Which of the following can be used as the test for unsaturation with regard to colour change of reaction?

In a bcc lattice having the edge length of 200 pm , the cation has the radius of 70 pm . The radius ratio of $\frac{r^{+}}{r^{-}}$is (Given, $\sqrt{2}=1.4, \sqrt{3}=1.7$ and $\sqrt{6}=2.4$ )

An aqueous solution of $98 \%(w / w) \mathrm{H}_2 \mathrm{SO}_4$ has density of $1.02 \mathrm{~g} / \mathrm{cc}$. The molality of the solution is

25 mL of 0.1 N NaOH solution neutralises 12.5 mL of HCl solution. The amount of water needed to convert 500 mL of such HCl solution to 0.1 N is