 This one says, because it has a non-fluidine combustion product, which is water vapor, hydrogen gas is used for fuel above the space shuttles in earthbound cars with prototype engines in the reaction that's shown up here. So let's express the rate in terms of changes of hydrogen gas concentration, oxygen gas concentration, and water gas concentration over time. So how do we do that? Remember, the rate itself, in this case, with hydrogen. Can you guys help me out? What is it going to be? Negative? 1 1⁄2. 1⁄2. Very good. Times what? H. The change of the concentration of H2 divided by? Shift of time. Yep. L of T. Everybody OK with doing something like that? So what would be, what's the next one it wants us to do? Oxygen? So what would oxygen be? Somebody else help me? L of T, over the change. Times, yeah, a negative 1, right? Like that. Is everybody OK with that? So notice what I'm doing here. This is equivalent to that. So we can actually set these two equal to each other if we want it to 2. And that's what we're going to do in half feet of this problem. OK, so let's do water vapor. What is it going to be? So it's going to be also equal to what? 1⁄2. Yes. And the change in? The water vapor. Over the change in time. OK. And then it says when oxygen is decreasing at 0.23 moles per liter second, so the concentration of oxygen is decreasing at 0.0, 0.23 moles per liter second. How fast is water increasing? So how do we do this problem? Well, let's just erase this part here, because we have an equation here. So this, this. So we're looking for, so delta T is going to be the same. So we can cancel that out on each side. You guys see that? Delta T is equivalent to delta T. So effectively, what do we got? We got negative change of 0.2 equals 1⁄2 times the change of H2. Is everybody OK with that? OK, what do we know about the change in O2 concentration? It's going to be negative 0.23 moles per liter second. So what would be the change in water concentration? So negative 2 times the change in O2 concentration. So everybody OK with us doing that? When we do that? Negative 2 times the negative 0.23 moles per liter second. Would you expect the water concentration to be increasing when the oxygen was decreasing? Yes. So is this going to be the correct answer? That's positive. It's going to be 0.46 moles per liter second. Is everybody OK with doing something like that? Any questions? That's wonderful. Good job, guys.