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Please, please help quickly!!!!!!!!!!!!!!!!!!!!!!

I have to do this planning exercise (only as practioce) but it is for tomorrow and as I have never done something like this before, because I am not from England, I find it quite difficult how to do it.

The Exercise asks me to:
Describe how the % purity of MgCO3 (as the only active ingredient) in an indigestion tablet is determined by a) thermal decomposition and b) reaction with an acid.

I know that this should include:
a method for each way with a diagram
* list of apparatus (just put above the diagram)
* how the % of MgCO3 would be found in the thermal decompostition method
* the method of how I decided how much acid and tablet to use in method b
* safety precautions( possible hazards )

and it all has to be in 500 words

That is what I wrote so far, but I really don't know how to do it right.

(a) Thermal decomposition

Method:
1. Crush the tablet and measure mass of crucible containing tablet residue.
2. Heat over a Bunsen burner with the lid off for 2 to 3 minutes.
3. Measure mass of crucible again
4. Heat again for a while
5. Remove from the heat and allow cooling. If the mass is still decreasing, heat it again. Repeat until mass remains constant.
6. - Calculate the mass difference and use the mass loss to calculate the number of moles of carbon dioxide given off.
- Use molar ratios to calculate how many moles of magnesium carbonate there were, and then calculate the % purity

Explanation of Method:
( gas collecting method)
The tablet has to be crushed up because otherwise too much gas would be produced AND THE TEMPERATURE HAS TO BE KEPT CONSTANT (WHHHHHHYYYYY?????)

On heating the MgCO3 with an acid all of it will decompose (WHYYYYY???)and the mass loss will be due to the given off CO2. You than know that the weight loss equals the mass of CO2 given off. To find the expected original mass I first need to find the moles of CO2 which are the mass of CO2 divided by 44(Lets say it's 0.05 moles).

It is needed to do a % purity calculation, so:

MgCO3 --> MgO + CO2
.`. 1:1 ratio between MgCO3:CO2
.`. Same amount of moles of MgCO3 (X)
Mass MgCO3 = (X) x 84.3 (RAM of MgCO3)
= in my example with 0.05 moles:
Mass MgCO3 = 0.05 x 84.3 (Mr of MgCO3)

So now I have the mass of MgCO3 in the crushed tablet.
To find the percentage purity I use the following equation:
Percentage purity = (Mass MgCO3/Mass of crushed tablet) x 100%

or % purity = mass difference/original mass of tablet x 100.% purity = mass difference/original mass of tablet x 100.

(and moles CO2 = moles MgCO3

use moles of MgCO3 to calculate expected original mass
purity = actual mass/theoretical mass x 100%

I used above the three weights from the experiment
a)empty crucible b)crucible and crushed tablet c)after heating

which gave:
mass of tablet = b - a
mass lost = b – c)

(b) Reaction with an acid

Method:
1. Crush tablet, weigh, and put into a sample tube.
2. Add a fixed volume of Hydrochloric acid (HCl) to a conical flask, and stand sample tube upright in the flask.
3. Add a bung with a delivery tube attached to it.
4. Fill a burette with water, and place it in a trough of water.
5. Turn the sample tube up side down to initiate the reaction. Collect the released CO2 gas in a syringe, waiting until fizzing stops, and note the volume the gas produced.
6. Use moles and molar ratios to determine how much magnesium carbonate there was.

MgCO3 is highly soluble in acids so every other substance, which must be insoluble, will be the impurity in the MgCO3/acid mixture after the reaction is completed (assuming the impurities present are non soluble). It is then possible to calculate the percentage purity through comparing the measured amount of MgCO3 before the reaction and after the reaction.

When dissolved with acid, magnesium carbonate decomposes with release of carbon dioxid, which is delivered to the cylinder via the delivery tube and water is displaced by the gas:

MgCO3 (s) + 2HCl(aq) → MgCl2(aq) + CO2(g) + H2O(l)
Magnesium Carbonate + Hydrochloric Acid ---> Carbon Dioxide + Magnesium Chloride + Water

So the mole ratio would be 1mol MgCO3 : 2mol HCL

It is needed to do a reactant quantity calculation.

As a gas sringe will only hold 100cm^3 of the CO2:

1 mol CO2 = 44g = 24000cm^3
100cm^3 = 0.18333g

84g MgCO3 -----> 44g CO2
0.35g MgCO3 -----> 0.183g CO2

...so I should use approximately 0.35g tablet. (This is a very small amount, which is why considering to use something bigger to collect the gas would be sensible).

But by using the gas sringe which can only hold 100cm^3 of the CO2, I need to use 0.35g of the tablet or even less, because if I use 0.35g of the tablet and it is 100% MgCO3, the max. ammount of CO2 I can get from it is 0.18333g which equals 100cm^3. So I need to make sure it is less than 100cm^3 because if its more, it won't fit into the gas syringe!

As the ratio is one to two, I double the amount of moles to get the amount in moles of acid used.
Then I multiply it by the RAM of HCL to get the amount in moles and I can do the same to find out how many grams of the tablet I should be using. But it is important that the HCL is in excess to make sure all of the tablet reacts.

What are the hazards of the experiments and how can I explain everything better??????? Are my calculations right or is everything rubbish????? Please, please help tonight or very early tomorrow morning. I will sit in front of the computer until I have finished.
Thanks a lot!!!

Reply 1

Eau

It looks very organised and fine to me.

Hazards - chemical and apparatus/methodology dangers.

Reply 2

Funny Mann

celeritas

It looks very organised and fine to me.

Hazards - chemical and apparatus/methodology dangers.

Thanks for answering, but please, please anyone tell me the hazards and answer my questions (can't fint it out, tried it honestly). Only 20 minutes, than I have to hand it in!!!!!!

Reply 3

Eau

Build up of pressure in gas syringe - make sure you allow for that. Acids may be corrosive. Bunsen burner may burn skin. That knd of thing.

Reply 4

Funny Mann

Alright, so there is nothing special except sucking up the acid would obviously be quite dangerous, thanks. That is a stupid question put I really don't know how to explain this: WHY does MgCO3 heated with an acid decompose???? What is the chemical explanation for this?? And my last question: WHY has the temperature to be kept constant???
Thank you so much for helping me so early in the morning, smile.

Reply 5

Eau

MgCO3 is a base.

Well a changing temperature will cause the reading on the syringe to deviate as pressure is inversely proportional to volume - pV = nRT

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Please, please help quickly!!!!!!!!!!!!!!!!!!!!!!

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