How do you calculate the concentration of the reactant when every reactant decompose completely to form the product?

To calculate the concentration of the product formed when reactants decompose completely, you first need to know the initial concentrations of the reactants and their stoichiometry in the balanced chemical equation. Assuming complete decomposition, the concentration of the product will equal the total moles of the reactants converted into product, divided by the total volume of the solution. Use the formula: [ \text{Concentration of product} = \frac{\text{Total moles of reactants}}{\text{Total volume}} ]. Adjust for any stoichiometric coefficients as necessary based on the balanced equation.