Question

Please answer and exaplain ALL 4 parts! Thank you!

A 3.00 mL aliquot of 0.001 M NaSCN is diluted to 25.0 mL with 0.2 M Fe(NO3)3 and 0.1 M HNO3.

1. How many moles of SCN- are present?

2. If all of the SCN- is complexed with Fe3+ to form FeNCS2+, what is the molar concentration of FeNCS2+?

3. For preparing a set of standard solutions of FeNCS2+, the equilibrium molar concentration of FeNCS2+ is assumed to equal the initial molar concentration of the SCN- in the reaction mixture. Why is this assumption valid?

4. The blank solution used to calibrate a spectrophotometer is 10.0 mL of 0.2 M Fe(NO3)3 diluted to 25.0 mL with 0.1 M HNO3. Why is this solution preferred to simply using de-ionized water for the calibration?

Answer 1

1. Molarity = number of moles X (1000/volume)

number of moles of SCN^- = 0 .001X3 / 1000 = 3 X10^{- 6} moles    in the 25 ml of solution

2) SCN^- + Fe³⁺ ===> FeNCS²⁺

1 mole SCN^- produces 1 mole of FeNCS²⁺

Therefore moles of FeNCS²⁺ in 25 ml = 3 X10^{- 6}

Molar Concentration of FeNCS²⁺ = (3 X10^{- 6} )X 1000 / 25 = 1.2X 10^-4 Moles / liter

3)One mole of FeNCS²⁺ is equal to exactly one mole of SCN^- .

Since we have standardized solution of NaSCN, molar concentration of SCN^- is taken in to consideration instead of FeNCS^{2+ to avoid} error

4) The reason you add the HNO3 to the Fe(NO3)3 is to make the Fe(NO3)3 absorb (it will be red in color after you add it)

several reasons you could do this. It could be done to subtract background in that absorbance region. The Fe⁺³ & NO^3- ions may absorb slightly in the range that the FeSCN⁺² ions do, whereas, the deionized water wouldn't. So to correct for that absorbance you need to use a blank that contains those ions.