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)?
Solute almost always changes the volume of final solution. The change (volume decrease or volume increase of final solution) of volume depends of solvent and solute chemicophysical properties, e.g. the full charge of each or the polar properties of each, or their molecular orbital structure and electron bond donors or acceptors. Two very non-interactive materials would not even mix, e.g. oil and water (
H2O), though tiny tiny amounts of oil will "dissolve" in water as micelles, and tiny tiny amount of water will infiltrate oil, humidity of oil, just like humidity of breathable air.
If solute is not charged and does not have hydrogen-bonding-ready open orbitals or lone pairs of electrons, then the volume increases, both solute and solvent usually do not mix well.
If the solute is a ionic compound (e.g. salt,
NaCl), then up to a certain amount of table salt the volume will decrease, the density will increase and reach the maximal amount of salt to reach the minimum amount of solvent,
H2O, at a specific temperature and pressure.
If you mix a polar compound ready to accept or donate hydrogen bonds to H2O, then you get a similar picture as for
H2O, but not so drastic http://butane.chem.uiuc.edu/pshapley/genchem1/l21/vol.png , http://butane.chem.uiuc.edu/pshapley/genchem1/l21/1.html.
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.