stoichiometric ratio

Examples of stoichiometric ratio in the following topics:

• Reaction Stoichiometry

  • In a balanced chemical equation, we can easily determine the stoichiometric ratio between the number of moles of reactants and the number of moles of products, because this ratio will always be a positive integer ratio.
  • Therefore, the stoichiometric ratio, oftentimes referred to simply as the "mole ratio" or "molar ratio," between N2(g), H2(g), and NH3(g) is 1:3:2.
  • For reactants, the stoichiometric number is the negative of the stoichiometric coefficient, while for products, the stoichiometric number is simply equal to the stoichiometric coefficient, remaining positive.
  • In the equation H2(g) + Cl2(g) → 2 HCl(g), what is the molar ratio (stoichiometric ratio) between H2(g) and HCl(g)?
  • The molar ratio between these two compounds is therefore 1:2.

• Calculating Theoretical and Percent Yield

  • It is the ratio between the actual yield and the theoretical yield.
  • A reaction should theoretically produce as much of the product as the stoichiometric ratio of product to the limiting reagent suggests.

• Limiting Reagents

  • However, if the reagents are not mixed or present in these correct stoichiometric proportions, the limiting reagent will be entirely consumed and the reaction will not go to stoichiometric completion.
  • One way to determine the limiting reagent is to compare the mole ratio of the amount of reactants used.
  • Next, convert all given information (typically masses) into moles, and compare the mole ratios of the given information to those in the chemical equation.
  • To determine how much NaOH is produced by each reagent, use the stoichiometric ratio given in the chemical equation as a conversion factor:

• Thermochemical Equations

  • A thermochemical equation is a balanced stoichiometric chemical equation which includes the enthalpy change.
  • $\Delta H$ is dependent on both the phase (solid, liquid, or gas) as well as the molar ratios of the reactants and products.
  • Therefore, all thermochemical equations must be stoichiometrically balanced.

• Solubility Product

  • An equilibrium constant is the ratio of the concentration of the products of a reaction divided by the concentration of the reactants once the reaction has reached equilibrium.
  • For substances in which the ions are not in a 1:1 ratio, the stoichiometric coefficients of the reaction become the exponents for the ions in the solubility-product expression:

• Molar Ratios

  • Molar ratios, or conversion factors, identify the number of moles of each reactant needed to form a certain number of moles of each product.
  • The numerical coefficient next to each entity denotes the absolute stoichiometric amount used in the reaction.
  • From this reaction equation, it is possible to deduce the following molar ratios:
  • These molar ratios can also be expressed as fractions.
  • Calculate the molar ratio between two substances given their balanced reaction

• Mole-to-Mole Conversions

  • The coefficients next to the reactants and products are the stoichiometric values.
  • Therefore, the ratio is one mole of O2 to two moles of H2O, or $\frac{1\:mol\:O_2}{2\:moles\:H_2O}$.
  • Each stoichiometric conversion factor is reaction-specific and requires that the reaction be balanced.

• Mass-to-Mass Conversions

  • The coefficients before the reactants and products are their stoichiometric values.
  • The ratio of the coefficients of two of the compounds in a reaction (reactant or product) can be viewed as a conversion factor and can be used to facilitate mole-to-mole conversions within the reaction.
  • Taking coefficients from the reaction equation (13 O2 and 2 C4H10), the molar ratio of O2 to C4H10 is 13:2.
  • But by converting the butane mass to moles (0.929 moles) and using the molar ratio (13 moles oxygen : 2 moles butane), one can find the molar amount of oxygen (6.05 moles) that reacts with 54.0 grams of butane.

• Redox Titrations

  • We know from our balanced equation above that permanganate and iron react in a 1:5 mole ratio.
  • From the balanced equation, Fe2+ and KMnO4 react in a 5:1 mole ratio.

• The Equilibrium Constant

  • Each of the concentrations is raised to a power equal to the stoichiometric coefficient for each species.
  • When looking at the Keq expression, we should notice that it is essentially a ratio relating the concentrations of products to the concentrations of reactants at equilibrium.
  • A Keq value $\approx$ 1 is indicative that the forward and reverse reactions are about equally favorable, for the ratio of concentrations of reactants and products is close to unity.