Question

for the lab of
SPECTROPHOTOMETRIC DETERMINATION OF IRON USING 1,10 PHENANTHROLINE
%Fe= c unknown(mol/L) X 1L X 80 X Molarmass Fe(g/mol)   X 100%
                                          Mass of Sample (g)

I am confused about how to find the concentration of the unknown

In this method equal volumes of the unknown are pipetted into several volumetric flasks. Then increasing volumes of the standard are added to each flask and each is diluted to the same final volume. Every flask now contains the same concentration of unknown and different concentrations of standard. (Remember that the standard is the same substance as the unknown). For each flask, a measurement of analytical signal is then made. The equation of the line is y= mx + b, where the x-intercept (the concentration of the unknown) is calculated from the expression x=−b/m

** if you need more information let me know

Answer 1

You should have provided the data; would have been easier to show you the calculations. However, I will show you a qualitative treatment.

You are following the method of standard addition because the given sample contains the analyte (your molecule of interest) in such a small concentration that it cannot be detected spectrophotometrically. You add the analyte (your molecule of interest) standard in increasing amounts and then measure the absorbances of the standard samples.

Suppose the unknown analyte concentration in the sample is C mol/L. Suppose you prepared the standard solution having x moles/L of the standard in a volume of V L. You add V₁, V₂, V₃ etc. liter of your known standard to the unknown sample. Then the total concentration of the analyte in your prepared standard samples is

(C + V₁x/V), (C + V₂x/V), (C + V₃x/V) etc. Let us denote the second terms as C₁ mol/L, C₂ mol/L, C₃ mol/L, etc. The total analyte concentrations are then (C + C₁) mol/L, (C + C₂) mol/L, (C + C₃) mol/L etc.

You measure the absorbances of your standard samples and the absorbance values (recorded by a spectrophotometer) are A₁, A₂, A₂ etc. If ε L/mol.cm be the molar absorptivity of the analyte and l cm is the path length of the solution, then,

A₁ = ε.(C + C₁).l

A₂ = ε.(C + C₂).l

A₃ = ε.(C + C₃).l

We do not know C; but we know C₁, C₂, C₃ etc (since we prepared these concentrations in the lab). We already know that C is small, hence we can say that A varies with C₁, C₂, C₃ etc. linearly. We plot a graph between absorbance A (recorded by the instrument) and C₁, C₂, C₃ etc. The graph is linear with the equation y = mx + b where m is the slope and b is the intercept.

The linear equation is used to find out the unknown concentration C. Since C is small, we know that the absorbance due to concentration C will be negligible and we can assume A = 0 when the unknown concentration is C. This must give

0 = m.C + b

====> C = -b/m

====> C = b/m (we neglect the negative sign since concentration cannot be negative).

The explanation is qualitative; actual data would have made understanding the procedure more easy.