I am pretty sure that the answer is A, because the element's atomic number is how many protons, maybe the neutrons, it has in it and all elements need 18 protons to make it stable. So the answer is A.
Hope that helps.
Answer: -
Chemical formula of sodium hydroxide = NaOH
Chemical formula of Nickel (II) Chloride = NiCl₂
The product is a solid compound of Ni²⁺ ions and hydroxide ions.
So the compound formed is Nickel (II) Hydroxide.
Chemical formula of Nickel (II) Hydroxide = Ni(OH)₂.
Since a double displacement reaction occurs, the other product should be NaCl.
The balanced chemical molecular equation for the reaction is thus
NiCl₂ + NaOH → Ni(OH)₂ + NaCl.
We must first write out the entire equation for this reaction which is as follows:
CO + Fe2O3 --> Fe + CO2
Now we must balance this equation which provides us with the following equation:
3 CO + Fe2O3 --> 2Fe + 3 CO2
We are told that we have excess Fe2O3, so that suggests that CO is the limiting reagent. We now simply convert the mass of Fe given to moles of Fe, and convert moles of Fe to moles of CO.
35.0 g Fe/ 55.845 g/mol = 0.627 moles Fe
0.627 moles Fe x (3 moles CO)/(2 moles Fe) = 0.940 moles CO
Now with the moles of CO present, we simply convert this back to mass using the molecular weight of CO.
0.940 moles CO x 28.01 g/mol = 26.3 g CO.
Therefore, 26.3 g of CO are needed to produce 35.0 g of Fe. Since we began with three significant figures in our starting mass, our answer must also have three significant figures.
Well the answer is 207.2. You take the lowest sig figs that you are using which would be 2500= 2 sig figs. 207.2 rounded to 2 sig figs would be 210