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

The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction, meaning the total mass of reactants should equal the total mass of products. Simply adding the masses of two reactants to determine the total mass of products is incorrect because it does not consider the stoichiometry of the reaction, which dictates the proportions of reactants used and products formed. To determine the correct mass of products, we must follow a series of steps involving the balanced chemical equation, molar conversions, identifying limiting reactants, and using stoichiometric coefficients to relate reactants and products. This approach ensures mass conservation in a chemical reaction.

Step-by-step solution

Introduction to the law of conservation of mass

The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction. In other words, the total mass of the reactants in a chemical reaction should be equal to the total mass of the products.

Understanding chemical reactions

In a chemical reaction, reactants are converted into products through the rearrangement of atoms, which involves breaking the existing bonds and forming new ones. It is essential to understand that the mass of atoms involved in a chemical reaction does not change; only the way they are connected or bonded changes.

Mismatch in mass while adding reactants

When we attempt to simply add the masses of the reactants to determine the total mass of products, we often find that there is a mismatch in the mass. This is because we are not taking into account the stoichiometry of the reaction, which is the balanced chemical equation that shows in what proportions the reactants are to be used exactly and how many products are formed. The stoichiometric coefficients in the balanced chemical equation play an essential role in how to obtain the correct mass of the products by relating the amount of reactants to the amount of products.

Stoichiometry and conservation of mass

By taking into account the stoichiometry of the reaction, we can properly understand the conservation of mass. To calculate the mass of the products, we need to follow these steps: 1. Write the balanced chemical equation for the reaction. 2. Convert the masses of the reactants into moles by dividing the masses by their respective molar masses. 3. Determine the limiting reactant (if there is one) by comparing the mole ratios of the reactants to their stoichiometric coefficients. 4. Calculate the moles of products formed by using the stoichiometric coefficients and the moles of limiting reactant. 5. Convert the moles of products into mass by multiplying them by their respective molar masses. Following these steps can help us understand the relationship between the masses of reactants and products and how they are related by the stoichiometry of the reaction. This will correctly show that the mass is conserved during a chemical reaction.