### Percentages When Solving for Molecular Formula

Posted:

**Sun May 06, 2018 5:10 pm**The following question is #22 from module 1:

Xylitol, a sugar substitute, has a mass composition of C 39.43% O 52.58% H 7.88% and a molar mass of 152.15 g.mol-1 What is its molecular formula?

My question is, how do we know when we should divide the percentages by 100? From what I've read online we can only assume that there are 100g of the sample when we are not given a specific molar mass and thus, use the numbers 39.43g C, 52.58g O, and 7.88g H. In this example however, we are still required to use 39.43g C, 52.58g O, and 7.88g H instead of the numbers we would get if we divide by 100 (.3943g C, .5258gO, & .0788gH) even though we are given the molar mass. Why are we assuming that the sample is out of 100g when we are explicitly told otherwise?????

Someone plz help sos

Xylitol, a sugar substitute, has a mass composition of C 39.43% O 52.58% H 7.88% and a molar mass of 152.15 g.mol-1 What is its molecular formula?

My question is, how do we know when we should divide the percentages by 100? From what I've read online we can only assume that there are 100g of the sample when we are not given a specific molar mass and thus, use the numbers 39.43g C, 52.58g O, and 7.88g H. In this example however, we are still required to use 39.43g C, 52.58g O, and 7.88g H instead of the numbers we would get if we divide by 100 (.3943g C, .5258gO, & .0788gH) even though we are given the molar mass. Why are we assuming that the sample is out of 100g when we are explicitly told otherwise?????

Someone plz help sos