ored complex when a solution of cobalt(II) ions is mixed with a solution of ammo
ID: 578701 • Letter: O
Question
ored complex when a solution of cobalt(II) ions is mixed with a solution of ammonium thiocyanate. The color of the tetraisothiocyanatocobaltaterII) ion solution is a dee curve for his unknown analysis, the student prepared a series of six standard solutions by adding a quantity of 0.0500 M Co(NO3)2 to a 50 ml. p blue. A student determined 605 nm to be its analytical wavelength. To obtain a standard ric flask that contained 10 mL of 1.0 M ammonium thiocyanate and 10 mL of DI water. Distilled water was added to make the total h solution 50.00 mL. The percent transmittance of each solution was read, using a blank that contained all of the same components as the sample solutions except for the cobalt(II) solution. Data for the determination are: Stock Solution (m) Concentration [Co(SCN)4]2 (M) Transmittance (96T) Absorbance 1.00 80.35 64.71 33.65 7.00 21.78 9.00 14.09 10.00 11.32 NG 1 NG 1Explanation / Answer
We start with 0.0500 M Co(NO3)2 and we dilute to a final volume of 50.0 mL. The concentration of ammonium thiocyante is 1.0 M and hence, ammonium thiocyante is much more concentrated than Co(NO3)2. Consequently, Co(NO3)2 is the limiting reactant and hence, the concentration of [Co(SCN)4]2- is given as
[Co(SCN)42-] = [Co(NO3)2]dil = (volume of stock Co(NO3)2 in mL)*(concentration of stock Co(NO3)2)/(50.0 mL)
Take solution #1 as an example; we have, volume of stock Co(NO3)2 = 1.00 mL; therefore,
[Co(SCN)42-] = (1.00 mL)*(0.0500 M)/(50.0 mL) = 0.0010 M.
The absorbance (A) and transmittance (%T) of the solutions are related as
A = 2 – log (%T)
Take solution #1 as an example; we have %T = 80.35 and we have
A = 2 –log (%T) = 2 – log (80.35) = 0.095 (ans).
Prepare the table as below.
Stock Solution (mL)
Concentration [Co(SCN)4]2- (M)
Transmittance (%T)
Absorbance
1.00
0.001
80.35
0.095
2.00
0.002
64.71
0.189
5.00
0.005
33.65
0.473
7.00
0.007
21.78
0.662
9.00
0.009
14.09
0.851
10.00
0.010
13.32
0.875
Stock Solution (mL)
Concentration [Co(SCN)4]2- (M)
Transmittance (%T)
Absorbance
1.00
0.001
80.35
0.095
2.00
0.002
64.71
0.189
5.00
0.005
33.65
0.473
7.00
0.007
21.78
0.662
9.00
0.009
14.09
0.851
10.00
0.010
13.32
0.875
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