 Okay, so let's try one of these law of definite composition problems. What is the mass percent of carbon and calcium carbon? So the first thing you want to do is write down the formula for calcium carbonate And what we're looking for So then we want to remember the formula for mass percentage, okay? So first mass percentage if you recall is going to be percent mass equals The mass of what you're looking for So you can take a bit as mass x in this case the general formula over the over what? total mass Times what? times 100% Right, very important that you put the percent there because the percent is your actual units for this problem Okay, if you don't put you remember in chemistry you don't put units then your answer is going to be wrong, okay? So in this case right x is going to be what? carbon and the total is going to be what? CaCO3 for calcium carbon, okay, so how will we figure out the masses of these two things? Can you guys help me out? Look at the periodic table, okay, so Periodic table tells us what the mass of carbon is what? 12.01, okay, what what are we missing here? It's not grams, right? Is that a huge carbon atom? Imagine something that's 12.01 good, AMU, okay? So remember just because the periodic table doesn't say it you got to remember in this case They're going to cancel out AMU is going to cancel out the AMU, but it's still important Okay, how will we figure out calcium carbonate? How do we do? Look at the periodic table and add all the stuff together, okay? So calcium carbonate is going to be calcium as what? 40.01, right, or 40.08, sorry Carbon, it can help me out anytime, point to it, and oxygen 16.00 AMU but Times three, we want to times that number by three, and I'm just using a periodic table that goes to two Decimal points are two numbers after the decimal point. Is that okay with everybody? Okay, so Significant figures on this will be two after the decimal point So three times And I get 100.09 AMU So is that the right number of sick things even though it's five sick things and every one of our numbers here has four? Yes, it is because it's the two numbers after the decimal that's important, okay? Okay, so that's the total mass And how many sick things would our answer be to in this case? 12.00% Everybody okay with that question?