Question

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Fe+3 + SCN-
---->
<----
FeSCN+2
You mix the following:
Volume of KSCN solution 4.27 mL
Original concentration of KSCN solution 0.00495 M
Volume of ferric ion solution 4.74 mL
Original concentration of ferric ion solution 0.02488 M
Voluume of distilled water 5.25 mL
You measure the %T of the blank and sample at 447 nm using the same cuvet. Only FeSCN2+ absorbs at 446 nm! KSCN and Fe+3 are transparent.
%T of blank 100.6
%T of sample 42.7
constant [Molarity FeSCN2+ per absorbance] 0.001264 M/A
Note: This constant is only for this pre-lab
Calculate:
a) Millimoles of SCN- originally put in solution _______________ mmoles
b) Millimoles of Fe+3 originally put in solution _______________ mmoles
c) Total volume of solution _______________ mL
d) Molarity of FeSCN2+ at equilibrium _______________ M
e) Millimoles of FeSCN2+ at equilibrium _______________ mmoles
f) Molarity of SCN- at equilibrium _______________ M
g) Molarity of Fe+3 at equilibrium _______________ M
h) Calculated equilibrium constant _______________

Answer 1

KSCN 4.27 ml at 0.00495M

a) Moles of KSCN= 0.00495*4.27/1000 = 2.11X10-5Moles= 2.11X10^-2millmoles

b) Moles of Ferric iron =0.02488*4.74/1000= 0.000117931 Moles = 0.1179 mill moles

c) Volume of water added = 5.25 ml .Total volume of solution = 4.27+4.74+5.25=14.26 ml

Moles of KSCN befores reaction (M1) can be calculated from 0.00495 * 4.27 = 14.26*M1

M1= 0.00495*4.27/ 14.26 =0.001482M

M2= Molarity of Ferric iron before reaction = 0.02488*4.74/14.26=0.00827M

After reaction

From Absobrance= 2-log%(T)= Absorbance = 2-log(42.7)=0.3696

d) Molarity of FeSCN+2= 0.3696*0.001264= 0.000467M ( This is the molarity at equilibrium)

e) Moles of FeSCN+2 = 0.000467*14.26/1000=6.089X10-6moles = 0.006089 mill moles

g) the reaction is Fe+3+ SCN- ----> FeCN+2

Inital mill moles Final miil moles ( KSCN is the limting reactant)

KSCN 2.11X10-2 0.006089   

Fe+3 0.1179 0.1179- 0.006089 =0.11181

FeSCN+2 formed = 0.006089 mill moles

Equilibrium constant = Molarity of FeSCN+2/ ( Molarity of Fe+3)* Molairty of KSCN

=(0.006089/14.26)/ {( 0.11181/14.26)* (0.006089/14.26)} =127