The correct answer is 12.2% BaO.
The solution is found by dividing the mass of the BaO, which is 25.8 grams, by the total mass of the solution, which is 212 grams, then multiplying it by 100 to get the percentage:
.5 mol of A will be left over since 1.5 mol of A will be used for every 3 mol of B due to the 2:1 ratio established by the formula.
The element that has the fewest valence electrons available for bonding I believe is Hydrogen.
7.5 is the answer. You have to move the decimal 2 places to the right.