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A student mixes 5.00 mL of 0.0020 M Fe(NO3)3 with 3.00 mL of 0.0020 M KSCN solution and 2.00 mL water at a certain temperature. He places a sample of the resulting solution in a spectrometer, and measures the transmittance as 23% at 447 nm. What are the initial concentrations of [Fe3 ] and [SCN-]

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Answer:

[Fe⁺³] = 0.0010 M

[SCN⁻] = 0.0006 M

Explanation:

First we calculate how many moles of Fe⁺³ and SCN⁻ were added, using the given volumes and concentrations:

  • Fe⁺³ ⇒ 5.00 mL * 0.0020 M = 0.010 mmol Fe⁺³
  • SCN⁻ ⇒ 3.00 mL * 0.0020 M = 0.006 mmol SCN⁻

Now we can calculate the initial concentrations, after calculating the new volume of the solution:

  • New volume = 5.00 mL + 3.00 mL + 2.00 mL = 10.00 mL
  • [Fe⁺³] = 0.010 mmol / 10.00 mL = 0.0010 M
  • [SCN⁻] = 0.006 mmol / 10.00 mL = 0.0006 M

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