' $X$ ' is a protecting colloid. The following data is obtained for preventing the coagulation of 10 mL of gold sol to which 1 mL of $10 \% \mathrm{NaCl}$ is added. What is the gold number of ' $X$ '?
$$ \begin{array}{ccl} \hline \text { Expt No. } & \begin{array}{c} \text { Weight of } X \text { (in mg) } \\ \text { added to gold sol } \end{array} & \text { Coagulation } \\ \hline 1 & 24 & \text { Not prevented } \\ \hline 2 & 23 & \text { Not prevented } \\ \hline 3 & 26 & \text { Prevented } \\ \hline 4 & 27 & \text { Prevented } \\ \hline 5 & 25 & \text { Prevented } \\ \hline \end{array} $$
The reactions which occur in blast furnace at $500-800 \mathrm{~K}$ during extraction of iron from haematite are
i. $3 \mathrm{Fe}_2 \mathrm{O}_3+\mathrm{CO} \longrightarrow 2 \mathrm{Fe}_3 \mathrm{O}_4+\mathrm{CO}_2$
ii. $\mathrm{Fe}_2 \mathrm{O}_3+3 \mathrm{C} \longrightarrow 2 \mathrm{Fe}+3 \mathrm{CO}$
iii. $\mathrm{Fe}_3 \mathrm{O}_4+4 \mathrm{CO} \longrightarrow \mathrm{Fe}+4 \mathrm{CO}_2$
iv. $\mathrm{Fe}_2 \mathrm{O}_3+\mathrm{CO} \longrightarrow 2 \mathrm{FeO}+\mathrm{CO}_2$
Which of the following reactions give phosphine?
i. Reaction of calcium phosphide with water
ii. Heating white phosphorous with concentrated NaOH solution in inert atmosphere
iii. Heating red phosphorous with alkali