a mixture of baking soda is prepared in water. the mass of this mixture is 225g. the original mass of baking soda was 50g. what is the original mass of water to which the baking soda was added?
a) 50g
b) 175g
c) 225g
d) 275g

It would have to be B 175. This is because if you subtract the baking soda from the total mass, you will get the original mass. 225-50=175. 

Remember that rust prevented by placing a sacrificial electrode in electrical contact with the iron. the sacrificial electrode must be composed of a metal that  oxidizes  more easily than iron (that is, it must be below iron in table 18.1).