Examples
Example one
Calculate the number of moles of oxygen required for the complete combustion of 132g of propane.
Step 1
Start with the balanced equation.
\({C_3}{H_8} + 5{O_2} \to 3C{O_2} + 4{H_2}O\)
Step 2
Use the information from the question to calculate how many moles of the known reactant or product are actually used.
\(number\,of\,moles = \frac{{mass}}{{formula\,mass}}\)
\(= \frac{{132}}{{44}}\)
\(= 3\,moles\)
Step 3
Use the balanced equation to work out how many moles of the reactant or product you are asked to calculate this equates to.
\({C_3}{H_8} + 5{O_2} \to 3C{O_2} + 4{H_2}O\)
From the equation, 1 mole of propane will react with 5 moles of oxygen.
\(\begin{array}{l} 1\,mole\,{C_3}{H_8} = 5\,moles\,{O_2}\\ 3\,moles\,{C_3}{H_8} = 3 \times 5\,moles\,{O_2}\\ = 15\,moles\,{O_2} \end{array}\)
Example two
If 32g of methane gas are burned in a plentiful supply of oxygen, calculate the mass of carbon dioxide produced.
Step 1
Start with the balanced equation
\(C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O\)
Step 2
Use the information from the question to calculate how many moles of the known reactant or product are actually used.
\(number\,of\,moles = \frac{{mass}}{{formula\,mass}}\)
\(= \frac{{32}}{{16}}\)
\(= 2\,moles\)
Step 3
Use the balanced equation to work out how many moles of the reactant or product you are asked to calculate this equates to.
\(C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O\)
From the equation, 1 mole of methane will produce 1 mole of carbon dioxide. So;
\(\begin{array}{l} 1\,mole\,C{H_4} = 1\,mole\,C{O_2}\\ 2\,moles\,C{H_4} = 2 \times 1\,moles\,C{O_2}\\ = 2\,moles\,C{O_2}\\ \end{array}\)
Step 4
Be careful - this question asked for the mass of carbon dioxide, not the number of moles. Use the formula triangle once more to convert this number of moles to a mass.
\(\begin{array}{l} mass = number\,of\,moles \times formula\,mass\\ = 2 \times 44\\ = 88\,grams \end{array}\)