Which of the following is the most acidic salt?

A 0.1 M solution of which salt is most acidic?

(a) Aluminum(III) nitrate $$\ce{Al(NO3)3}$$;
(b) Magnesium(II) bromide $$\ce{MgBr2}$$;
(c) Sodium bicarbonate $$\ce{NaHCO3}$$;
(d) Sodium formate $$\ce{NaHCO2}$$.

I know that it is either a or b. However, they are both product of a strong acid and a weak base reaction. How do you determine the answer?

• sorry I made a mistake. I meant MgBr2 – Jaehyun Ahn Jan 10 at 4:02
• I've edited you question so that now it's a bit more consistent and makes a bit more sense. Feel free to verify that my edit complies with what's been written where you took the question from; also please pay more attention when asking questions: we are all people and we do make mistakes, but correctly written question is going to be answered quicker and more precisely. – andselisk Jan 10 at 5:24

2 Answers

All alkali metals, and all alkaline earth metals except beryllium, form hydroxides that are stronger bases than what you get with just about any other metal. Magnesium hydroxide is weaker than heavier alkaline earth hydroxides (such as calcium hydroxide), but it still follows that rule. So work from there.

You would check the Ka values of both ions, Aluminum and Magnesium. I found a sheet listing both of them here. Even in this case, the aluminum ion is still a stronger acid.

• a) As the OP decided to change the question, so you might want to rewrite the part of your answer; b) "Aluminum is still a stronger acid": aluminium is a metal, aluminium(III) is probably what you should've written; c) Compound d exists all right, it's sodium formate. – andselisk Jan 10 at 5:20