3.1.7 Using Moles to Balance Equations

• Stoichiometry refers to the numbers in front of the reactants and products in an equation, which must be adjusted to make sure that the equation is balanced
• These numbers are called coefficients (or multipliers) and if we know the masses of reactants and products, the balanced chemical equation for a given reaction can be found by determining the coefficients
• First, convert the masses of each reactant and product in to moles by dividing by the molar masses using the Periodic Table
• If the result yields uneven numbers, then multiply all of the numbers by the same number, to find the smallest whole number for the coefficient of each species
  • For example, if the resulting numbers initially were 1, 2 and 2.5, then you would multiply all of the numbers by 2, to give the whole numbers 2, 4 and 5

• Then, use the molar ratio to write out the balanced equation

Answer

• • Calculate the molar masses of the substances in the equation

CH₃OH = 32 g mol^-1         O₂ = 32 g mol^-1        

CO₂ =  44 g mol^-1             H₂O =  18 g mol^-1  

• • Divide the masses present by the molar mass to obtain the number of moles

CH₃OH = 64 g ÷ 32 g mol^-1  =  2 mol 

O₂ = 96 g ÷ 32 g mol^-1          =  3 mol   

CO₂ =  88 g ÷ 44 g mol^-1      =  2 mol

H₂O =  72 g ÷ 18 gmol^-1       =  4 mol

• • The mole ratios are the same as the coefficients in the balanced equation

2CH₃OH  +  3O₂  ⟶ 2CO₂ + 4H₂O

Answer