1. a. FeS + 2HCl --> FeCl2 + H2S
b. This is a double- replacement reaction. (The elements "switch partners".)
2. a.2Na + F2 --> 2NaF
b. This is a composition/synthesis reaction. (The two reactants are combining or synthesizing to make one product.)
3. a. 2HgO --> 2Hg + O2
b. This is a decomposition reaction. (The single reactant is breaking down or "decomposing" into multiple reactants.)
4. Hydrogen gas reacts with oxygen gas to create water in a composition/synthesis reaction.
I hope these answers will help you! If you need any explanation, ask and hopefully I can get back to you. ;)
Each compound will be neutral
There are 5 valence electrons in a atom of phosphorus.
Answer:

Explanation:
2NO₂ ⇌ N₂O₄
E/mol·L⁻¹: 0.058 0.012
K_{\text{eq}} = \dfrac{\text{[N$_{2}$O$_{4}$]}}{\text{[NO$_{2}$]$^{2}$}} = \dfrac{0.012}{0.058^{2}} = \mathbf{3.6}
\\\\
\text{The $K_{\text{eq}}$ value would be $\boxed{\mathbf{3.6}}$}