# Which is more soluble in water and why: K₂HPO₄ or CaHPO₄?

If I dissolve $\ce{K_2HPO_4}$ and $\ce{CaHPO_4}$ in water (two different reactions), the latter shows better solubility. Is it because $\ce{Ca^{2+}}$ has ha higher valence of $2$ than $\ce{K^+}$, which has only $1$? Or has it something to do with the lattice enthalpy?

The size of the potassium ion is around 152 pm, the size of the calcium ion is around 114 pm. So the calcium ion is both the smaller and the more highly charged of the two. Therefor we would expect the calcium ion to have the less stable lattice and be more highly solvated (stabilized) when in solution. Both of these factors would lead us to expect $\ce{CaHPO4}$ to be more soluble than $\ce{K2HPO4}$