Start with a balanced equation.
"2C"_6"H"_14 + "19O"_22C6H14+19O2rarr→"12CO"_2 + "14H"_2"O"12CO2+14H2O
Use the balanced equation to determine the mole ratios between "C"_6"H"_14"C6H14 and "O"_2"O2.
("2 mol C"_6"H"_14)/("19 mol O"_2")2 mol C6H1419 mol O2 and ("19 mol O"_2)/("2 mol C"_6"H"_14)19 mol O22 mol C6H14
Determine the molar masses of "C"_6"H"_14C6H14 and "O"_2"O2.
"C"_6"H"_14:C6H14:"86.178 g/mol"86.178 g/mol
https://www.ncbi.nlm.nih.gov/pccompound?term=C6H14
"O"_2:O2:"31.998 g/mol"31.998 g/mol
Determine the moles of "C"_6"H"_14"C6H14 by dividing its given mass by its molar mass.
(11.5 cancel"g C"_6"H"_14)/(86.178cancel"g"/"mol")="0.13344 mol C"_6"H"_14"
Determine moles of "O"_2" by multiplying mol "C"_6"H"_14" by the mol ratio that has "O"_2" in the numerator.
0.13344cancel"mol C"_6"H"_14xx(19"mol O"_2)/(2 cancel"mol C"_6"H"_14)="1.2678 mol O"_2"
Determine the mass of "O"_2" that will react with 11.5"g C"_6"H"_14" by multiplying the moles "O"_2" by its molar mass.
1.2678cancel"mol O"_2xx(31.998"g O"_2)/(1cancel"mol O"_2)="0.0400 g O"_2" (rounded to three significant figures)