The symbol for hydrogen is H
Answer: The missing coefficient is 2.
Explanation:
According to the law of conservation of mass, mass can neither be created nor be destroyed. Thus the mass of products has to be equal to the mass of reactants. The number of atoms of each element has to be same on reactant and product side. Thus chemical equations are balanced.

As in the products, there are 2 atoms of sodium, thus there will be 2 atoms of of sodium in the reactant as well. This will balance the number of hydrogen and oxygen atoms as well.
Thus the missing coefficient is 2.
I believe that would be oil and vinegar.
Chlorobenzene would essentially be a benzene ring that is substituted with chlorine. Thus it would have 6 carbon atoms, in its molecular formula.
Answer:
250 mmol are left after 26.60 secs ≅ 26.6 seconds
Explanation:
zero order kinetics formula is:
[A] = [A₀] - kt
where [A] = amount left; [A₀] = amount remaining; k = rate constant; t = time
concentration [A₀] = mole/volume = 0.5 mole/2.0L = 0.25
[A] = 0.25 mole/2.0L = 0.125 M
k = 0.0047 Ms⁻¹
t = {[A]-[A₀]}/-k = (0.125 - 0.25)/(-0.0047) = 26.60 seconds = 26.6 seconds
the amount is reduced by half after 26.6 seconds