CHEMISTRY stoichiometry and moles and combustion problem

Question

Methanol has the chemical formula CH₃OH. If 4 moles of methanol are combusted, how many moles of oxygen will be consumed? 

a- 2mol

b- 3 mol

c- 4 mol

d- 6 mol

e- not enough information

Answer 1

Methanol would burn with O₂ to produce CO₂ and H₂O. That we know. Let us try to write the equation tentatively:

CH₃OH + O₂ → CO₂ + H₂O .... But the equation is not balanced yet.

We see, for each C there must be one O₂. Therefore, for the C we have sufficient O₂ on Left hand side to form CO₂.

Look at H atoms - we have 4 in one methanol molecule. To completely oxidize the 4H we need 2O. Interestingly the methanol molecule already has one. So the air need to provide just one atom of oxygen.

Then the 4H and 2O will completely burn to produce 2H₂O.

So let us add an atom of O in our equation and balance it:

CH₃OH + O₂ + 0.5O₂→ CO₂ + 2H₂O

Now the equation is perfectly balanced. But it is not customary to write molecules in fractions. We must write it as whole numbers. Multiplying the whole equation by 2 solves the problem:

2CH₃OH + 3O₂→ 2CO₂ + 4H₂O

Obvious that 4 moles of methanol would need 6 moles of oxygen.

Answer 2

The first step in problems of this sort is to write a balanced chemical equation.

__ CH3OH + ___ O2 -------> ___ CO2 + ___ H2O

This is the unbalanced equation, showing the reactants (methanol and oxygen) and the products (carbon dioxide and water). Now, you need to determine the coefficients to balance the equation. Start with the carbon. As it stands now, there is one carbon on the left, and one carbon on the right. You can tentatively put 1 as the coefficient for the methanol and carbon dioxide.

1 CH3OH + ___ O2 -------> 1 CO2 + ___ H2O

Now, look at the hydrogen. You have four on the left but only two on the right. To fix this, you need to (tentatively) put a 2 in front of the water. This gives you

1 CH3OH + ___ O2 -------> 1 CO2 + 2 H2O

Finally, look at the oxygen. You have four on the right but only three on the left. In order to fix this, you would need to put a coefficient of 3/2 in front of the oxygen, to get

1 CH3OH + (3/2) O2 -------> 1 CO2 + 2 H2O

Since a balanced chemical equation needs integer coefficients, you must multiply all the coefficents by 2. This gives you the final balanced chemical equation,

2 CH3OH + 3 O2 -------> 2 CO2 + 4 H2O

Now you are in a position to answer the original question. If you have four moles of methanol, this can be represented by multiplying this balanced chemical equation by another factor of 2, to get

4 CH3OH + 6 O2 -------> 4 CO2 + 8 H2O

As you can now see, four moles of methanol will react with six moles of oxygen, so the answer is D.