Question

Number of molecules in 100 ml of each of o2 nh3 and co2 at stp are

Answer 1

The no. of molecules will be equal for all because according to AVAGADRO's Law " equal volume of all gases have same no. of molecules"

Answer 2

According to Avogadro’s Law, gases with equal volumes at equal temperature and pressure consists of equal number of molecules which is equal to $\bold{6.023 \times 10^{23}}$.

Therefore, number of molecules of $\bold{\mathrm{O}_{2}, \mathrm{NH}_{3} \text { and } \mathrm{CO}_{2}}$ in 100 ml each at STP (same conditions) is the same.  

This number also applies to various other units of a gas such as electrons, protons, neutrons, etc. However, this depends on the substance properties and its reactivity.