#194.2/97~~2#, so we have to multiply the numbers in the empirical formula by 2 to get our molecular formula:įor a start we establish the #"empirical formula"# of caffeine.Īnd to this end we assume #100*g# of compound and interrogate its atomic composition by dividing the mass of the individual elements by the atomic mass of each element: #(12*4)+(1*5)+(14*2)+(16*1)=97# which is not what we were given, so we take what we were given and divide it by this number: Now we have to check the molar mass of the empirical formula to make sure it matches up with the molar mass given: Since all the numbers are already whole we can skip the last part and go to an empirical formula: Now we divide all the moles by the smallest number of moles, in this case the 1.03 from Oxygen. Now we take those numbers and convert them to moles of the element we are dealing with, so: We assume we have a 100g sample of caffeine, and so 49.48% Carbon is equal to 49.48 g of Carbon in the sample, 5.15% Hydrogen is equal to 5.15 g of Hydrogen in the sample, and so on. So first we want to take the percents and make them masses. Percent to mass mass to mole divide by small multiply 'til whole
The way I tackle these problems is using a process that goes like this:
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