Homework Answers
An unknown Fe(II) sample is added to 2.50 mL of a 0.0300 M potassium dichromate solution (in acidic medium) in a 25 mL volumetric flask. 2 M H2SO4 is added to fill the flask to the calibration mark. After the reaction proceeds to completion, the absorbance of the solution at 447 nm is measured to be 0.550 using a 1.00 cm path length. Calculate the following and enter your answers with correct significant figures.
Our reaction :
14 H+ (aq)+ 6Fe2+ (aq)+ Cr2O7 2- (aq) ==> 6Fe3+ (aq)+
2Cr3+ (aq) + 7 H2O (l)
the molar absorptivity of dichromate at 447 nm is 5.10 x 102 M-1 cm-1.
A) Molarity of Cr2O7 in the solution after reaction.
we have, A = e*C*l
C = A / e*l = 0.550 / 5.10 x 102 M-1 cm-1*1 cm = 0.00108 M
B) Moles of Cr2O7 that reacted :
volume of solution : 25 mL
moles added : 0.0300 mol/L * (2.50 ml/1000 ml/L ) = 7.5*10-5 moles
moles remaining : 0.00108 mol/L * (25.0 ml/1000 ml/L ) = 2.7*10-5 moles
thus, moles reacted : 4.80*10-5 moles
C) Moles of Fe (II) reacted :
1 mol of dichromate reacts with : 6 moles of Fe (II)
thus, moles of Fe (II) reacted : 4.80*10-5 moles*6 =
2.88*10-4 moles
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