this is a simple scaling problem. If 3 moles of H_{2} produce 2moles NH_{3. } We can write that as a ratio 2/3 but since we only have 2.5 moles H_{2} we need to find how many create the same ratio 2/3=x/2.5. if we multiply both sides by 2.5 we find x= 5/3 . So this would mean that we would get 5/3 moles NH_{3} when we used 2.5moles H_{2}.

For the second problem you need to compare the moles of N_{2} used to moles of H_{2} used --> so again we have 2/3=x/2.5.

The last two problems require that you convert grams into moles before finding the ratios that you need to solve and back to grams once you have found the moles....