Question

When 125.0 g of a compound is decomposed, it is found to yield

50.00 g of C, 8.25 g of H, and 66.75 g of O. The molecular weight
is found to be 90.0. What is the molecular formula of this
compound?
A

Answer 1

First, we need to find the number of moles of each element in the compound:

- For Carbon (C): 50.00 g / 12.01 g/mol = 4.16 mol

- For Hydrogen (H): 8.25 g / 1.008 g/mol = 8.18 mol

- For Oxygen (O): 66.75 g / 16.00 g/mol = 4.17 mol

Next, we divide each of these by the smallest number of moles to get the ratio of the elements:

- For C: 4.16 mol / 4.16 = 1

- For H: 8.18 mol / 4.16 = 1.97 (which we can round to 2)

- For O: 4.17 mol / 4.16 = 1

So, the empirical formula (the simplest, most reduced ratio) is CH2O.

The empirical formula mass of CH2O is (12.01 g/mol * 1) + (1.008 g/mol * 2) + (16.00 g/mol * 1) = 30.026 g/mol.

The molecular formula is a multiple of the empirical formula. We find this multiple by dividing the molecular weight by the empirical formula mass: 90.0 g/mol / 30.026 g/mol = 2.99, which we can round to 3.

Therefore, the molecular formula, which is 3 times the empirical formula, is C3H6O3.