Posted by Timofey on Saturday, May 25, 2013 at 2:08pm.
What you have is mathematically correct; however, I think it is mixed up. You have made the factor (0.625/1 mol O2).
The factor is (2 mols NO/1 mol O2) and I would write, to convert O2 to NO (to use all of the O2) we would need the following NO.
mols NO needed = 0.625 mols O2 x (2 mols NO/1 mol O2) = 1.25 mols NO. However, we have only 0.333 mols NO, therefore, NO is the limiting reagent.
Here is what you are doing with this method. You ask yourself, how much NO do I need to react with 0.625 mols O2. Then you start with what you are given (that's 0.625 mols O2) and convert that using the dimensional method to mols NO.
Notice how the units cancel.
0.625 mols O2 x (2 mols NO/1 mol O2) = 0.625 x (2/1) = 1.25 mols NO.
This ALWAYS follows the pattern of
what is given x (factor) = what is wanted.
We are given O2, we multiply that by a factor and that changes it to what we want(mols NO).
Notice in the above that the mols O2 (bolded in the numerator and denominator cancel to leave a unitless number) and the italicized numerator (mols NO) is the unit we want to keep and the unit we want for the answer.
Changing feet to inches is the same kind of problem but we mix in the word chemistry and students think it's hard. Suppose we have 24 inches and we want to convert to feet. The factor is (1 foot = 12 inches)
We are given 24 inches so we follow the
given x factor = what we want
24 inches x (1 foot/12 inches) = 2 feet.
Note the inches (bolded) cancel and the expression leaves the unit we want to keep (feet) to stay so the answer has the unit of feet.