Question

According to the law of conservation of mass, mass cannot be gained or destroyed in a chemical reaction. Why can't you simply add the masses of two reactants to determine the total mass of product?

Short answer

In a chemical reaction, atoms within reactant molecules rearrange themselves to form product molecules, so merely adding the masses of reactants will not give the exact mass of products. We must balance the chemical equation, use mole ratios, and consider the rearrangement of atoms to determine the total mass of products accurately. The law of conservation of mass holds true at an atomic level, meaning the number and type of atoms remain constant in a closed system throughout the reaction.

Step-by-step solution

: In a chemical reaction, atoms within reactant molecules rearrange themselves to form product molecules. The total mass of atoms remains conserved, but the individual masses of different types of molecules may change. Therefore, merely adding the masses of reactants will not give you the exact mass of products. #Step 2: Balancing the chemical equation#

: Before determining the mass of products, we need to balance the chemical equation to ensure the number of atoms in reactants and products is the same for each element. By following the law of conservation of mass, we can ensure that the correct amount of reactants is used, and the correct mass of products is formed. #Step 3: Using mole ratios to find the total mass of product#

: Once the chemical equation is balanced, we can use the mole ratio of reactants and products to determine the mass of each product formed. We establish these relationships by converting the masses of reactants into moles, using the molecular weights of each substance. For example, if the balanced equation is: \[A + 2B \to C + 3D\] Suppose the mass of 'A' and 'B' reactants are given. To determine the mass of products 'C' and 'D', you would: 1. Convert the mass of reactant 'A' to moles using its molecular weight 2. Convert the mass of reactant 'B' to moles using its molecular weight 3. Compare the mole ratios in the balanced equation and see which reactant is in excess or limiting 4. Determine the moles of products 'C' and 'D' according to the balanced equation 5. Convert the moles of product 'C' back into mass using its molecular weight 6. Convert the moles of product 'D' back into mass using its molecular weight 7. Sum the masses of 'C' and 'D' to find the total mass of products #Step 4: Recognizing that mass is conserved at an atomic level#

: In conclusion, the reason why we can't simply add the masses of reactants to determine the total mass of products is that a chemical reaction involves the rearrangement of atoms, not just the combination of masses. We must consider the balanced chemical equation and the corresponding mole ratios to accurately determine the total mass of product. The law of conservation of mass still holds true but at an atomic level, meaning the number and type of atoms remain constant in a closed system throughout the reaction.