To determine the total number of atoms present in the reaction based on the law of conservation of matter, we need to count the atoms of the reactants and the products.
In the reaction:
- 10 atoms of carbon (C)
- 20 atoms of oxygen (O)
When these react to form carbon dioxide (CO₂), each CO₂ molecule contains 1 carbon atom and 2 oxygen atoms.
For 10 molecules of CO₂:
- Carbon atoms: 10
- Oxygen atoms: 10 (because \(10 \text{ CO₂} \times 2 \text{ O} \text{ per molecule} = 20 \text{ O atoms}\))
Adding the atoms from the reactants:
- C: 10 atoms
- O: 20 atoms
Total number of atoms in the reactants: 10 (C) + 20 (O) = 30 atoms.
Since the reaction is balanced, the total number of atoms in the products will also be 30 atoms.
Thus, the answer is: 30.