When ammonia gas, ${\mathrm{NH}}_3$ reacts with hydrogen chloride gas, $\mathrm{HCl}$ the white solid ammonium chloride, ${\mathrm{NH}}_4\mathrm{Cl}$ is formed:

The apparatus shown in figure was set up to test Graham's law. A metre stick is placed beside the tube. Cotton wool soaked in ${\mathrm{NH}}_3$ is placed in the left-hand end and cotton wool soaked in $\mathrm{HCl}$ is placed in the right-hand end. A white cloud of solid appears where the $\mathrm{HCl}$ and ${\mathrm{NH}}_3$ meet. Assuming the NH3 end to be $0.00$ m and the $\mathrm{HCl}$end to be $1.00$ m on the metre stick, calculate the position at which you would expect the solid $\mathrm{NH}4\mathrm{Cl}$ to appear.

A modern solution to the provision of power for remote places is the dye-sensitized solar cell (DSSC). A Gratzel DSSC contains an organic dye molecule on the surface of a titanium dioxide, ${\mathrm{TiO}}_2$, semiconductor and an electrolyte containing iodide ions.

Explain its operation, including the importance of nanotechnology in its construction and its advantage compared with silicon-based photovoltaic devices.

Discuss the use of silicon in photovoltaic cells, with reference to the following:

Why pure silicon is a better electrical conductor than non-metals such as phosphorus and sulphur.

Discuss the use of silicon in photovoltaic cells, with reference to the following:

How a $\mathrm{p}$-type semiconductor made from silicon is different from pure silicon.

Discuss the use of silicon in photovoltaic cells, with reference to the following:

How sunlight can produce an electric current in a photovoltaic cell.

Bromothymol blue is an acid-base indicator that is blue in base and yellow in acid:

$\left(\mathrm{Yellow}\right) \mathrm{HB}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{B}}^{-} \left(\mathrm{Blue}\right)$

 Deduce which of the two forms of the indicator has the higher degree of conjugation. Explain it in terms of the wavelength of light absorbed and molecular structure of an organic indicator.