Now stoichiometric rate calculations happen when the rate of one compound is known and the rate of another can be calculated using a rate to rate comparison. So here in this example it says if the rate of decomposition of H2 is 1.54 molarities per minute at a particular time, what will be the rate of formation of nitrogen gas at that same time? So they're giving us the rate of one compound and asking for the rate of another within a balanced chemical equation. This is a key giveaway that you're dealing with a stoichiometric rate calculation.
Now here step one. If the rate of change for one compound is not given, then first calculate it using information provided here. We don't have to worry about that because we're given the rate of H2 right from the beginning. It's 1.54 molarities per minute. Now Step 2, using the rate of one compound, perform a rate to rate comparison using stoichiometric coefficients. Now this is similar to a mole to mole comparison that we've used before in stoichiometry.
Now the way we'd set it up is we have 1.54 molarity per minute and this is for H2. And remember in stoichiometry we do a multiple comparison, but now we're doing a rate to rate comparison. So we're going to say according to our balanced chemical equation, we have two here for H2. That two would mean 2 molarities per minute for H2 and then for N2. There's a one here, so that's one molarities per minute for N2. So here our units will cancel out and look, we're going to have the rate of N2.
So when we plug that in, we get 0.770 molarities per minute for N2. So this would be the rate of nitrogen gas. So again it's kind of reminiscent a little bit of stoichiometry, but instead of doing a mole to mole comparison, we're doing a rate to rate comparison. So approach it in the same manner, and you'll always be able to find the rate of any compound within a balanced equation if they give you a rate of another compound within that same balanced equation.
