Find the weight percent of iron (55.85 g/mol) in a rock. Suppose that a 0.1773 g rock is crushed and dissolved in nitric acid. The solution is diluted to 100.00 mL. Then 15.00 mL of this solution is mixed with 10 mL 0.200 M chromophore. The resulting Fe complex has an absorption of 0.431 at 508 nm. A second solution is prepared exactly the same as above but spiked with 1 mL of 750 ppm Fe solution, which then gives an absorption of 0.808.
The instrumental analysis of the rock is carried out following the standard addition method. In this technique, the sample without standard solution has an absorbance given by the equation:
In this form of the Beer's Law, we have:
ε : molar absorptivity of the substance
b: optical path
Vx: volume of the sample
Cx: concentration of the substance in the sample
Vt: volume of the sample after dilution.
With the first addition of standard solution, the absorbance of the sample will be given by the equation:
In this equation
Vs: volume of standard added
Cs: concentration of the standard.
If we divide equation (2) by equation (1), we have:
Solving for the concentration of the substance in the sample (Cx):
We have the data:
Absorbance without standard: A1 = 0.431
Absorbance with standard: A2 = 0.808
Volume of the sample: Vx = 25.00mL
Volume of the standard: Vs = 1.0mL
Concentration of standard: Cs = 750 ppm Fe+3
Substituting all of these values in equation (3), the concentration of Fe+3 in the measured sample is:
But this sample was prepared by mixing 15.00 mL of the diluted solution of rock with 10 mL of the chromophore. To calculate the concentration of Fe+3 in the 15.00mL we will use:
Where C is the concentration and V is the volume, the subindex -c is referred to the concentrated solution (15.00 mL) and the subindex -D is referred to the diluted solution (25.0 mL). Solving for the concentration in concentrated solution (Cc):
This is the same concentration for the 100 mL stock solution prepared at dissolving the rock in nitric acid. Remember that ppm units can be written as:
Converting 100 mL to liters:
The mass of Fe+3 in that volume is given by:
Converting the mass into gram units:
The weight percent of iron in the rock is given by:
Subtituting known data:
Find the weight percent of iron (55.85 g/mol) in a rock. Suppose that a 0.1773 g...
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