In a chemical equation, what do the stoichiometric coefficients represent?

• A. The exponents in a chemical formula that show the number of atoms.
• B. The numbers in front of chemical formulas that balance the equation; they indicate the mole ratio.
• C. The subscripts in a chemical formula that show the number of atoms.
• D. The coefficients of a catalyst that speed up the reaction.

Answer: (B)

In a chemical equation, the numbers in front of the formulas are coefficients that show how many moles of each substance participate or are formed, giving the mole ratio between reactants and products. They are used to balance the equation so the same total number of atoms of each element appear on both sides, reflecting conservation of mass. The subscripts inside a formula tell how many atoms are in a single molecule, not how much of that substance you have; coefficients scale those amounts overall to represent quantities of reactants and products.

For example, in 2 H2 + O2 -> 2 H2O, the coefficients indicate you need 2 moles of hydrogen gas and 1 mole of oxygen gas to produce 2 moles of water. Catalysts, if present, speed up the reaction but are not part of the balancing in the equation.