Question

Doing a Chem Review, these are just study scenarios that I need to have a good understanding of

6. To determine the concentration of SCN in our saliva, we reacted the SCN in the saliva with Fe' according to the following reaction: We measured the absorbance of the FeSCN and used the absorbance value of the FesCN2 to obtain the concentration of the FesCN2*. This was achieved by measuring the absorbance values of standard FeSCN2+ solutions and constructing a calibration curve. We then related the FeSCN concentration to the original SCN concentration in our saliva. Answer the following questions from this experiment (a) Define Kf for the chemical reaction in the form of an equation. Why is it important that it is large for this experiment. (5 POINTS) (b) Calculate Kf from the data below knowing that eb is 6120 L/mol for FeSCN2*. Show your work. (10 POINTS) Absorbance FeSCN 0.579 mL of Fe" solution mL SCN 2.50 x 10 20.00 2.50 x 10% 5.00 (Hint: Calculate the initial concentrations of Fe and SCN first and then determine the equilibrium concentrations of Fe3*, SCN, and FeSCN2+. Calculate K, from the equilibrium concentrations) (c) Suppose a student took 400 μL of saliva and added 10.0 mL of Fe" solution in excess. If the absorbance of the sample was 0.935 and the calibration curve from my standards (in mM concentration) was y 6.204x +0.00123, calculate the approximate concentration of SCN (in mM) in the saliva. Do you think this person is a smoker? Show all calcuations. (5 Points)

Answer 1

(a) The reaction is: Fe³⁺(aq) + SCN^- (aq) <---> Fe(SCN)²⁺(aq)

K_f = [Fe(SCN)²⁺]/[ Fe³⁺][ SCN^-]

To determine the concentration by measuring the absorbance it is necessary that all the SCN^- ions in the solution reacts with Fe³⁺ to give Fe(SCN)²⁺. Large K_f value indicates that [Fe(SCN)²⁺] is large in the solution and practically all of the SCN^- reacts in presence of excess Fe³⁺.

(b) To calculate the value of K_f we need to determine the equilibrium concentrations of all the three species.

We can calculate the equilibrium concentration of Fe(SCN)²⁺ from the value of the absorbance from the equation,

Absorbance, A = ?cl          (with the knowledge of A and ? and l for a particular cell we can calculate c)

To determine the equilibruim concentration of others:

                              Fe³⁺(aq)                +             SCN^- (aq) <--->    Fe(SCN)²⁺(aq)

Initial:                   C⁰_Fe3+                                   C⁰_SCN-                    0

At eqm:                C⁰_Fe3+ - x                              C⁰_SCN- - x               x

where, C⁰_SCN- = initial concentration of SCN- = 5*0.0025/25 = 0.0001 M

C⁰_Fe3+ = initial concentration of Fe³⁺ = 20*0.0025/25 = 0.002 M

x = equilibrium concentration of Fe(SCN)²⁺ = A/?l

K_f = [Fe(SCN)²⁺]/[ Fe³⁺][ SCN^-] = x/[ C⁰_Fe3+ - x][ C⁰_SCN- - x]

(c) You can calculate the concentration of Fe(SCN)²⁺ from the calibration curve of absorbance vs concentration. A calibration curve looks like:

Beer's law relationship between absorption and concentration: 0.045 0.04 0.035 0.03 E 0.025 2 0.02 0.015 0.01 0.005 0 4 10 concentration of Fe(SCN)2+ (M)

Absorbance of the unknown sample is 0.935, locate the point in your calibration curve and get the corresponding concentration value. This is the concentration of Fe(SCN)²⁺ in the unknown sample.

If this value exceeds the normal range of Fe(SCN)²⁺ in saliva then the person is probably a smoker.