What mass of carbon dioxide is produced from burning 100 grams of ethanol in oxygen according to this reaction: C2H5OH + 3 O2 --> 2 CO2 + 3 H2O

When approaching a question like this I always find it easier to write out the equation with the known quantities written underneath each individual compound.  This allows us to easily check whether the equation is balanced

                                   C₂H₅OH          +         3 O₂     -->       2 CO₂                         +          3 H₂O

We can see that this equation is balanced as it has 2 carbons, 6 hydrogens, and 7 oxygens on each side.

We are trying to work out the mass of CO₂ and we already know what mass of ethanol (C₂H₅OH) we have.  We can also work out the gram formula mass of each compound we are interested in. 

gfm CO₂ = (1 x 12) + (2 x 16) = 44 g/mol

gfm C₂H₅OH = (2 x 12) + (6 x 1) + (1 x 16) = 46 g/mol

Next we need to decide what formula we can use to figure out the mass of CO₂ from the data that we have. 

                                   C₂H₅OH          +         3 O₂     -->       2 CO₂                         +          3 H₂O

                       m = 100 g                                            m = ?

                       gfm = 46 g/mol                                gfm = 44 g/mol

Using n = m/gfm we can figure the number of moles of ethanol.  We can use this to work out the number of moles of carbon dioxideas these can be directly compared. 

                                  C₂H₅OH          +         3 O₂     -->       2 CO₂                         +          3 H₂O

                      n = m/gfm                      

                      = 100/46                         

                      =2.174 moles                 

The mole ratio of ethanol to carbon dioxide is 1:2. So by multiplying our number by 2 we get the number of moles of CO₂.

                                    C₂H₅OH          +         3 O₂     -->       2 CO₂                         +          3 H₂O

                n = moles of C₂H₅OH x 2

           = 2.174 x 2

           = 4.349 moles

Now that we have the number of moles of carbon dioxide we can figure out the mass produced by using the formula m = n x gfm

                                  C₂H₅OH          +         3 O₂     -->       2 CO₂                         +          3 H₂O

                                                                                    m = n x gfm                       

                                                                                        = 4.349 x 44                 

                                                                                        = 191.3 g

Therefore, the mass of carbon dioxide is produced from burning 100 grams of ethanol in oxygen is 191.3g.