A gas tank holds a homogeneous mixture of 0.1 mol neon, 0.2 mol helium, O.2 mol oxygen and 0.8 mol argon. Which of these gases is the solvent in this mixture?
As you know, a solution is made up of a solvent and one or more solutes.
You can distinguish between a solvent and a solute by looking at how much of each constituent substance you get in the mixture. The substance that's present in the larger amount will be your solvent.
Consequently, the substances that are present in smaller amounts will be your solutes.
In this case, you know that a homogeneous mixture, which is usually another term used for a solution, contains
#0.1#moles of neon, #"Ne"#
#0.2#moles of helium, #"He"#
#0.2#moles of oxygen gas, #"O"_2#
#0.8#moles of argon, #"Ar"#
As you can see, argon is present in the larger amount, since you have more moles of argon than of the other gases, so this gas will be your solvent.
This means that your solution has neon, helium, and oxygen gas as solutes, and argon as solvent.