## M 19

Ian Morris 3C
Posts: 101
Joined: Wed Sep 18, 2019 12:18 am

### M 19

I am not sure how to get the mass of Oxygen. I was able to get the mass of C, H, and N, but am confused on what to do from here.

Hannah Lee 2F
Posts: 117
Joined: Thu Jul 11, 2019 12:15 am

### Re: M 19

After finding the mass of C, H, and N, add them all together to find the total mass. Then subtract that total mass from 0.376, which is the mass of caffeine burned. So, roughly: 0.376 - (0.1861 + 0.01947 + 0.1100) = 0.06042 g. This amount left over should be the mass of O, which you can then convert into moles by dividing by molar mass: 0.06042 g O / 16.00 g/mol = 0.0038 mol O.

Then you would divide all the moles by the smallest number (which in this case is O) and adjust to find the ratios of the empirical formula. Compare the empirical molar mass to the actual molar mass given in the problem—since it's different, you would divide the given molar mass by the empirical molar mass, then adjust the ratios of the empirical formula to find the molecular formula.

Rachel Yu 1G
Posts: 113
Joined: Fri Aug 09, 2019 12:15 am

### Re: M 19

You can get the mass of oxygen by subtracting from the total mass, since caffeine is made up of oxygen, carbon, hydrogen, & nitrogen and only the mass of oxygen is unknown. In the problem the given mass for caffeine is 0.376g. After finding the mass of C, H, and N (0.186g + 0.0195g + 0.110g), we can subtract this by 0.376g to isolate the mass of oxygen, 0.376g - (0.186g + 0.0195g + 0.110g) = 0.060g O