The standard solution of FeSCN2+ (prepared by combining 9.00 mL of 0.200 M Fe(NO3)3 w/1.00 mL of 0.0020 M KSCN) has an absorbance of 0.510. If a trial's absorbance is measured to be 0.250 and its initial concentration of SCN– was 0.00050 M, the equilibrium concentration of SCN– will be
what formulas are there that relate absorbance and concentration ?
Use Beer's Law, A=abc for the standard to determine the constant, a. Then use A=abc on the unknown to determine c and use that and ICE to arrive at the amount of SCN^- at equilibrium.
There are two common formulas that relate absorbance and concentration: Beer-Lambert Law and the molar absorptivity formula.
1. Beer-Lambert Law:
According to Beer-Lambert Law, the absorbance (A) of a solution is directly proportional to the concentration (C) of the absorbing species and the path length (b) of the light through the solution. Mathematically, it can be expressed as:
A = ɛbc
Where:
A = Absorbance
ɛ = Molar absorptivity (also known as the molar absorptivity coefficient or extinction coefficient)
b = Path length (typically measured in centimeters)
C = Concentration (typically measured in molarity, M)
2. Molar Absorptivity Formula:
The molar absorptivity (ɛ) is a constant that represents the intrinsic ability of a substance to absorb light. It is dependent on the specific compound, the wavelength of light used, and the solvent. The molar absorptivity can be determined using the following formula:
ɛ = A / (bc)
Where:
ɛ = Molar absorptivity (typically measured in L·mol^(-1)·cm^(-1))
A = Absorbance
b = Path length (typically measured in centimeters)
C = Concentration (typically measured in molarity, M)
Note: The molar absorptivity (ɛ) is specific to a particular compound at a given wavelength and is commonly provided in references or can be determined experimentally.
By using either of these formulas, you can relate the absorbance and concentration of a solution.