For $\mathrm{As}_2 \mathrm{~S}_3$ sol, the most effective coagulating agent is

Which of the following is true for spontaneous adsorption?

The charge of " $\mathrm{SnO}_2-\mathrm{sol}^{\prime}$ in alkaline and acidic medium are respectively

(Hint $\mathrm{SnO}_2$ is amphoteric oxide)

To resist the coagulation of 100 cc gold sol; 1 cc of $10 \% \mathrm{NaCl}$ is added to it in the presence of $10^{-4} \mathrm{~g}$ gelatin. The gold number of gelatin is