What mass of oxygen (in grams) is needed to burn 3.5g of propane (C3H8) according to the following equation

Question

What mass of oxygen (in grams) is needed to burn 3.5 g of propane (C3H8) according to the following equations: 

C3H8 + 5 O2 → 4 H2O + 3 CO2

Answer 1

Complete combustion of propane:

C3H8 + 5O₂ ==> 3CO₂ + 4H₂O ... balanced equation

Use dimensional analysis:

3.5 g C3H8 x 1 mol C3H8/44 g x 5 mol O₂/mol C3H8 x 32 g O₂/mol O₂ = 12.7 g O₂ = 13 g O₂ (2 sig.figs.)