How many moles of oxygen are required to complete the combustion of 0.050 mL of liquid ethanol (C2H5OH)?  The density of ethanol is 0.789 g/mL.Please explain using conversion factors thank you :)

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ncchemist | eNotes Employee

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The balanced chemical equation for the combustion of ethanol is:

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

So for every mole of ethanol we need three moles of oxygen.  Let's start with the volume of ethanol and convert it to a mass using the density:

0.05 mL (0.789 g/mL) = 0.03945 grams ethanol

Now divide by the molecular weight of ethanol to get the moles of ethanol:

0.03945 g (mole/46 grams) = 8.6 x 10^-4 moles ethanol

We stated earlier that we need 3 moles of oxygen per mole of ethanol:

8.6 x 10-4 * 3 = 2.6 x 10^-3 moles O2

So 2.6 x 10^-3 moles of O2 are required for the complete combustion of 0.05 mL of ethanol.