when water decomposes into oxygen and hydrogen the mass

Question

a. remains constant 

 

b. varies according the number of hydrogen and oxygen molecules that form.

c. increases

d. decreases

Answer 1

In the balanced reaction

 

2H₂O → 2H₂ + O₂

 

the total masses on the left hand and right hand sides are the same, but as the reaction progresses, the mass of water is decreasing while the masses of hydrogen and oxygen are increasing.  

 

I would say there is a matter of interpretation here.  Are they asking whether the mass of just the water changes, or do they mean the combined water-hydrogen-oxygen mass?  Depending on how you interpret the question, you could answer differently.