What is unchanged when a solution is diluted by the addition of solvent?

The moles of solute

Explanation:

When you make a dilution, moles of solute does not change when you add solvent.

In concentrated solution, you have x moles of solute, that will be the same in the diluted solution.

Relation between molarity or volume of concentrated and diluted solutions is called, "dilution factor".

When you calculate the dilution factor: you use this formula

Molarity concentrated. Concentrated volume = M diluted. Diluted volume

M concentrated / M Diluted = Diluted volume / Concentrated volume