Calculate the minimum mass of hexane that could be left over by the chemical reaction. Be sure your answer has the correct number of significant digits.

Question

Liquid hexane CH3CH24CH3

 will react with gaseous oxygen O2

 to produce gaseous carbon dioxide CO2

 and gaseous water H2O

. Suppose 6.89 of hexane is mixed with 35. g of oxygen. Calculate the minimum mass of hexane that could be left over by the chemical reaction. Be sure your answer has the correct number of significant digits. 

Answer 1

This is a combustion reaction. The first thing to do when solving these problems is to setup the chemical equation and balance it.

CH₃CH₂CH₂CH₂CH₂CH₃ + (13/2) O₂ --> 6 CO₂ + 7 H₂O

From there, using the molar mass of each reactant, and the amount of starting material, determine the limiting reactant. For complete combustion, the fuel is completely consumed and oxygen is in excess.

Molar mass of hexane: 86.18 g/mol starting mass of hexane: 6.89 g starting moles: 0.07994 mol

Molar mass of oxygen: 32 g/mol starting mass of oxygen: 35. g starting moles: 1.09375 mol

Use dimensional analysis to determine how many moles of oxygen will be required to react all of the hexane.

0.07994 mol hexane * ((13/2 mol oxygen) / (1 mol hexane)) = 0.51961 mol oxygen required to react all of the hexane.

I have 1.09375 mol of oxygen, an excess, meaning all of the hexane should react and not leave any fuel leftover.