Question

A different brand of iron tablet was analysed by Hitration with 0.0093 mol.L" potassium

dichromate via the reaction listed below;
Cr2O72- + 6Fe2+ + 14H+ + 2Cr3+ + 6Fe3+ + 7H20
In this experiment 4 tablets were crushed and dissolved in 200 cm3 of dilute acid. 25cm3
of this solution was used in the titration. The results of the titrations were as follows;
Titration Volume dichromate (cm)
1
33.0
2
32.05
3
32.15
4
32.10
Calculate the concentration of iron used in the titration (4 dp).
C1V1/C2V2 = n/n2

Answer 1

The concentration of iron used in the titration : 0.009 M

Further explanation

Given

Reaction

Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ ⇒ 2Cr³⁺ + 6Fe³⁺ + 7H₂O

0.0093 mol/L potassium dichromate

200 cm³ of dilute acid, 25cm³ was used in the titration.

Required

the concentration of iron

Solution

Titration formula

C₁V₁n₁=C₂V₂n₂⇒ From equation : n₁=6n₂(1=Cr₂O₇, 2=Fe)

titration average : 33+32.05+32.15+32.1 / 4 = 32.325 cm³(ml)

25 cm³ of iron solution used in titration :

`\tt V_1=32.325~ml\\\\V_2=25~ml\\\\C_1=0.0093~M\\\\\\\\C_1.V_1.n_2=C_2.V_2.n_1\\\\0.0093* 32.325* 6=C_2* 25* 1\\\\C_2=0.07215~M`

Dilution(25 ml from 200 ml iron solution)

`\tt C_2.V_2=C_1.V_1\\\\0.007215* 25=C_1* 200\\\\C_1=0.009~M`