We know that the equation for density is:

where D is the density, m is the mass in grams, and V is the volume.
Given two of the variables, we can then solve for density:

So therefore, we now know that the density of carbon dioxide gas is 0.00196g/mL.
For the answer to the question above, well presumably because the exact concentration of the composition KMnO4 solution doesn't matter. <span>If the concentration of the KMnO4 solution is important (usually in titrations etc.) then it is not allowed to use a wet bottle. The water in the bottle will dilute the KMnO4 solution and change the concentration of the said compound.</span>