[LiquidSword]

Prepare a solution as follows:
Pipette 10.00 mL of 0.150 M Fe3+ solution, and 5.00 mL of 5.00×10-4 M SCN− solution in a 25.00 mL volumetric flask.
Dilute to the mark with 0.100 M HNO3 and mix well.

Calculate the concentration of FeSCN2+ in this solution.


I'm not exactly sure how to go about solving this question.
I know M1*V1=M2*V2 but how would I use this equation here?
I can find the moles and divide by the total volume, but then what do I do with the 0.100M HNO3?  I may be over thinking this problem, but it has me very confused.
Help is very much appreciated.  Thanks.

[Arkcon]

I know M1*V1=M2*V2 but how would I use this equation here?
I can find the moles and divide by the total volume, but then what do I do with the 0.100M HNO3?  I may be over thinking this problem, but it has me very confused.

There's a few givens that you may not be aware of.  Let's try to work with what you know, and see if any insights jump out at you.  You have molarities, and dilution volumes, instead of using M1V1=M2V2, actually tru to work out the molarities of each species.  See if you can work out the result that way.

but then what do I do with the 0.100M HNO3?  I may be over thinking this problem, but it has me very confused.

Let's not worry about this for now.  After all, they don't ack what the resulting H+ or NO3 concentrations are, do they?  I mean, they're still there, even if they don't ask, but not really part of the question.  So yeah, maybe you did overthink just a bit.  But I'm of the opinion, that that's never bad. 

[LiquidSword]

Thanks for the help, I got the answer. 

Fe3+ + SCN- --> FeSCN2+

I found the limiting reagent/reactant by calculating the moles of Fe3+ and SCN-

moles Fe3+: 0.150M*0.010L = 0.0015 moles
moles SCN-: 5.00x10^-4M*0.005L = 0.0000025 moles

As we can see from the moles, SCN- is limiting.

[SCN-] = (5.00x10^-4M*5.00mL)/25.00mL
          = 0.0001M

[SCN-] = [FeSCN2+] = 0.0001 M = 1.00x10^-4M

Thanks again Arkcon!

[Borek]

HNO₃ is used just to acidify the solution. Usually HCl would be used, but Cl^- complexes iron and you want to avoid interference.

[Arkcon]

Thanks for the help, I got the answer. 

Fe3+ + SCN- --> FeSCN2+

Very good.  I wanted you to to figure out for yourself, that this was a synthesis reaction, creating the iron thiocyanate complex.  And you did the math that was needed.  I don't know why it calls for nitric acid.  Like Borek: said, we need it to be acid, to avoid side reactions.  It seems that many of these preparations call for nitric -- nitrates are all mostly soluble, so it may avoid some other, competing, precipitation reactions.  Nitric acid is known to be an oxidizer, but when very dilute, we usually discount that, and just treat it as a good acid.