stoichiometric number

Examples of stoichiometric number 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.
  • As we will see, through balancing chemical equations and determining the stoichiometric coefficients, we will be able to determine the number of moles of product(s) that can be produced in a given reaction, as well as the number of moles of reactant(s) that will be consumed.
  • Occasionally, you might come across the term stoichiometric number, which is related to the stoichiometric coefficient, but is not the same.
  • 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.
  • Therefore, for our example here, the stoichiometric number for H2(g) is -2, and for O2(g) it is -1.

• Electrolysis Stoichiometry

  • The amount of chemical change that occurs in electrolysis is stoichiometrically related to the amount of electrons that pass through the cell.
  • The extent of chemical change that occurs in an electrolytic cell is stoichiometrically related to the number of moles of electrons that pass through the cell.
  • The equivalent weight of a substance is defined as the molar mass divided by the number of electrons required to oxidize or reduce each unit of the substance.
  • Most stoichiometric problems involving electrolysis can be solved without explicit use of Faraday's laws.

• Mole-to-Mole Conversions

  • The coefficients next to the reactants and products are the stoichiometric values.
  • They represent the number of moles of each compound that needs to react so that the reaction can go to completion.
  • On some occasions, it may be necessary to calculate the number of moles of a reagent or product under certain reaction conditions.
  • Each stoichiometric conversion factor is reaction-specific and requires that the reaction be balanced.
  • This video shows how to determine the number of moles of reactants and products using the number of moles of one of the substances in the reaction.

• Writing Chemical Equations

  • The coefficients next to the symbols of entities indicate the number of moles of a substance produced or used in the chemical reaction.
  • The stoichiometric coefficients (the numbers in front of the chemical formulas) result from the law of conservation of mass and the law of conservation of charge (see the "Balancing Chemical Equations" section for more information).

• Gas Stoichiometry

  • Stoichiometric calculations involving gases allow us to convert between mass, number of moles, and most importantly, volume of gases.
  • From the periodic table, we can determine that the molar mass of ammonia, NH3(g), is 17 g/mol, and perform the following stoichiometric calculation:

• 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.
  • This method can be extended to any number of reactants more easily than the previous method.
  • To determine how much NaOH is produced by each reagent, use the stoichiometric ratio given in the chemical equation as a conversion factor:

• 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.
  • The molar ratios identify how many moles of product are formed from a certain amount of reactant, as well as the number of moles of a reactant needed to completely react with a certain amount of another reactant.

• Thermodynamics of Redox Reactions

  • The uppercase letters are concentrations and the lowercase letters are stoichiometric coefficients for the reaction $aA + bB \rightarrow cC + dD$
  • This equation allows the equilibrium constant to be calculated just from the standard reduction potential and the number of electrons transferred in the reaction.

• The Nernst Equation

  • n is the number of moles of electrons transferred in the reaction
  • ln Q is the natural log of $\frac{C^cD^d}{A^aB^b}$, where the uppercase letters are concentrations, and the lowercase letters are stoichiometric coefficients for the reaction: $aA + bB \rightarrow cC + dD$
  • The number of moles of electrons transferred is 2 and Q is $\frac{[Ni^{2+}][Pb]}{[Pb^{2+}][Ni]}$, where Pb and Ni are pure solids whose concentrations remain constant, so they are dropped from the equation.

• Solubility Product

  • 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:
  • The solubility product of a number of substances have been experimentally determined and can be used to predict solubility at a specified temperature.
  • The solubility product constants of a number of substances.