What are the answers to the following chemistry questions? 3c) Main ore of Aluminum? 3d) Products of brine electrolysis? 3e) Element atomic number 17? 3f) % alumina in a mixture? 3g) electrolysis of N

$$\text{Total mass} = \text{Mass of alumina} + \text{Mass of cryolite}$$ $$\text{Total mass} = 20.0 \text{ g} + 20.0 \text{ g} = 40.0 \text{ g}$$

$$\text{Percentage of alumina} = \frac{\text{Mass of alumina}}{\text{Total mass}} \times 100\%$$ $$\text{Percentage of alumina} = \frac{20.0 \text{ g}}{40.0 \text{ g}} \times 100\%$$ $$\text{Percentage of alumina} = 0.5 \times 100\% = \boxed{50\%}$$

3g) Nitrogen (III) chloride ($\text{NCl}_3$) is a covalent compound and does not typically undergo electrolysis in the same way ionic compounds do, as it does not readily dissociate into ions. If an electrical current were forced through it, decomposition would occur. At the anode, oxidation takes place. In $\text{NCl}_3$, the chlorine atoms are in the $-1$ oxidation state. If oxidation were to occur, chloride would lose electrons to form chlorine gas.

3h) The Contact Process is an industrial method for the manufacture of $\boxed{\text{Sulfuric acid}}$. The "percentage of Nitrogen" likely refers to the Haber process for ammonia synthesis, where nitrogen gas typically constitutes $\boxed{25\%}$ of the reactant mixture (1 part $\text{N}_2$ to 3 parts $\text{H}_2$ by moles).

3i) In the reaction $2\text{H}_2\text{S} + \text{SO}_2 \rightarrow 3\text{S} + 2\text{H}_2\text{O}$:
The oxidation state of sulfur in $\text{H}_2\text{S}$ is $-2$.
The oxidation state of sulfur in $\text{SO}_2$ is $+4$.
The oxidation state of sulfur in elemental $\text{S}$ is 0.
$\text{H}_2\text{S}$ is oxidized (sulfur goes from $-2$ to 0), so $\text{H}_2\text{S}$ is the $\boxed{\text{reducing agent}}$.

3j) The oxidation number of Sulfur in $\text{SO}_4^{2-}$ is:
Let the oxidation number of Sulfur be $x$.

$$x + 4(-2) = -2$$ $$x - 8 = -2$$ $$x = -2 + 8$$ $$x = \boxed{+6}$$

3k) Which halogen is a gas at room temperature?
The correct option is $\boxed{\text{(C) Chlorine}}$. (Iodine is a solid, Bromine is a liquid at room temperature).

3l) The colour in transition metal complexes is due to $\boxed{\text{absorption of specific wavelengths of light, causing d-d electron transitions}}$.