Which among the following salts turns red litmus blue in its aqueous solution?

4 gram of NaOH is added in water to form 500 mL solution at 298 K . What is pH of solution? (Molar mass of $\mathrm{NaOH}=40 \mathrm{~g} \mathrm{~mol}^{-1}$ )

The solubility of salt $\mathrm{BA}_2$ is $4 \times 10^{-4} \mathrm{~mol} \mathrm{dm}^{-3}$.

What is solubility product of the salt?

Solubility of $\mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2$ is ' S ' $\mathrm{mol} \mathrm{dm}{ }^{-3}$. Find solubility product.