[A] após t (ord 1)
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
About this calculator
This calculator determines the concentration of a reactant [A] after time t in a first-order reaction. It uses the formula [A] = [A]₀·e^(−kt), where [A]₀ is the initial concentration, k is the rate constant, and t is time. First-order reactions are common in processes like radioactive decay or chemical decomposition.
The exponential function e^(−kt) represents the continuous decrease in concentration over time. To use it correctly, ensure the units of the rate constant k (e.g., s⁻¹ or min⁻¹) match the time input. Negative values for k or t should be reviewed, as they are not physically valid.
Apply this tool to chemical kinetics calculations, such as predicting drug shelf-life or monitoring reaction rates in labs. Avoid using it for non-first-order reactions, as the formula does not apply. Always cross-check input data with experimental results or scientific literature.
Frequently asked questions
Why does the formula use a negative exponent?
The negative exponent (-kt) represents the continuous decrease in concentration over time in first-order reactions.
How to apply this to reactions with multiple reactants?
This calculator is specific to first-order reactions involving a single reactant. For complex systems, refer to appropriate models.
What if the reaction order is different?
For zero or second-order reactions, use formulas specific to those orders, as the first-order equation does not apply.
Do I need to convert units before using it?
Yes. Ensure k and t are in the same units (e.g., seconds or minutes) to avoid calculation errors.