Yes, and if you posted the question about Cr and Al, I gave specific details there. In general, however, the mole ratio is nothing more than a conversion factor. Let me give an example but in inches to cm. How many cm in 10 inches?
We know there are 2.54 cm in 1 inch, so the conversion factor is one of these two factors
(1 inch/2.54 cm) or (2.54 cm/1 inch).
We start with what we have, in this case, 10 inches. We want to convert 10 inches to cm.
10 inches x factor = ?? cm. All we need to do is to insert one of the two factors, but how do we know which to use? Easy. We want the inches unit to cancel (the unit we don't want) and we want to have cm left (the unit we want to keep).
10 inches x (2.54 cm/1 inch) = 25.4 cm. Note that we placed the 1 inch in the denominator so it will cancel with the inches (from 10 inches) in the numerator. So the inches unit is gone and we are left with cm which is what we want. What if we decide to place the factor the other way? 10 inches x (1 inch/2.54 cm) = 3.94 inches2/cm. The unit is not cm, the unit is some strange duck called inches2/cm and I have no idea what that unit is. That's how these are done. For your question, where do you get the factor? The factor is the ratio of the coefficients in the balanced equation.
The equation 2H2 + O2 ==> 2H2O and we have 3 moles H2 and want to convert to moles O2.
3 moles H2 x (1 mole O2/2 moles H2) =
3 x (1/2) = 1.5 mols oxygen.