IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => IGCSE/ GCSE => Sciences => Topic started by: june on April 24, 2009, 05:43:07 pm

Title: help with preperation of salts question
Post by: june on April 24, 2009, 05:43:07 pm
I have no idea how these type of questions that come up in paper 6 are to be answered! could anyone please explain? I have the answer but i dont know why it is that answer..
November 2008 paper 6..
Page11 Q7..
A solution of magnesium sulphate can be made by reacting magnesium oxide with warm sulphuric acid.
a) Describe how you could make a solution of magnesium sulphate starting with magnesium oxide powder and dilute sulphuric acid.
b) Describe how you would obtain pure dry crystals of hydrated magnesium sulphate, MgSO4.7H2O from the solution of magnesium sulphate in (a).
Thank you!
Title: Re: help with preperation of salts question
Post by: sweetsh on April 24, 2009, 06:15:43 pm
Okay..What I took with my GREAT teacher  8) God bless him for me  :P
The following.
Magnesium sulphate is a soluble salt so I think we should use Mg(OH)2 (Magnesium hydroxide) not MgO ...
1)So we make titration to Mg(OH)2 with H2SO4 using an indicator..
H2SO4+Mg(OH)2 --> MgSo4 +H2O
2)repeat titration without indicator
3)heat till saturation
4)make crystillization (cooling,filtering,wash crystals with small amount of water)
Title: Re: help with preperation of salts question
Post by: SGVaibhav on April 24, 2009, 07:01:56 pm
i never understood why wash crystals. (i neveer did that too while making crystals)
Title: Re: help with preperation of salts question
Post by: emi on April 24, 2009, 08:13:02 pm
Quote
1)So we make titration to Mg(OH)2 with HCl using an indicator..
HCl+Mg(OH)2 --> MgSo4 +H2O

you mean with H2SO4 not HCL ??
Title: Re: help with preperation of salts question
Post by: sweetsh on April 24, 2009, 08:24:53 pm
Sorry H2SO4
I was confused I was fighting with someone while writing this sorry guys  :)
Title: Re: help with preperation of salts question
Post by: june on April 24, 2009, 09:33:47 pm
This is the answer to part B right?
Im sorry im kinda thick, im not sure i get it.. im really bad at this salt making sh*t..
Why is it that you use magnesium hydroxide?
Title: Re: help with preperation of salts question
Post by: june on April 24, 2009, 10:04:17 pm
By the way i just checked the mark scheme.. thats not the answer .. it should be as follows:
1) heat
2) cool to form crystals
3)filter off crystals
4) method of drying crystals eg. pressed filter papers
Im confused now ;s
Title: Re: help with preperation of salts question
Post by: sweetsh on April 25, 2009, 06:59:31 am
What you are talking about is the method CRYSTILLIZATION...
I answered the question the way I will answer as I studied
Title: Re: help with preperation of salts question
Post by: hyebyekadal on April 26, 2009, 08:26:43 am
guys its really simple..
(I just did that question and got it rite!)
a. ok MgO is insoluble rite? (all oxides are insoluble except Na,K and NH4 salts..)
so 1. add excess of MgO to the H2SO4
    2.then wait till reaction completes(I.e. no more bubbling)
    3. then filter off the excess Mgo and u get ur solution.
B.1.heat the solution till crystallization point.
   2.filter out the extra water.
   3. dry the crystals by pressing with tissue paper or towel it off.