Question

10.0 g of alloy containing only Fe, Cr and Ni when heated with dil. HNO3 dissolved to give Fe3+, Cr3+ and Ni2+ respectively. The resulting solution was diluted to a total volume of 250.00 cm3 by adding distilled water. 25.00cm3 of the diluted solution was treated with excess NaOH and H2 O2 . The obtained precipitate P was fltered and yellow colour fltrate Q was acidifed with dil. H2 SO4 acid, and titrated with 1.0mol dm-3 Fe2+ solution. The volume of Fe2+ required to reach the end point was 30.00 cm3 .

(i) Name the species responsible for yellow colour in the above fltrate Q.

(ii) Write balanced ionic equation for the reaction of metal ion in fltrate Q.

(iii) Give the colour of the solution formed by acidifying the fltrate Q and chemical species responsible for this colour.

(iv) Write balanced ionic equations for the reactions between chemical species given in (iii) above with Fe2+ in acidic medium.

(v) Calculate the mass percentage of the metal in the alloy given in part (ii).

(vi) Precipitate P was dissolved completely in H2 SO4 and excess KI was added. The volume of 0.20 mol dm-3 Na2 S2 O3 required to titrate I2 evolved was 20.00 cm3 . Write balanced chemical equations for all reactions that occur in this process.

(vii) Calculate mass percentages of the remaining two metals in the alloy separately.

(viii) How you would confrm the presence of Ni metal in alloy by using a solution / precipitate in above experiment.

Answer 1

Fe³⁺ and Ni²⁺ will not react with NaOH and H₂O₂ while, Chromium would react as follows,

2Cr(OH)₃ + 4NaOH + 3H₂O₂ → 2Na₂CrO₄ + 8H₂O, to give sodium chromate (Na₂CrO₄) which is yellow in colour. So, this is product Q.

(i) the yellow colour species in filtrate Q is sodium chromate.

(ii) 2Cr(OH)₃ + 4NaOH + 3H₂O₂ → 2Na₂CrO₄ + 8H₂O.

(iii) The solution will be yellow in colour due to sodium dichromate (Na₂Cr₂O₇).

2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O
(iv) Cr₂O₇^2- (aq) + 6 Fe²⁺ (aq) + 14H⁺(aq) → 2 Cr³⁺ (aq) +6Fe³⁺ (aq) + 7 H₂O(l)

(viii) The presence of Ni metal can be confirmed by using dimethylglyoxime as a complexing agent.