Consider separate samples of water and carbon dioxide, each with the same mass. which contains the greater number of molecules?

The sample that contains the greater number of molecules is water.

Explanation: To calculate the number of molecules, it is used the Avogadro's number ( particles/molecules). So, considering that the mass to water and carbon dioxide is 1g (it can be any other number), the relationship between moles and molar weight is:

To water: = 0.05 moles.

Therefore,

1 mol ---- molecules

0.05 moles ---- x

x = molecules of water.

To carbon dioxide: = 0.02 moles.

Therefore,

1 mol ---- molecules

0.02 moles ---- y

y = molecules of carbon dioxide.

the answers would be

the first one and the second one