Answer: The substance Y in the given equation is
![O_2](https://img.qammunity.org/2021/formulas/chemistry/college/g1yc0vvlky5k42cnhv052uznavotjq980k.png)
Step-by-step explanation:
Law of conservation of mass states that mass can neither be created nor be destroyed but it can only be transformed from one form to another form.
It means that total number of individual atoms on the reactant side is always equal to the total number of individual atoms on the product side.
For the given chemical equation:
![C_2H_4+3Y\rightarrow 2CO_2+2H_2O](https://img.qammunity.org/2021/formulas/chemistry/high-school/ckntoxvfwutboq27fu0eu05yoxe0fuulau.png)
On the reactant side:
Number of carbon atoms = 2
Number of hydrogen atoms = 4
Number of oxygen atoms = 3Y
On the product side:
Number of carbon atoms = 2
Number of hydrogen atoms = 4
Number of oxygen atoms = 6
Evaluating the subscript of oxygen atom:
![\Rightarrow 3Y=6\\\\\Rightarrow Y=2](https://img.qammunity.org/2021/formulas/chemistry/high-school/rje1zxiklkcoya5jrqnuu199fepjzfcxav.png)
So, the missing substance in the above equation is
![O_2](https://img.qammunity.org/2021/formulas/chemistry/college/g1yc0vvlky5k42cnhv052uznavotjq980k.png)
The equation now becomes:
![C_2H_4+3O_2\rightarrow 2CO_2+2H_2O](https://img.qammunity.org/2021/formulas/chemistry/high-school/nkymb6jck063tsfazz04epo9cid3fte3g4.png)
Hence, the substance Y in the given equation is
![O_2](https://img.qammunity.org/2021/formulas/chemistry/college/g1yc0vvlky5k42cnhv052uznavotjq980k.png)