Sodium sulphate is soluble in water whereas barium sulphate is sparingly soluble because

$BaS{O}_4$ is sparing;-:; soluble in water because its hydration energy is lesser than the lattice energy and thus ions are not separated from each other. On the contrary in $N{a}_{2}S{O}_4$, the hydration energy is more than its lattice energy. Thus ions are separated from each other and pass in solution state.