![]() ![]() This is repeated until there is a term for each isotope. The weighted average is determined by multiplying the percent of natural abundance by the actual mass of the isotope. The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. Why? The reason is that we need to take into account the natural abundance percentages of each isotope in order to calculate what is called the weighted average. And we are done.= 35.968 \,u \nonumber \]Īs you can see, the average atomic mass given in the last column of the table above (35.453) is significantly lower. Or we could say 40% or 40.00% carbon by mass when we round to four significant figures. So this will be, I'll say, approximately equal to 0.4000. 72.06 divided by 180.156 is equal to, and if we round to four So we just have to calculate this and round to four significant figures. So this four significant figures is our significantįigures limiting factor. Than the hundredths place, but even if we rounded over there for significant figures purposes, we would still have at least four, we'd actually have five To the hundredths place, and so when we add things together, we should get no more Because even down here, if we were just doing thisīlue calculation here, that should only haveįour significant figures, it would have gotten us So we can type this into a calculator but we should remind ourselves that our final answer should have no more than four significant figures. Did I do that right? If I were just to add up everything, not even think about significant figures. Six times 16 is 96.00, and this will be equal to 72, if we're just thinkingĪbout the pure calculation, before we think about significant figures, 72.06 divided by, let's see, if I add 72 to 12, I get 84, plus 96, I get 180.156. So in the denominator, we have 72.06 plus, let's see, 12 times 1.008 is 12.096, and then we have plus Pure calculation first, and then I'm gonna worryĪbout significant figures. And then in the denominator, I'm just going to do the ![]() So let's see, in the numerator, six times 12.01 is 72.06. The units will cancel out, and we'll get a pure Grams in the numerator, grams in the denominator, Here, the units cancel out, so we're left with just Plus six moles of oxygen, times the molar mass of oxygen, which is going to be 16.00 So 12 moles of hydrogen times the molar mass of hydrogen, which is going to be 1.008 To that, we are going to add the mass of 12 moles of hydrogen. We just had up here, it's going to be six moles of carbon times the molar mass of carbon, 12.01 grams per mole of carbon. Mass of six moles of carbon, 12 moles of hydrogen,Īnd six moles of oxygen. ![]() And then in the denominator, what are we going to have? Well, the mass of one mole of glucose, for every glucose molecule, we have six carbons, 12 If the average atomic mass is 12.01 universal atomic mass units, the molar mass is going to be 12.01 grams per mole of carbon. Well, what's that going to be? Well, we can get that from the average atomic mass of carbon. Now, what is this going to be? Well, this is going to be equal to, it's going to be in our numerator, we're going to have six moles of carbon times the molar mass of carbon. Times as many carbon atoms or six moles of carbon. Glucose is because this, if we assume this is a mole of glucose, every molecule of glucose has six carbons. Why it's six moles of carbon divided by one mole of Is going to be the mass of six moles of carbon divided by the mass of one mole of glucose? And once again, the reason So one way we could think about it is, we say okay, for every mole of glucose, we have six moles of carbon. Let's just assume that this is a mole, this is a mole of glucose. But to help us think this through, we can imagine amount. Of glucose doesn't matter is because the percent carbon by mass should be the same You the average atomic masses of carbon, hydrogen, and oxygen. Percentage of carbon by mass of my sample? Pause this video and think about it. I'm not even gonna tell you its mass, but based on the molecular formula, can you figure out the And so let's just say that I had a sample of pure glucose right over here, this is my little pile of glucose. So right over here, I have the molecular formula for glucose. ![]()
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