Question

Fe2O3 + CO = Fe + CO2 How to balance the above equation?

Answer 1

Fe2O3 + CO ==> Fe + CO2
Rule: balance by changing coefficients. You may not change subscripts.
Look at the Fe atoms. There are 2 Fe atoms on the left and only 1 on the right; therefore, we place a 2 in front of the Fe on the right. Like so.
Fe2O3 + CO ==> 2Fe + CO2

Count O on the left. I see 4. That means I need a 2 in front of the CO2 on the right. Like so.
Fe2O3 + CO ==> 2Fe + 2CO2

But we see that by changing CO2 on the right, we automatically mess up the C on the left. So we place a 2 in front of the CO on the left and TRY IT. It may not work.
Fe2O3 + 2CO ==> 2Fe + 2CO2

Now we check. Fe balances with 2 on the left and right. C balances with 2 on the left and right. O does not balance because there are 5 on the left and 4 on the right. Alas! What to do? Since there is an odd number of O atoms on the left, sometimes it helps to try the next higher number; i.e., instead of 2 on the CO2, let's try 3 for CO2 (and of course that makes us use 3CO on the left. Like so.
Fe2O3 + 3CO ==> 3Fe + 3CO2