Question

The decomposition of acetaldehyde has a rate constant of 4.71 × 10⁻⁸ L/mol/s. What is the instantaneous rate of decomposition of acetaldehyde in a solution with a concentration of 5.55 × 10⁻⁴ M?

Answer 1

Here is what I would do.
The reaction here is a second order reaction. I know that because of the units given for k. The equation is (1/A) - (1/Ao) = kt. You know Ao. I would make a table and calculate (A) vs various times. Plot those with A vs time. The instantaneous rate of decomposition is the slope of the tangent drawn to the line at any time, but usually at time = 0. Here is the best description I've seen in a long time. It discusses both rate of decomposition and instantaneous rate. Purdue University does a great job with these informative posts.
https://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/rate.html