In Part 1 of the lab you will make the following four solutions: Given the data
ID: 1053503 • Letter: I
Question
In Part 1 of the lab you will make the following four solutions:
Given the data in the table above, determine the initial concentrations of [Fe3+] and [SCN-]. Since one of the reactants is in excess, you can determine [FeSCN2+]eq as well. You will also want to write these in your laboratory notebook as you will need them to determine the value of the molar absorptivity constant for FeSCN2+. Enter the concentrations in the table below.
Solution Number 0.500 M Fe3+ 3.00 x 10-4 M SCN- water 1 5.00 mL 5.00 mL 0 mL 2 5.00 mL 4.00 mL 1.00 mL 3 5.00 mL 3.00 mL 2.00 mL 4 5.00 mL 2.00 mL 3.00 mLExplanation / Answer
Solution Number
0.500 M Fe3+ (mL)
3.00*10-4 M SCN- (mL)
Water (mL)
Total volume (mL)
[Fe3+] initial (M) (see sample calculation below)
[SCN-] initial (M) (see sample calculation below)
[FeSCN2+] equilibrium (M) (see sample calculation below)
1
5.00
5.00
0.00
10.00
0.25
1.5*10-4
1.5*10-4
2
5.00
4.00
1.00
10.00
0.25
1.2*10-4
1.2*10-4
3
5.00
3.00
2.00
10.00
0.25
9.0*10-5
9.0*10-5
4
5.00
2.00
3.00
10.00
0.25
6.0*10-5
6.0*10-5
Sample calculations:
[Fe3+] initial (M):
Use the dilution equation and write
(5.00 mL)*(0.500 M) = (10.00 mL)*[Fe3+]initial
===> [Fe3+]initial = 5.00*0.500/10.00 M = 0.25 M (ans)
[SCN-] initial (M):
Use the dilution equation to obtain
(5.00 mL)*(3.0*10-4 M) = (10.00 mL)*[SCN-]initial
===> [SCN-]initial = 5.00*3.0*10-4/10.00 M = 1.5*10-4 M (ans)
[FeSCN2+] equilibrium (M):
Write down the balanced equation for the reaction as
Fe3+ + SCN- <=====> FeSCN2+
Note that [Fe3+]initial >> [SCN-]initial. Therefore, SCN- is the limiting reactant and the reaction essentially goes to completion.1
Therefore, SCN- reacts completely and [SCN-]initial = [FeSCN2+]equilibrium (ans)
1 This is an approximate calculation. For finding out the true equilibrium concentration of [FeSCN2+], we need to know the equilibrium constant, K for the equilibrium reaction written above. Since, K is not provided, we will work with the approximation that the reaction goes to completion.
Solution Number
0.500 M Fe3+ (mL)
3.00*10-4 M SCN- (mL)
Water (mL)
Total volume (mL)
[Fe3+] initial (M) (see sample calculation below)
[SCN-] initial (M) (see sample calculation below)
[FeSCN2+] equilibrium (M) (see sample calculation below)
1
5.00
5.00
0.00
10.00
0.25
1.5*10-4
1.5*10-4
2
5.00
4.00
1.00
10.00
0.25
1.2*10-4
1.2*10-4
3
5.00
3.00
2.00
10.00
0.25
9.0*10-5
9.0*10-5
4
5.00
2.00
3.00
10.00
0.25
6.0*10-5
6.0*10-5
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