Final answer:
In UV-Vis spectroscopy experiments, absorbance is often considered more important than transmittance, especially for quantifying the concentration of substances. Absorbance directly correlates with substance concentration, utilizing the Beer-Lambert law, while transmittance inversely relates to absorbance.
Step-by-step explanation:
When conducting an experiment involving UV-Vis spectroscopy, both absorbance and transmittance can provide vital information about a sample. However, in the case of measuring the concentration of a substance in a sample, absorbance is more commonly used and often considered more important. Absorbance is measured as a unitless number, and it is directly proportional to the concentration of the absorbing species in the sample when the path length is constant (typically 1 cm in cuvettes). By measuring the absorbance at the wavelength of maximum absorbance (αμaμmαξ), one can determine the concentration using the Beer-Lambert law, with the molar absorptivity (ε) known for the compound of interest. Transmittance provides the percentage of light that is not absorbed and passes through the sample, which inversely relates to absorbance.
An example from the provided text is when NAD* has a max absorption at 260 nm, and the observed absorbance (A) is 1.0. Knowing the molar absorptivity for NAD* at 260 nm is 18,000 L*mol⁻¹*cm⁻¹, one can calculate the concentration to be 5.6 x 10⁻⁵ M. Thus, by measuring the absorbance, we gather more precise information for quantifying the concentration of a substance in a sample, making it generally more significant than transmittance in such experimental setups.