# Equal volumes of 0.02M CaCl2 0.0004M Na2SO4 are mixed. Will precipitate of CaSO4 form?

You need to quote a $\text{K"_"sp}$ value for ${\text{CaSO}}_{4}$, and this should have been supplied with the question.
If $\left[C {a}^{2 +}\right] \left[S {O}_{4}^{2 -}\right] > \text{K"_"sp}$, then precipitation will occur until $\left[C {a}^{2 +}\right] \left[S {O}_{4}^{2 -}\right] = \text{K"_"sp}$.