Citric acid, the compound responsible for the sour taste of lemons, has the following elemental composition: C, 37.51%; H, 4.20%; O, 58.29%.
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Ok so basically you're going to pretend you have 100 grams of the stuff.
That means in the pile of 100 grams:
37.51g is C
4.2g is H
58.29g is O
Now divide each number of grams by the molecular mass of the atom. Doing this you end up with:
37.51/12.01 = 3
4.2/1.008 = 4
58.29/ 15.999 = 3.65
Now the values above are not exact, but close enough to the ratio of the molecular formula of citric acid. To obtain the formula you want all whole numbers, because 3.65 is roughly 3.5 you can multiply all the values by 2 (so that 3.5 becomes a whole number; 7) to yield:
3x2 = 6 C
4x2 = 8 H
and 3.5x2 = 7 O
C6H8O7 is your answer.
That means in the pile of 100 grams:
37.51g is C
4.2g is H
58.29g is O
Now divide each number of grams by the molecular mass of the atom. Doing this you end up with:
37.51/12.01 = 3
4.2/1.008 = 4
58.29/ 15.999 = 3.65
Now the values above are not exact, but close enough to the ratio of the molecular formula of citric acid. To obtain the formula you want all whole numbers, because 3.65 is roughly 3.5 you can multiply all the values by 2 (so that 3.5 becomes a whole number; 7) to yield:
3x2 = 6 C
4x2 = 8 H
and 3.5x2 = 7 O
C6H8O7 is your answer.