How can the number of molecules and atoms be the same in a gas?

A sample of certain mass of gas was collected and it was found that the number of atoms and the number of molecules in the collected sample are equal. You are required to state the conclusion drawn from this observation.

I am not sure of the answer. I think it is related to the quantity of a mole being constant.

3 Answers

The gas was one (or a mixture) of the noble gases, as every other gas is composed of more atoms than molecules.

Example

• Helium: $1~\text{mol}~\ce{He}~\text{gas} = 1~\text{mol}~\ce{He}~\text{atoms}$
• Nitrogen: $1~\text{mol}~\ce{N2}~\text{gas} = 2~\text{mol}~\ce{N}~\text{atoms}$

The conclusion is that the gas is monoatomic - helium, neon, argon, krypton, xenon; or something clever done in a physics lab with low temperature, high vacuum, and lasers. It is a lot of fun to do a little chair parade, learn new things, and present a second answer that is beyond your teacher's understanding. Then, smile. Knowledge wants to propagate.

If in a given volume of gas the number of atoms and molecules is same ,then the only conclusion that can be drawn is that the mixture was of the noble gases.E.g.- He,Ne,Ar,Kr,Xe etc.