Calculating molar mass and number of moles (worked example) (video) | Khan Academy

Video transcript

– [ Instructor ] We are asked to calculate the numeral of moles in a 1.52 kilogram sample of glucose. So comparable always, pause this video recording and try to figure this out on your own and this periodic table of elements will prove utilitarian. All right, now if we’re trying to figure out the count of moles, remember, mole is in truth, you can view it as a measure of something. If I said a twelve of something, you ‘d say oh, that ‘s 12 of that thing. If I say a mole of something, I ‘m saying that ‘s Avogadro’s number of that thing. And so we have a 1.52 kilograms sample distribution of our molecule in motion, of glucose so if we can figure out the mass per gram molecule, or another manner to think about it, the molar mass of glucose, well then we just divide the mass of our sample by the bulk per mole and we ‘ll know how many moles we have. So what is the molar bulk of glucose ? Well to figure that out, and that ‘s why this periodic table of elements is utilitarian, we merely have to figure out the molar batch of the component elements. then if we first look at carbon paper, carbon paper, we see from this periodic table of elements, has a molar mass of 12.01 grams per mole. We ‘ve talked about it in early videos, you could view this 12.01 as a relative atomic multitude of a carbon atom, of as the average nuclear mass of a carbon paper atom, or what ‘s useful, and this is where Avogadro ‘s Number is valuable, if you have Avogadro ‘s Number of carbons, it is going to have a batch of 12.01 grams. So carbon paper has a molar multitude of 12.01 grams per mole and now we can think about hydrogen in the same direction. Hydrogen has a molar mass of 1.008 grams per mole, 008 grams per counterspy. And then last but not least, we have oxygen here. Oxygen, we can see from our periodic table of elements, it has a molar mass of 16.00 grams per mole. And so nowadays we have all the information we need from our periodic table of elements. So the molar mass of glucose is going to be six times the molar mass of carbon plus 12 times the molar bulk of hydrogen plus six times the molar mass of oxygen. So it ‘s going to be six times 12.01 grams per mole plus 12 times 1.008 grams per gram molecule plus every atom of glucose has six oxygen plus six times 16.00 grams per gram molecule. Six times 12.01 plus 12 times 1.008 plus six times 16 is equal to, and if we ‘re thinking about meaning figures here, the molar mass of hydrogen goes to the thousandth rate but we only go to the hundredths for carbon and for oxygen, we ‘re adding all of these up together so it ‘s going to be 180. I can alone go to the hundredth place for significant figures, so 180.16. So that ‘s adequate to 180.16 grams per breakwater. And we could say grams of glucose, C6H12O6 per gram molecule of glucose, C6H12O6 and then we can use this 1.52 kilograms to figure out how many moles we have. sol if we start off with 1.52 kilograms of glucose, so that ‘s C6H12O6, well first we can convert it to grams ’cause hera, our molar bulk is given in terms of grams, so time, we ‘re going to want kilograms in the denominator and grams in the numerator, so how many grams are there per kilograms ? well, we have 1,000 grams for every one kilogram. so when you multiply these two out, this is going to give you the total of grams we have of glucose which would be 1,520 and if you have your batch in terms of grams, you can then divide by your molar mass or you can view it as multiplying it by the moles per gram. therefore for every one gram molecule of glucose, C6H12O6, we have 180.16 grams of glucose, C6H12O6, and this is going to get us, we get 1.52 times 1,000 is equal to, this is the number of grams of glucose we have, and then we ‘re going to divide by 180.16, separate by 180.16, gives us this number, and let ‘s see, if we see significant figures, we have three meaning figures here, we have five hera so we wan na round it to three meaning figures, so it will be 8.44 moles of glucose. indeed our kilograms cancel with our kilograms and then our grams of glucose natural with our grams of glucose and we are left with 8.44 moles of glucose, moles of C6H12O6. And we are done.

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