Calculate the minimum concentration of sulphate ion required to precipitate $BaSO_4$ in a solution containing $10^{-4} \ mol/L$ of $Ba^{2+}$. $(K_{sp} \ BaSO_4 = 4 \times 10^{-10})$

Formula	Solubility product $(K_{sp})$
$AgBrO_3$	$2.5 \times 10^{-10}$
$MgSO_4$	$2.7 \times 10^{-27}$
$CaCO_3$	$2.7 \times 10^{-18}$
$FeC_2O_4$	$2.4 \times 10^{-15}$

Which of the following salts has the maximum solubility?

$A$ white powder soluble in $NH_4OH$ but insoluble in water is:

If the solubility product of $Ni(OH)_2$ is $4.0 \times 10^{-15}$, the solubility (in $mol \ L^{-1}$) is

Concentration of $H_2SO_4$ and $Na_2SO_4$ in a solution is $1 \ M$ and $1.8 \times 10^{-2} \ M$,respectively. Molar solubility of $PbSO_4$ in the same solution is $X \times 10^{-Y} \ M$ (expressed in scientific notation). The value of $Y$ is. . . .
[Given: Solubility product of $PbSO_4$ $(K_{sp})=1.6 \times 10^{-8}$. For $H_2SO_4$,$K_{a1}$ is very large and $K_{a2}=1.2 \times 10^{-2}$]

Which of the following has maximum solubility at low $pH$?