Final answer:
The chemical reaction between dichromate and thiosulfate forms Cr3+ and other products. When balanced in acid, it is necessary to add hydrogen ions for charge and atom balance. Finally, counting O atoms on the right side yields the total number present from all products.
Step-by-step explanation:
The reaction between dichromate (Cr2O72-) and thiosulfate (S2O32-) involves a redox process where Cr2O72- is reduced to Cr3+ and S2O32- is oxidized. When the chemical equation is balanced in an acidic solution, we must add hydrogen ions (H+) to balance both the charge and the atoms on both sides. Once the equation is balanced, we can count the number of oxygen (O) atoms on the product side which will be determined by the total number produced from both the resultant Cr3+ and water (H2O) molecules, and any additional oxyanions formed as products.