In a compound, carbon and oxygen react in a ratio 3:8 by mass to form carbon dioxide. The mass of oxygen required to react completely with 9g carbon is?

Given,
The ratio in which carbon and oxygen combine is 3:8 by mass
I.e , 3g carbon reacts with 8g oxygen atoms by mass
∴ 1g carbon atom react with $\frac{8}{3}$ g oxygen atoms.
Hence, 9g carbon atoms react with $\frac{8}{3}$ $\times$9 
                                                             =24g oxygen atoms.
The law of constant proportion is associated here which states that, “ In a pure chemical compound, elements are always present in definite proportions by mass.”

 In a compound, carbon, and oxygen react in a ratio of 3:8 by mass to form carbon dioxide. The mass of oxygen required to react completely with 9g carbon is 24g. And the law associated is the law of constant proportion.