, you need to follow the logic very carefully from above and onwards!
m3 MCO3 (initial) ==> m5 [MO formed + MCO3 undecomposed] residue + m6 CO2 lost
m3 we know and m6 we no from the weighings.
So how do we sort out the mixture of m5?
Well from m6 we get moles of CO2 which equals moles of MO
We can then compare moles MO formed with theoretical moles for 100% decomposition, hence calculate the % conversion.
Atomic masses: Ca = 40, C = 12, O = 16, formula masses: CaCO3 = 100, CaO = 56, CO2 = 44
I'll now outline the calculation with limestone, but ignoring the 'real' weighings, that's hardly the difficult part of the calculation!
Suppose after heating 5.00g of calcium carbonate (limestone) the residue in the crucible is 3.20g
Mass of CO2 lost = 5.00 - 3.20 = 1.80g,
so mole CO2 formed = 1.80/40 = 0.045
and from CaCO3 ===> CaO + CO2 (1 : 1 : 1 mole ratio)
we can argue that mol CaO formed = 0.045
and initial mol CaCO3 = maximum theoretical mol CaO = 5.00/100 = 0.05
Therefore % conversion of carbonate to oxide = 100 x 0.045 / 0.05 = 90%
So, it wasn't too bad a calculation after all, was it?!
This is another example of gravimetric analysis - analysis from 'weighings'!
(i) How to determine the number of moles of water of crystallisation in a hydrated salt by titration
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