Question

For the reaction shown, 38.25 grams of silicon is combined with 14.33 grams of nitrogen gas. How many grams of silicon nitride can be formed if nitrogen is the limiting reactant? 3Si + 2N2 yields Si3N4 17.94 g 21.23 g 35.88 g 63.69 g

Answer 1

From the periodic table:
molar mass of silicon = 28 grams
molar mass of nitrogen = 14 grams

From the balanced chemical equation, 
3 x 28 = 84 grams of silicon react with 2 x 2 x 14 = 56 grams of nitrogen to produce 3(28) + 4(14) = 140 grams of silicon nitride.

Since it is mentioned that nitrogen is the limiting reagent, therefore, the reaction will stop once the amount of reacting nitrogen is consumed.
Now we know that 56 grams of nitrogen produce 140 grams of silicon nitride. Thus, we can use cross multiplication to calculate the amount of silicon nitride produced when using 14.33 grams of nitrogen as follows:

amount of produced silicon nitride = (14.33 x 140) / 56 = 35.8 grams