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Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: Science geek on July 16, 2015, 05:41:19 PM

Title: A complexed problem (solutions)
Post by: Science geek on July 16, 2015, 05:41:19 PM

Could you please help me solve this problem
In  a 20 ml of solution zinc chloride concentration of 0,02 mol/dm3 a 15 ml solution of nickel(II) chloride of unknown concentration is added and the solution is diluted in a normal dish of 100 ml volume.For analysis 25 ml of that solution is taken.Then in that 25 ml solution 30 ml of EDTA solution of concetration of 0,0225 mol/dm3 is added. The formed solution is then titrated with standardized solution of magnesium sulfate of concentracion of 0,0505 mol/dm3 with the indicator eriochrome black T which requires a consumption of 5,10 ml of solution magnesium sulfate. Calculate the concentration of nickel (II) chloride.
Ka(ZnY2-)=3*10(16) alfay=0.052( pH=9) alfam=8*10(-6) (pH=9)

Title: Re: A complexed problem (solutions)
Post by: Science geek on July 18, 2015, 02:53:38 AM

It is similar to icho preparatory problem number 27

Title: Re: A complexed problem (solutions)
Post by: Borek on July 18, 2015, 06:51:30 AM

Wouldn't hurt to start writing reaction equations.

Title: Re: A complexed problem (solutions)
Post by: Science geek on July 18, 2015, 07:41:31 AM

Theseare reaction I know
MgSO4 + EDTA----> Mg(EDTA) + SO4
Zn2+ + Y4- ---------> ZNY2-
I am trying to solve this problem for several days but i can't
Here is a similar problem on icho
http://icho2015.msu.az/uploads/files/Preparatory_problems_IChO_2015_July_8.pdf       (here is the 27 problem )
http://icho2015.msu.az/uploads/files/Solutions_Preparatory_problems_IChO_2015_June_15.pdf (and here is the solution)
Can you help me solve this particular problem above

Title: Re: A complexed problem (solutions)
Post by: Borek on July 18, 2015, 08:47:39 AM

This is probably because you are making the problem harder than it really is. It is a simple back titration.

Do you know how the EDTA reacts with metals in general?

Title: Re: A complexed problem (solutions)
Post by: Science geek on July 18, 2015, 09:00:23 AM

EDTA4-+Mg2+--->[Mg-EDTA]2-

ErioT +Mg2+---->ErioT-Mg
Yes i know,but again i do not know what to do with all these informations, i Tried like this
CNi+= [VEDTA*CEDTA-VMgSO4*CMgSO4]/Valfa

alfaY=[ZnY2]/[EDTA]

Please can you explain this problem or do it step by step. I would be so grateful,please.

Title: Re: A complexed problem (solutions)
Post by: Borek on July 18, 2015, 12:23:02 PM

Looks to me like this information:

Quote from: Science geek on July 16, 2015, 05:41:19 PM

Ka(ZnY2-)=3*10(16) alfay=0.052( pH=9) alfam=8*10(-6) (pH=9)

is there just to confuse you. Ignore it.

Think in terms of a simple stoichiometry. Eriochrome black is just an indicator, its presence (nor the reaction with Mg²⁺) is not important for the titration result.

Title: Re: A complexed problem (solutions)
Post by: Science geek on July 18, 2015, 02:42:13 PM

I Must use all the informations that are given in the problem

Title: Re: A complexed problem (solutions)
Post by: Borek on July 18, 2015, 04:48:45 PM

Good luck then.

Title: Re: A complexed problem (solutions)
Post by: Science geek on July 19, 2015, 02:44:57 AM

Please somebody help me?

Title: Re: A complexed problem (solutions)
Post by: Borek on July 19, 2015, 03:24:10 AM

You have refused to even try to solve the problem following hints given. Nobody is going to solve the problem for you, especially when you are not showing any efforts.

Title: Re: A complexed problem (solutions)
Post by: Science geek on July 23, 2015, 08:32:11 AM

Can somebody just direct me what should I do?

Title: Re: A complexed problem (solutions)
Post by: Science geek on July 23, 2015, 09:05:14 AM

Should I ivolve relative activity constant KΘ