The point is, in this case, you need to pay attention to which isotope of the compound you are using because each one would give a different final molar mass. on second thought, I don't think I should teach you how to purify Uranium for your home atomic bomb shenanigans so I will omit that section here. It contains Uranium which has an average weight closer to 235 g/mol. Sometimes you build compounds with an unnatural isotope distribution, for example, a nuclear bomb. formula weight, monoisotopic mass, molecular mass, molecular weight, atomic weight, etc.Strictly speaking what we have been discussing (and what you need for your exam) is the molar mass taking the actual formula of the compound you are interested in and adding up atomic masses. You might have even heard about some of them e.g. There are actually a whole bunch of different types of weights and masses used to describe atoms and it can be important to know the difference between them. The units for both measures are atomic mass units. Then we can calculate how heavy a mole of molecules is by adding up atomic masses. It's called the atomic mass (like the weight of an average grain of sand in the example above) and we use it to calculate how heavy a mole of those atoms is, i.e. So we invented something to deal with the issue. In this case, you would need 10 kg of sand per batch of cement.A chemist faces this issue on a daily basis: we know how many atoms we need, but it is impossible for us to count them out one by one. Then you can just calculate how many kilos of sand you need. Let's say you get 10 mg which means it's 1/10th of a mg per grain of sand. You can probably figure out how much an average grain of sand weighs by measuring 100 of them on a scale. Now you might be tempted to start and actually count it out because you are paid an hourly wage, but if you need to do it efficiently there is another way. Imagine you are working at a construction site and the engineer is a very precise man: he comes up with the estimate that you need to use 100 million grains of sand per batch of cement mixed.
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