 Okay, so let's try this problem again. I'm just going to reread it and we have all the information there So it says a sample of mercury with a mass of 114.0 grams is combined With a 12.8 grams of oxygen gas and the resulting reaction gave 123.1 grams of mercury 2-Oxygen Oxygen was left over after the reaction was complete. Okay, so this is conservation of mass problem And we just did this problem in kind of a long step-by-step way. Okay, but we can do it a much quicker way Because we know in the conservation of mass the mass of the products has or the mass reactants has to equal the mass of the products right so so We know what the mass of the products is okay And we know that all of the oxygen gas didn't react Okay, is that cool? So let's just add these two things together and figure out what that mass is So 114 plus 12.8 So 126.8 grams does that equal 123.1 grams? No Okay, so Since we know that it's oxygen that's gone That's missing all we really have to do in this problem because of what they told us is take 126 and subtract 123.1 grams from it 0.7 grams So there's a 3.7 gram difference between the mass of the products and the mass of the reactants Does that make sense? Okay, so Since that's the case and we know that all the oxygen didn't react but all of this reacted then this difference must be that oxygen so this is how much Was left over or unreacted. I can't exactly remember what the problem says, but that's how much O2 is left unreacted Make sense? Okay, yeah, I told you It's good to go the other way too, you know because it's like a step by step and you're just like okay Now I understand this part of the process now understand this part of the process because they won't always tell you That oxygen was the way first then the other way is more comprehensive. Yeah, exactly. You can Switch over back and forth go back and forth watch these videos a couple times. Okay, any other questions on this one? I'm glad I'm glad we kind of knocked it out though