Need help with a pHquestion

Ba2+ has a larger radius than Mg2+: it has a weaker attraction to oxygen which means that less energy is needed to break the bond between BaO, hence more OH- can be made when reacted with water, so it has a higher pH.

F2 is the most reactive halogen because F2 has the strongest ability to attract electrons from other atoms, due to each F having the smallest radius and fewest electron shells.

Oh okay, I get the halogen part now. So for the pH part, it is all down to how much energy is needed to overcome the bond. So if less energy is needed, then the easier bonds are broken. But MgO can also form OH- ion though. I dont really get what it means by a high pH in this context. Thanks.

Basically, MgO is only partially soluble, which means that when it's dissolved in water an equilibrium reaction is set up between MgO forming Mg(OH)2. I think it's the same for all the metal oxides.

Mg(OH)2 can dissociate into Mg2+ and OH-, so you're right, it does form OH- which is what makes the solution an alkaline!

The higher the concentration of OH- ions, the higher the pH. More BaO becomes Ba(OH)2 because less energy is needed. More Ba(OH)2 means more OH- and a higher pH.

Say you had 10 moles of MgO, but only 2 become Mg(OH)2, so you could have 4OH-.
But if you have 10 moles of BaO, 4 might become Ba(OH)2 because it needs less energy, so you have 8OH-. There will be a higher pH in this for the same amount of BaO as there was MgO if you see what I mean.

How did you get Ba(OH)2? Sorry I don't think i have learnt that in my chemistry lesson yet so can you just explain briefly? So Ba(OH)2 is formed and it can dissociates into OH- ions as you have mentioned. More OH- ions= more alkaline solution?

Ba(OH)2 is just what you would get if the Ba2+ + 2OH- ions bonded ionically. I don't know what your specific exam board wants, but Ba(OH)2 is a strong base, so it dissociates fully into Ba2+ and 2OH-. Ba(OH)2 is kind of just an intermediate, sorry and you don't have to worry about it

BaO+H2O⇌Ba²⁺ + 2OH^-

Last question, this is something completely different. Say if the mole of Na2CO3 is 0.4 mole. What is the mole for the Na+ ions in this compound? Thanks.

There would be 0.8 moles.

Think about it like this:
1Na₂CO₃->2Na+1C+3O
0.4Na2CO3-> 0.8Na