[ola]

Please help me with these calculations
the aim of the experiment was to detrmine the extent of oxidation of magnesium metal by heating in air

2Mg + O2 = 2 MgO
mass of magnesium = 1.07, mass of MgO was 1.66 (weighted after burning)
taking into account the stechiometry of the reaction, the mass of MgO should be 1.78g,
I obtained in the experiment 1.66,  so -  mass of Mg + mass O2= mass MgO, therefore mass of O2 = mass of MgO - mass of Mg - mass O2= 1.66 -1.07 = 0.56 g

calculations:
1) initial mass of magnesium  - 1.07
2) No. of Moles of Magnesium -
3) No. of moles of Oxygen required -???
4) Final mass of Mg/MgO - it is 1.66
5) weight of Oxygen used in the reaction
6) No.of moles of oxygen used - ??
7) % of Magnesium Oxidised -?
% of Magnesium not Oxidised-???
 
If somone could help me , I would be happy  

[AWK]

2) 1.07/24

[chronictonic]

it weighed 1.66g (MgO) after burning. and the theoretical amount that you should obtain (according to you) is 1.78g. sooooo (1.66g/1.78g)*100% = 93.3%
correct me if i read it wrong but there's no need to do all those conversions when you already have all amounts of MgO. You're over thinking it, mang

[Borek]

You are wrong - 1.66 g is not a mass of MgO, it is a mass of post reaction mixture.

[chronictonic]

Please help me with these calculations
the aim of the experiment was to detrmine the extent of oxidation of magnesium metal by heating in air

2Mg + O2 = 2 MgO
mass of magnesium = 1.07, mass of MgO was 1.66 (weighted after burning)
taking into account the stechiometry of the reaction, the mass of MgO should be 1.78g,
I obtained in the experiment 1.66,  so -  mass of Mg + mass O2= mass MgO, therefore mass of O2 = mass of MgO - mass of Mg - mass O2= 1.66 -1.07 = 0.56 g

calculations:
1) initial mass of magnesium  - 1.07
2) No. of Moles of Magnesium -
3) No. of moles of Oxygen required -???
4) Final mass of Mg/MgO - it is 1.66
5) weight of Oxygen used in the reaction
6) No.of moles of oxygen used - ??
7) % of Magnesium Oxidised -?
% of Magnesium not Oxidised-???
 
If somone could help me , I would be happy  

moles of Mg = (1.07g/24.31gmol^-1)= 44.01mmol
1 mole of MgO = 24.31gmol^-1 + 16.00gmol^-1 = 40.31gmol^-1
expected moles of MgO = 0.04401mol * 40.31gmol^-1 = 1.77g (Not 1.78 though i realise that's just a rounding error and depends on where you got your data so no marks lost there)

Number of moles of oxygen required to complete this reaction is equal to 1/2*moles of Mg.
According to the equation

Mg + 1/2O2 --> MgO

Is 44.01mmol/2 = 22.01mmol

weight of oxygen required for reaction to go to completion = 22.01mmol*16.00gmol^-1 = 0.352g

Without knowing whether the reaction went to completion or not we can't figure out any of the other questions. Did the experiment involve separating the MgO and unreacted Mg? you need to know the individual weights otherwise there's a missing variable. Perhaps you are supposed to get the Gibbs energy data and calculate the equilibrium point for the reaction at atmospheric temperature? That seems a little confusing. I've burnt Mg in the lab and I gotta say there's rarely anything left behind once you get it going. Arre you absolutely positive that the final mass is of both reacted and unreacted Mg? I feel like we're missing some info...

[Borek]

Without knowing whether the reaction went to completion or not we can't figure out any of the other questions. Did the experiment involve separating the MgO and unreacted Mg? you need to know the individual weights otherwise there's a missing variable.

We don't miss anything, there is enough information to calculate everything. What is 1.66g-1.07g?

[DrCMS]

Number of moles of oxygen required to complete this reaction is equal to 1/2*moles of Mg.
According to the equation

Mg + 1/2O2 --> MgO

Is 44.01mmol/2 = 22.01mmol

weight of oxygen required for reaction to go to completion = 22.01mmol*16.00gmol^-1 = 0.352g

Try again. 

If you are going to try to answer the whole question, for somebody else, to show how super smart you are learn some chemistry first.

[chronictonic]

Well i'm sorry I tried to help. there is nothing wrong with my calculations, as far as i can see. if there is you should probably point out specifics Drdouche, perhaps even correct them so that ola can hand their prac report on time. As for learning chemistry I am in my final year of a bachelor of medicinal chemistry and the university of new south wales, australia. so, yeah, i'm working on it.

[DrCMS]

Well i'm sorry I tried to help.

Trying to help is great we all come here to do that but doing the whole thing for the OP is not part of the deal.  We point them in the right direction so they can answer the question themselves and learn how to solve such questions.  Doing the whole thing for them and getting it wrong is even worse because the OP learns nothing and fails their homework.

there is nothing wrong with my calculations, as far as i can see. if there is you should probably point out specifics

There is something wrong with your calculation which is why I said so; the molecular weight of oxygen gas is 32 not 16. 

If you use the correct figure you'd see that the amount of O₂ that could react with 1.07g of Mg is 0.704g.  As the total after the reaction is 1.66g rather than 1.774g (1.07+0.704) not all the Mg reacted with O₂.  Which is what Borek has tried to point out to you twice but you did not take the hint.

As for learning chemistry I am in my final year of a bachelor of medicinal chemistry and the university of new south wales, australia. so, yeah, i'm working on it.

Well I've got a BSc, a PhD and over 15 years in the chemical industry and I'm still working on it.   As far as I'm concerned getting a degree in chemistry should only be the start of your journey into chemistry not the end.  So next time try not to be quite so cock sure of yourself.

[chronictonic]

See now wasn't that easy, that is so much more constructive. i now see where i made errors. I did stoichiometry for a single carbon atom and then did it again. now, seeing as you are the leading authority in chemistry STICK TO THE POINT OF THE QUESTION

[chronictonic]

help sort out the real problem which is how we determine what was reacted vs. unreacted. the only way i can think is with some equilbrium minimum gibbs' energy calculations, assuming standard temp etc. but im also positive that that is well beyond the scope of the question. so im totes lost. apply your phD, oh mighty lord!!

[sjb]

help sort out the real problem which is how we determine what was reacted vs. unreacted. the only way i can think is with some equilbrium minimum gibbs' energy calculations, assuming standard temp etc. but im also positive that that is well beyond the scope of the question. so im totes lost. apply your phD, oh mighty lord!!

Why does the mass increase?

[vmelkon]

So the mass of the final product is 1.66 g. We are assuming it is a mix of MgO and Mg. (Note that it could also form Mg3N2).

1) 1.07 g of Mg
2) 1.07 g/24.305 g per mol = 0.0682987 mol ~ 0.0683 mol of Mg
3) You need half. 0.0682987 mol/2 = 0.0341493 mol ~ 0.0341 mol of O2
4) Final product mass = 1.66 g
5) Final mass - initial mass = O2 in grams = 1.66 g - 1.07 g = 0.59 g
6) 0.59 g/(15.9994 *2) g per mol = 0.01843819 mol ~ 0.0184 mol of O2

7) % of Magnesium Oxidised
0.01843819 mol of O2 combines with 0.03687638 mol of Mg.
0.03687638 mol of Mg / 0.0682987 mol = 53.9928% ~ 54.0%

% of Magnesium not Oxidised-???
uhmmm, use a calculator or a super cray.

[DrCMS]

So the mass of the final product is 1.66 g. We are assuming it is a mix of MgO and Mg. (Note that it could also form Mg3N2).

1) 1.07 g of Mg
2) 1.07 g/24.305 g per mol = 0.0682987 mol ~ 0.0683 mol of Mg
3) You need half. 0.0682987 mol/2 = 0.0341493 mol ~ 0.0341 mol of O2
4) Final product mass = 1.66 g
5) Final mass - initial mass = O2 in grams = 1.66 g - 1.07 g = 0.59 g
6) 0.59 g/(15.9994 *2) g per mol = 0.01843819 mol ~ 0.0184 mol of O2

7) % of Magnesium Oxidised
0.01843819 mol of O2 combines with 0.03687638 mol of Mg.
0.03687638 mol of Mg / 0.0682987 mol = 53.9928% ~ 54.0%

% of Magnesium not Oxidised-???
uhmmm, use a calculator or a super cray.

Correct approach but the wrong answer, try step 2 again.