Stanton D. answered • 02/26/21
Tutor to Pique Your Sciences Interest
Hi Joana S.,
The short answer is, follow the units. This has the fancy name "dimensional analysis". But essentially, if you take care of the units in a problem, the units take care of you.
How it works: always start with what you are given, here 10.0g ethanol. Now, what units does molar mass come with? It's g/mol, isn't it. So if you take (g)/ (g/mol) you end up with moles (the same thing as mol, units can take the "s" when you're talking English, but you never add the "s" or "es" when you're talking calculations, so that you don't ever confuse with s (time in seconds!). So divide your given mass, by the molar mass of ethanol, to get mol ethanol.
When you have your value for mol ethanol, you can go any direction! Multiply by Avogadro's Number to convert to #molecules, and then count the carbon atoms in the ethanol formula (there are 2 of them, I hope you agree?) and multiply by that factor to convert to total carbon atoms.
Key take-aways:
1) always carry your units through your calculations, they confirm that you've done the right thing;
2) Most chemistry problems require converting everything to moles, ratioing through using the coefficients of a balanced reaction, and possibly re-converting to mass for an answer.
-- CHeers, -- Mr. d.
