[jerrinl]

Hello,

I've just finished a lab involving some spectrophotometry, and now I'm working on the calculations in the lab book.

The problem I'm having is those of spectator ions and understanding how they affect concentrations.

We have a solution involving some mL of Fe(NO₃)₃, NaSCN, and HNO₃. However, the lab book is asking for the Initial [SCN^-] and Equilibrium [FeNCS²⁺].

When calculating the initial concentration, how do I go at finding the moles of the various substances given only the substance in mL/its molarity, and do I calculate a mole of NaSCN or the moles of SCN from NaSCN?

And when is the solution in equilibrium? Is it simply after all other the substances have been added?

Sorry if I'm unclear in any of this.

Thank you,
Jerrin

[Borek]

The problem I'm having is those of spectator ions and understanding how they affect concentrations.

You are definitely confused about something, no idea what it is. Spectators are just spectators,they don't affect concentration of other ions.

When calculating the initial concentration, how do I go at finding the moles of the various substances given only the substance in mL/its molarity, and do I calculate a mole of NaSCN or the moles of SCN from NaSCN?

You are most likely expected to do the dilution calculations. In an NaSCN solution analytical (AKA formal, total) concentrations of NaSCN, Na⁺ and SCN^- are all identical. Equilibrium concentrations - especially after taking into account side reactions - can be different.

And when is the solution in equilibrium? Is it simply after all other the substances have been added?

It depends on the reaction, but in most inorganic reactions equilibrium is reached very fast, so the solution is at equilibrium almost instantly. There are exceptions, but FeSCN^2- is not one of them.

[jerrinl]

Thank you for the reply!

So if I'm getting this right, to find the Initial [SCN]  in M, I can simply calculate it off the molarity and how much I used in the solution right?

For example, the molarity of my NaSCN was 2 * 10^-3, and I used 1 mL/.001 L of it in the first solution. That equals 2 *  10^-6 moles, and divided by the amount of solution all together (.05 L), my concentration of SCN is equal to 4 * 10^-5M.

Am I on the right track?

[Borek]

my concentration of SCN is equal to 4 * 10^-5M.

Am I on the right track?

Yes, just remember it is the analytical concentration, not equilibrium concentration. Analytical in this context means sum of concentrations of all forms (so you should include HSCN, FesCN²⁺, 2Fe(SCN)₂⁺ - note the coefficient - and so on).

[jerrinl]

my concentration of SCN is equal to 4 * 10^-5M.

Am I on the right track?

Yes, just remember it is the analytical concentration, not equilibrium concentration. Analytical in this context means sum of concentrations of all forms (so you should include HSCN, FesCN²⁺, 2Fe(SCN)₂⁺ - note the coefficient - and so on).

Eh, I just got an email back from my TA and he's telling me I'm off. He's telling me to refer to a help guide (this link: https://docs.google.com/file/d/1UE_h7wVftA9fdg7E7b7CUX-vlp_mFG7r_3cteIEsW1PaP4jSFt2zwldYkFBv/edit?usp=sharing ) that he sent out, but I don't see the relevancy as it talks about the ideal gas law. EDIT: My apologies. He sent out a different guide: http://no_way/SbYV9B , and I think I see where he's going with the M1V1 = M2V2. This modifies my question: Am I calculating something else entirely when I think of initial concentration?
---

My lab book has this to say about calculating the FeNCS²⁺ concentration: "Use the information in table 1 to calculate the FeNCS²⁺ concentration in each flask assuming that all of the SCN- is present as FeNCS²⁺.

The second solution consists of 10mL of .200 M Fe(NO3)3, 1mL of 2 * 10^-3M NaSCN, and the 39 mL of HNO3 to total a 50 mL solution.

I've exchanged a few emails with him but I never really feel like I've gotten far. Do you have an idea of what he's trying to convey?

Or is there any other information from my lab manual that I can provide to clarify the issue?

Thank you for your patience.

[Borek]

Please describe exactly what you are trying to do. As of now there are bits of information scattered throughout your posts, but not enough to build the whole picture.

Note that if you assume all SCN^- is present in FeSCN²⁺, amount of FeSCN²⁺ is equal to the total amount of SCN^- introduced into the solution. It typically means [SCN^-] is orders of magnitude lower than [FeSCN²⁺].