what is meant by an empirical formula?

Answer 1

It's a mathematical formula that is derived from observation and experience, and not by "pure" mathematics.

Example: to send any of the various spacecraft to Mars, one could 'elegantly' try and solve the physics of the gravitational field for a 1000+ body model (sun, planets, moons, asteroids, comets, unknown bodies) to find a trajectory, OR one can use an empirically derived formula based on past experience of sending probes to Mars, to calculate new trajectories (and make small course corrections along the way).

Example: one could solve for the n-point structural bonding of water, cement, sand, and gravel at the molecular level to achieve the "perfect" mix for a required strength of concrete, or one can use an empirical formula: 1 part portland cement, 2 parts sand, 3 parts gravel (or some similar formula) to make concrete.

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Answer 2

It is the formula where all of the subscripts are reduced to their smallest possible values. For example, glucose (a sugar) has a molecular formula of C6H12O6. Its empirical formula would be CH2O.