# After 20.0g of Na_2SO_4 are added to a 0.5 L saturated solution of CaSO_4, does the concentration of Ca^(2+) increase, decrease, or stay the same?

$\left[C {a}^{2 +}\right]$ will reduce.
$C {a}^{2 +} + S {O}_{4}^{2 -} r i g h t \le f t h a r p \infty n s C a S {O}_{4} \left(s\right)$ is at equilibrium. The addition of more sulfate anion will drive the equilibrium to the right. Calcium sulfate will precipitate, with reduction in $\left[C {a}^{2 +}\right]$, so that the ion product resatisfies the solubility expression, ${K}_{s p} = \left[C {a}^{2 +}\right] \left[S {O}_{4}^{2 -}\right]$.