Absorbance (A) is a measure of how much light a solution absorbs at a specific wavelength, given by the Beer-Lambert Law A = εbc, where ε is molar absorptivity, b is path length, and c is concentration. On the AP Chem exam, absorbance is the experimental handle for finding an unknown concentration.
Absorbance is a unitless number that tells you how much light a sample soaks up instead of letting through. Shine light of a specific wavelength through a colored solution in a spectrophotometer, and the instrument compares the light going in to the light coming out. More absorbing particles in the beam means more light gets caught and a higher absorbance reading.
The AP exam cares about absorbance because of the Beer-Lambert Law, A = εbc (EK 3.13.A.1). Each variable answers a simple question. Molar absorptivity (ε) asks how strongly this particular species absorbs at this wavelength. Path length (b) asks how far the light travels through the solution. Concentration (c) asks how crowded the absorbing particles are. In a typical experiment, ε and b are held constant, so absorbance becomes directly proportional to concentration (EK 3.13.A.2). That direct proportionality is the whole game. Double the concentration, double the absorbance. It's what lets you turn a number from a machine into the molarity of an unknown.
Absorbance lives in Unit 3 (Properties of Substances and Mixtures), specifically Topic 3.13 (Beer-Lambert Law) and Topic 3.11 (Spectroscopy and the Electromagnetic Spectrum). It directly supports learning objective 3.13.A, which asks you to explain how the amount of light absorbed relates to concentration, path length, and molar absorptivity. It also connects to 3.11.A, since the wavelength you choose matters. Visible and UV light cause electronic transitions, which is why colored ions like Cu²⁺ and MnO₄⁻ absorb in the visible range. Spectrophotometry is one of the official AP Chem labs, so absorbance shows up constantly in experimental-design FRQs. It's the bridge between something you can measure (light) and something you actually want (concentration).
Transmittance (Unit 3)
Transmittance is the fraction of light that makes it through the sample, and absorbance is built from it (A = -log T). They move in opposite directions, so a high-absorbance solution has low transmittance. Exam questions sometimes give you one and ask for the other, like the practice problem that hands you A = 0.45 and asks for percent transmittance.
Calibration curve (Unit 3)
A calibration curve is absorbance put to work. You measure A for several solutions of known concentration, plot absorbance versus concentration, and get a straight line whose slope is εb. Then one absorbance reading of your unknown drops it right onto the line. The 2026 short FRQ Q6 is built entirely on this move with V²⁺ solutions.
Spectroscopy and the EM Spectrum (Unit 3)
Why does a solution absorb at one wavelength and not another? Topic 3.11 answers that. UV/visible photons match the energy gaps of electronic transitions, infrared matches vibrations, microwaves match rotations. You pick the wavelength where your species absorbs strongly (its color complement), which is why purple MnO₄⁻ gets measured with green light.
Photon energy, E = hν (Unit 1/Unit 3)
Absorbance experiments only work because photons carry quantized energy. A species absorbs a photon when E = hν matches the energy of an allowed transition. This ties the lab technique back to the atomic structure ideas from earlier in the course.
Absorbance shows up two ways. Multiple-choice questions test the proportionality directly. You might be asked which combination of concentration and path length gives the highest absorbance (answer: maximize both, since A = εbc), or to scale absorbance with concentration, like predicting A for a 0.025 M solution when a 0.015 M solution reads 0.450. FRQs use absorbance in lab contexts. The 2021 LRFRQ had a student determine CuSO₄ concentration by spectrophotometry, the 2022 short FRQ used colorimetric analysis of purple MnO₄⁻, and the 2026 short FRQ asked about a calibration curve for V²⁺. Be ready to do three things. Solve A = εbc for any variable, read or build a calibration curve, and explain experimental choices (why this wavelength, why a blank, what happens if absorbance is too high to read accurately).
Absorbance measures light captured; transmittance measures light that gets through. They're inverses connected by A = -log T, so an absorbance of 1 means only 10% of the light is transmitted, and an absorbance of 2 means just 1%. Critical exam point: absorbance is linear with concentration (that's why calibration curves use it), but transmittance is not. Plotting %T versus concentration gives a curve, not a line.
Absorbance follows the Beer-Lambert Law, A = εbc, where ε is molar absorptivity, b is path length, and c is concentration.
When wavelength and path length are held constant, absorbance is directly proportional to concentration, so doubling concentration doubles absorbance.
Absorbance and transmittance are inverses related by A = -log T, and only absorbance gives a straight-line calibration curve with concentration.
Spectrophotometers measure absorbance at a wavelength where the species absorbs strongly, which corresponds to electronic transitions caused by UV/visible light.
On FRQs, absorbance data plus a calibration curve is the standard method for finding the molar concentration of an unknown colored solution like CuSO₄ or MnO₄⁻.
Absorbance is a unitless measure of how much light a solution absorbs at a given wavelength. The Beer-Lambert Law, A = εbc, relates it to molar absorptivity, path length, and concentration, which is why one absorbance reading can reveal an unknown concentration.
No. Transmittance is the fraction of light that passes through the sample, while absorbance measures the light that gets absorbed. They're related by A = -log T, so an absorbance of 0.45 corresponds to about 35% transmittance, not 55%.
More dissolved absorbing particles in the light path means more photons get captured. Since ε and b are held constant in a typical experiment, A = εbc reduces to absorbance being directly proportional to concentration (EK 3.13.A.2).
No, absorbance is unitless. The units cancel in A = εbc because molar absorptivity carries units of L·mol⁻¹·cm⁻¹, path length is in cm, and concentration is in mol/L.
Released FRQs from 2021 and 2022 had students find the concentration of CuSO₄ and MnO₄⁻ solutions using spectrophotometry and colorimetric analysis. You typically measure absorbance, use a calibration curve or solve A = εbc, and justify experimental choices like wavelength selection.