the amount of equivalents is the amount of mols multiplied with the coefficient of the ionspecies mulitplied with the carge of the ion.
5% = 5g / 100 mL --> thus 10 mL contains 0,5 g MgSO4
--> 0,5 g MgSO4 is equal to 4,2 mmol
--> this is equal to 4,2 * 1 * 2 = 8,4 mEq MgSO4 (1 for the ionspecies Mg and 2 for the 2+-charge of Mg)
You didn't bring the charge into account
