I have to calculate the concentration of sucrose in water just by knowing their masses (and their temperature). Now, if the volume doesn't change when the sugar is being dissolved, I can do that quite simply over density. But I am not sure if it changes or not. Does it? And is that generally true (e.g. for electrolytes)?
This can be seen from a very simple experiment. Take water in a measuring cylinder and note the volume. Add 5 grams of NaCl and dissolve. Note the new volume.
You will observe that the final volume is greater than the initial volume. Hence the NaCl HAS added to the volume of the solution.
That being said, we generally make the assumption that the mass of the solute does not significantly alter the volume of the solution which is a reasonable approximation in the case of dilute solutions.
Your treatment will depend on how rigorous you want to be.
The calculation of concentration of sucrose in water is a comparison of masses and does not involve volume (or density) unless the volume of the final solution is specified, as in molarity or normality, both of which involve moles or equivalents of solute in a given volume of solution (1 L).
Combination of units like 20% w/v usually means a weight in grams over a volume in mL. This is done for convenience and does not usually give solutions which are used for further reactions or calculations. Such solutions would be used for washing, rinsing, etc. Sucrose solutions might be used for hummingbird feeding.
As you've described the situation, the concentration may be expressed as mmolality (or molality), (i.e. moles/kg) without making any assumptions likely to introduce errors.
Unless you are able to measure the density (as you noted), expressing concentration as moles per volume will err to the extent that the solution departs from ideality.
Binary solutions have volumes larger, smaller or the same as the sum of the volumes of the individual components. The differences - excess molar volumes - are negative when solutions contract (e.g. 2-propanol + water), positive when they expand.