Concentration and particles in solution
Molar concentration is the amount of solute per volume of the final solution: c=n/V, with c in mol dm-3 and V in dm3. Distinguish solvent volume from solution volume. During dilution the moles of solute transferred remain constant, so c1V1=c2V2.
Hydrated ions
When an ionic solid dissolves in water, polar water molecules orient around separated ions. The oxygen end, with partial negative charge, points toward cations; hydrogen ends, with partial positive charge, point toward anions. Ion-dipole attractions form dynamic hydration shells. Dissolution depends on the balance among lattice disruption, hydration and entropy; not every ionic solid is highly soluble.
Crystal hydrates
A crystal hydrate contains a fixed stoichiometric number of water molecules in its crystal structure, written salt·xH2O. Water of crystallization is not simply liquid trapped on a surface. Controlled heating may remove it and form an anhydrous salt.
Finding x by mass loss
- Measure hydrated sample and container.
- Heat gently, cool and reweigh; repeat to constant mass.
- Mass water = hydrated mass - anhydrous mass.
- Convert water and anhydrous salt masses to moles.
- Divide by the smaller amount and obtain the simplest whole-number ratio.
| Process | Quantity conserved | Critical condition |
|---|---|---|
| Dilution | Moles of solute transferred | No solute lost or reacted |
| Hydration | Charge and atoms | Water molecules remain dynamic |
| Dehydration | Salt framework composition may remain | Avoid decomposition; heat to constant mass |