Titration Question

Question

Titration question

 An 0.0151 L acidic solution of potassium dichromate is titrated with a solution of 0.125 mol/L iron (II) chloride. The concentration of potassium dichromate according to the experimental evidence shown for the amount of iron (II) chloride used is a.bcde mol/L

1.  Input your value for a.bcde in the blank
2. Include the decimal 
3. Do not include units

I	II	III
Final Buret Reading (mL)	12.6	26.8	38.7
Initial Buret Reading (mL)	0.400	12.7	26.5
Color at Endpoint	Light Pink	Bright Pink	Light Pink

Answer 1

The very first thing you MUST do is to write the correctly balanced equation for the reaction taking place. If you don't know how to do this, you need to learn about balancing redox reactions. In this case we have...

Cr₂O₇^2- + 6 Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6 Fe³⁺ + 7H₂O

NOTE: it takes 6 mols Fe²⁺ for each 1 mol of Cr₂O₇^2- 

Take the average volume of the 3 experiments:

I: 12.6 ml - 0.400 ml = 12.2 mls

II: 26.8 ml - 12.7 ml = 14.1 mls

III: 38.7 ml - 26.5 ml = 12.2 mls

Average volume FeCl₂ = 12.8 mls

mols Fe²⁺ used for equivalence = 12.8 mls x 1 L / 1000 ml x 0.125 mol/L = 0.0016 mols Fe²⁺

mols Cr₂O₇^2- present = 0.0016 mols Fe²⁺ x 1 mol Cr₂O₇^2- / 6 mol Fe²⁺ = 2.67x10^-4 mols Cr₂O₇^2-  present

Concentration of potassium dichromate = 2.67x10^-4 mols Cr₂O₇^2- / 0.0151 L = 0.0177 M (3 sig.figs.)