make sense. equilibrium constants PS, the calculated equilibrium constants will
ID: 493795 • Letter: M
Question
make sense. equilibrium constants PS, the calculated equilibrium constants will not calculated A. Preparation and Analysis of Calibration Samples In order to relate the absorbance from the iron-thiocyanate complex formed equilibrium mixtures to specific concentrations of iron-thiocyanate, solutions containing known concentrations of both iron and e will have to be examined. To save time in this part, you ill work in Each pair in the laboratory section will prepare two calibration samples from the table below. The TA will assign the samples to prepared by each pair Calibration Volume of Volume of Volume of Total Sample 0.200 M Fe (NO3) 3 4.00 x 10 M KSCN 0.5 M HNO Volume in M HNO, 3 mL 0.5 (mL) in 0.5 M HNO3 (mL mL 5.00 5.00 1.50 10.00 500 2.00 3.00 10.00 2.50 5.00 5.00 3.00 2.00 L1000 5.00 3.50 4.00 5.00 1.00 10.00 9 5.00 4.50 0.50 00 10 5.00 5.00 0.00 1000 The TA will then list on the board, the absorbance values obtained for each calibration sample from the entire class. Be sure to record these values into your notebook. This data will then be used to construct the calibration curves needed. 1. Solutions containing known iron-thiocyanate concentrations are prepared by first pipetting 500Explanation / Answer
The concentration of Fe3+ is 0.200 M and the concentration of SCN- is 4.00*10-4 M KSCN; therefore, the concentration of Fe3+ is essentially 500 times higher than the concentration of KSCN. Thus, KSCN is the limiting reactant and the reaction goes to completion. Thus, SCN- is completely consumed to produce [Fe(SCN)2+]; therefore, [SCN]0 = [FeSCN2+]. Also, note that [Fe(SCN)2+] = 2*[SCN-] as per the equations below.
Calibration Sample #1
Volume of 4.00*10-4 M KSCN in 0.50 M HNO3 (mL)
Total volume (mL)
[SCN]0*10-4 M
Concentration of [FeSCN2+]*10-4 M
Concentration of [Fe(SCN)2+]*10-4 M
1
0.50
10.00
0.20
0.20
0.40
2
1.00
10.00
0.40* (check calculation below)
0.40* (check calculation below)
0.80* (check calculation below)
3
1.50
10.00
0.60
0.60
1.20
4
2.00
10.00
0.80
0.80
1.60
5
2.50
10.00
1.00
1.00
2.00
6
3.00
10.00
1.20
1.20
2.40
7
3.50
10.00
1.40
1.40
2.80
8
4.00
10.00
1.60
1.60
3.20
9
4.50
10.00
1.80
1.80
3.60
10
5.00
10.00
2.00
2.00
4.00
a) Find the initial concentration of SCN- after the solutions are mixed, [SCN-]0. Use the dilution law:
M1*V1 = M2*V2 where M1 = 4.00*10-4 M; V1 = 1.00 mL and V2 = 10.00 mL; plug in:
(1.00 mL)*(4.00*10-4 M) = [SCN-]0*(10.00 mL)
===> [SCN-]0 = (1.00*4.00*10-4)/(10.00) M = 4.0*10-5 M = 0.40*10-4 M
Write down the equation for the reaction between Fe3+ and SCN-
Fe3+ (aq) + SCN- (aq) ------> [Fe(SCN)2+]
As per the balanced stoichiometric reaction,
1 mole SCN- = 1 mole [Fe(SCN)2+] and hence [SCN-]0 = [FeSCN2+]; therefore, [FeSCN2+] = 0.40*10-4 M (ans)
b) Write down the balanced equation for the formation of the di-thiocyano complex as below:
Fe3+ (aq) + 2 SCN- (aq) -------> [Fe(SCN)2+]
As per the balanced stoichiometric reaction,
2 moles SCN- = 1 mole [Fe(SCN)2+] and hence [Fe(SCN)2+] = 2*[SCN-]0; therefore, [Fe(SCN)2+] = 0.80*10-4 M (ans)
Calibration Sample #1
Volume of 4.00*10-4 M KSCN in 0.50 M HNO3 (mL)
Total volume (mL)
[SCN]0*10-4 M
Concentration of [FeSCN2+]*10-4 M
Concentration of [Fe(SCN)2+]*10-4 M
1
0.50
10.00
0.20
0.20
0.40
2
1.00
10.00
0.40* (check calculation below)
0.40* (check calculation below)
0.80* (check calculation below)
3
1.50
10.00
0.60
0.60
1.20
4
2.00
10.00
0.80
0.80
1.60
5
2.50
10.00
1.00
1.00
2.00
6
3.00
10.00
1.20
1.20
2.40
7
3.50
10.00
1.40
1.40
2.80
8
4.00
10.00
1.60
1.60
3.20
9
4.50
10.00
1.80
1.80
3.60
10
5.00
10.00
2.00
2.00
4.00
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