How many moles of silicon are in 4.60 g of Si? This is the first venture into one of the most common chemical calculations, finding the number of moles present in a given mass of substance. Restate it: 4.60 g Si <=> ? mol Si The atomic mass (you need this for each element in your problems, too) of silicon, 28.09 (rounded from 28.0855), gives us the needed tool, because we know from the definition of the mole that the following is true about silicon. 28.09 g Si <=> 1 mol Si Like any equivalency, this one makes available two conversion factors. One will be the tool we need for our grams-to-mole conversion. 28.09 g Si/1 mol of Si and 1 mol Si/28.09 g Si or:
28.09 g Si and 1 mol Si
1 mol Si 28.09 g Si
Thus if we multiply the given, 4.60 g Si, by the second conversion factor, we will obtain the answer. Solution: We carry out the following calculation. Notice how the units, g Si, cancel to leave the correct unit, mol Si (Draw in the cancel lines yourself).
4.60 g Si x 1 mol Si/28.09 g Si = 0.164 mol Si
In other words, 4.60 g Si <=> 0.164 mol Si. Now, check to see if the answer makes sense. Is its numerical size in the right range? Yes, because we began with 4.60 g Si, clearly less than a whole mole of silicon, so the answer has to reflect this obvious fact. The atomic mass=1 mole.
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