Please help me with some compounds

Jun 2016
I need to choose at least 1 compound from the following(The question relates to the compounds only at room temperature):

1. weak base in aqueous solution
2. weak acid in aqueous solution
3. room temperature solid compound with characteristics of Ionic compounds

now I know that : H2,He,H2O,H2S,H2Se,
,CH4,NH3. are gases in room temp

so I'm left with MgH2 and Hg that are both solid.
now the problem is that MgH2: Metal hydrides are very strong bases and Mercury does not dissolve in water to act as acid/base.

I know that from the 2 options MgH2 is the right answer but how do I explain MgH2 is a weak base if its a strong one

I would love if someone can give me a source (website) that I can use in order to answer this question. thank
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