Hydrogen was the standard in the  1800s.
Then it was changed  to oxygen 16 because oxygen forms many more compounds. Later, it was discovered that oxygen had isotopes that varied with geographical region.
Thereafter a new standard was debated over between fluorine which had one isotope and carbon which had two, but the C13 was very low percent and in fixed proportion. Carbon 12 was chosen because it was safer to handle than fluorine.

hi i am having trouble getting the correct answer to this question.
the molar mass of aspirin is 180.2g/mol. how many aspirin molecules are present in one 500 miligram tablet.
here is how i tried to solve it:
1mol aspirin/180.2g aspirinx1g aspirin/1000 mg aspirinx500mg aspirin= 2.77x10^3.

I have no idea what you did

0.5 g aspirin is 0.5/180.2 mole. Once you know number of moles, multiply this number by Avogadro constant.