The first step here is to convert the MgO to mol, so we divide by the RMM of MgO.

Then we divide by the volume in Litres to get molarity.

In solution, the MgO splits up into and ions while the splits up into and ions.

Since the ion readily accepts 2 protons, we need to double the molarity to get the number of protons accepted in each Litre of solution. This gives 1.902.

Since the original concentration of was more than 1.902, the solution is acidic, so we are interested in [ ]

This is simply 2.40-1.902 = .4977M = .50M to the correct number of significant figures.