The salt having the least solubility in water is

$K_{sp}$ for $BaSO_4$ at $25 \, ^oC$ is $1.1 \times 10^{-10}$. To make the new solubility equal to $1.1 \times 10^{-8} \, M$,it is necessary to use a solution of $Na_2SO_4$ of the following concentration $....... \ M$

The solubility of $BaSO_4$ in water is $2.33 \times 10^{-3} \ g/L$. Its solubility product will be (molecular weight of $BaSO_4 = 233$).

$H_{2}S$ is passed into $1 \ L$ of a solution containing $0.1 \ mole$ of $Zn^{2+}$ and $0.01 \ mole$ of $Cu^{2+}$ until the sulphide ion concentration reaches $8.1 \times 10^{-19} \ M$. Which one of the following statements is true? $[K_{sp}$ of $ZnS$ and $CuS$ are $3 \times 10^{-22}$ and $8 \times 10^{-36}$ respectively.$]$

Solubility of calcium phosphate (molecular mass,$M$) in water is $W \ g$ per $100 \ mL$ at $25^{\circ} C$. Its solubility product at $25^{\circ} C$ will be approximately.

When equal volumes of the following solutions are mixed,precipitation of $AgCl$ will occur in - ($K_{sp}$ of $AgCl = 1.8 \times 10^{-10}$)