According to the law of conservation of mass, if an element a has an atomic mass of 2 mass units and element b has an atomic mass of 3 mass units, what mass would be expected for compound ab? for compound a2b3?

The law of conservation of mass states that mass is neither created nor destroyed. Since we have 2 g/mol of A and 3 g/mol of B then AB should be equal to the sum of their molar mass that is 

2 g/mol + 3 g/mol = 5 g/mol AB

for the case of A2B3

A2 = 2 * 2 = 4 g/mol
B3 = 3 * 3 = 9 g/mol

therefore A2B3 = 13 g/mol

Ithink 2 is the correct

The product formed when bao and h2o  react together is                        ba(oh)2that is  bao + h2o = ba(oh)21 mole of    bao    react  1 mole of h2o  to    form  1 mole  of ba(oh)2