According to Avogadro's law, why dies the ideal gas constant has the same value for all gases?

assumes that all gases are ideal. Ideal gases have the following properties:

• They experience no intermolecular forces.
• They experience totally elastic collisions.
• They are infinitely small.
• Their kinetic energy is directly proportional to the temperature in Kelvin.

In other words, all ideal gases behave identically under the same conditions. As a result, the ideal gas constant is the same for all gases, as there's no need to alter it.

Of course, in the real world this isn't true. However, it is accurate to about 5%, which is good enough for most purposes.

OK.