# Amount of Substance Q

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#1
The following two-stage method was used to analyse a mixture containing the solids magnesium, magnesium oxide and sodium chloride.
Stage 1A weighed sample of the mixture was treated with an excess of dilute hydrochloric acid.
The sodium chlorid*e dissolved in the acid. The magnesium oxide reacted to form a solution of magnesium chloride. The magnesium also reacted to form hydrogen gas and a solution of magnesium chloride. The hydrogen produced was collected.

When a 2.65 g sample of the mixture of the three solids was analysed as described above, the following results were obtained.
Hydrogen obtained in Stage 1 0.0528mol
Mass of magnesium oxide obtained in Stage 2 6.41 g

Use these results to calculate the number of moles of original magnesium oxide in 100 g of the mixture.

I am not in need of the working out, rather a sequence of steps and the reasoning behind:

A) Form equations
B) Find moles of hydrogen (=moles of Mg Stage 1)
C) Find moles of MgO (Stage 2)
D)
E)
F)
etc.
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#2
Thank you very much 🙏 . I got the question from an exam pack my school gave...

If you don't mind, I have another Q I had great difficulty working out:

A student carried out an experiment to find the mass of FeSO4.7H2O in an impure sample, X.
The student recorded the mass of X. This sample was dissolved in water and made up to 250 cm3 of solution.
The student found that, after an excess of acid had been added, 25.0 cm3 of this solution reacted with 21.3 cm3 of a 0.0150 mol dm–3 solution of K2Cr2O7
(a) Use this information to calculate a value for the mass of FeSO4.7H2O in the sample of X.

Main thing I really don't understand is how you form the equation

(Original post by jb2510)
Attachment 803046
Hope This helps. Very good question where did you find it.
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2 years ago
#3

Hope this helps.
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#4
(Original post by jb2510)

Hope this helps.
thank you so much! can I ask how you knew to form a Redox equation?
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2 years ago
#5
There is a change in ox state for both transition metals of the Fe2+/3+ and Cr6+/3+ and the wording and style of question looks similar to the redox q's that I have practised so I just put two n two together, that all.

Hope that helps
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#6
thanks, what in particular in regards to the wording of the q?
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2 years ago
#7
It's hard to say it's just once you do a lot of these redox titration q's you get used to how examiner word the question. I had to practice redox titration a lot because I kept getting the wrong answer in the lesson I did alot of q's and so I am familar iwth the wording they use. It basicaly come with practice that all mate.
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#8
cheers mate, you sitting exams this year or have you already done so? and if so, any tips?
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2 years ago
#9
nah sitting the exams this year
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#10
quick q, How do you know the Cr +6 was reduced to Cr +3?
(Original post by jb2510)
nah sitting the exams this year
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2 years ago
#11
You just have to learn that info: 6e+14H+ + (Cr2O7)-2------->2(Cr)3+ + 7H20 **** in acidic condition
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#12
thanks again, helped me more than I could've hoped for. Is there an easy way to learn this (and the rest), a website? or just exam practice
(Original post by jb2510)
You just have to learn that info: 6e+14H+ + (Cr2O7)-2------->2(Cr)3+ + 7H20 **** in acidic condition
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2 years ago
#13
just practice mate. I am just making flashcards of each of these TM reactions and their colours change because they're sooooo many too, learn.
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11 months ago
#14
(Original post by jb2510)
There is a change in ox state for both transition metals of the Fe2+/3+ and Cr6+/3+ and the wording and style of question looks similar to the redox q's that I have practised so I just put two n two together, that all.

Hope that helps
Hi, I know this thread is from a while ago but just wondering how you knew the oxidation state changed? I'm not sure if I'm just missing something but I can't see anywhere in the question where is gives a clue to that happening. (I'm pretty stuck on the equation part.)
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