Question

A) Standard solution contains 2.50 mL 0.20 M Fe³⁺ (in 0.50 M HNO₃), 0.60 mL 2.0 x 10^-3 M HSCN (in 0.50 M HNO₃), and 6.90 mL of 0.50 M HNO₃). Before any reaction occurs, what are the concentrations of Fe³⁺ and HSCN. What is the concentration of HNO₃?

B) For the standard solution described in (A) above,

- What is the maximum concentration of complex that can be produced by reaction I?

- What is the maximum concentration of complex that can be produced by reaction II?

Reactions:

Reaction I.    Fe(H₂O)₆³⁺_(aq) + HSCN_(aq) Û Fe(H₂O)₅(SCN)²⁺_(aq) + H₃O⁺_(aq)

Reaction II.   Fe(H₂O)₆³⁺_(aq) + 2 HSCN_(aq) Û Fe(H₂O)₄(SCN)⁺_(aq) + 2H₃O⁺_(aq

Answer 1

Before any reaction, the concentrations of:

Fe³⁺ = 0.20 M

HSCN = 2 * 10^-3 M

HNO₃ = 0.5 M

The maximum concentration of the resulting solution ,M₃ from the mixing of two solutions each of volumes V₁, V₂ and molarities M₁, M₂ is given by-

M₁V₁ + M₂V₂ = M₃ (V₁+V₂)

2.50 mL * 0.20 M + 0.60 mL 2.0 x 10^-3 M =  M₃ *(2.50 ML+ 0.60 ML)

0.5 + 1.2 * 10^-3 = M₃ * 3.10

M₃ = 0.1616 M is the concentration of the solution formed in reaction I.

For reaction II

the molar equivalents of the reactants are given by -

Fe³⁺ = 2.50 mL * 0.20 M = 0.5 moles

HSCN =  0.60 mL 2.0 x 10^-3 M =  1.2 * 10^-3 moles

Given that 1 mole of Fe³⁺ complex reacts with 2 moles of HSCN, here HSCN is the limiting reagent as it is present in lower quantities than the required stoichiometry.

1.2 * 10^-3 moles of HSCN can react with 0.6 * 10^-3 moles of Fe³⁺ to form 0.6 * 10^-3 moles of the complex.The maximum concentrstion of the complex is -

0.6 * 10^-3 moles * 3.10 ml = 1.86 * 10^-3 M