Answer:
 128g
Step-by-step explanation:
Given parameters: 
Mass of carbon = 48g 
Mass of carbon dioxide = 176g 
Unknown: 
Mass of oxygen that reacted = ? 
Solution: 
Every chemical reaction must obey the law of conservation of mass. It states that "in a chemical reaction, matter is neither created nor destroyed" . 
So; 
 Mass of carbon + Mass of oxygen = Mass of carbon dioxide 
Mass of oxygen = Mass of carbon dioxide - Mass of carbon 
 Mass of oxygen = 176 - 48 = 128g