An unknown organic compound is 40.00% C, 6.67% H and 53.33% O by mass. The formula weight of the unknown is 90.0g/mole. Find the empirical and molecular formulas for the unknown compound.

Take a 100 g sample which gives you

40.00 g C
6.67 g H
53.33 O
Convert to mols.
40.00/atomic mass C = ?
6.67/atomic mass H = ?
53.33/atomic mass O = ?

Now find the ratio of thre elements to one another with the smallest number being 1.000. The easy way to do that is to divide the smallest number by itself, then divide the other two numbers by the same small number. That will give you the empirical formula. Calculate the mass of the empirical formula. Then empirical formula mass x ?whole number = 90.0. Solve for the whole number (if it isn't EXACTLy whole then round to the nearest whole number, and multiply the empirical formula by that number; i.e., (CxHyOz)whole number
I believe that whole number will be 3 and that gives you the molecular formula.

To find the empirical formula for the unknown compound, we need to determine the ratio of elements in the compound using the mass percentages given.

1. Assume we have a 100g sample of the compound. This means we have 40.00g C, 6.67g H, and 53.33g O.

2. Convert the masses of each element into moles using their respective molar masses. The molar masses are 12.01 g/mol for carbon, 1.01 g/mol for hydrogen, and 16.00 g/mol for oxygen.

- Moles of C = 40.00g C / 12.01 g/mol = 3.33 moles
- Moles of H = 6.67g H / 1.01 g/mol = 6.60 moles
- Moles of O = 53.33g O / 16.00 g/mol = 3.33 moles

3. Divide the number of moles of each element by the smallest number of moles calculated in step 2 to get the simplest whole-number ratio.

- Simplest ratio of C = 3.33 moles / 3.33 moles = 1
- Simplest ratio of H = 6.60 moles / 3.33 moles = 1.98 (approximately 2)
- Simplest ratio of O = 3.33 moles / 3.33 moles = 1

4. The empirical formula is C₁H₂O₁.

To determine the molecular formula, we need to know the formula weight of the compound.

The formula weight given is 90.0 g/mol.

5. Calculate the empirical formula mass of the compound. The empirical formula mass is the sum of the atomic masses in the empirical formula.

- Empirical formula mass = (C₁H₂O₁) = (12.01 g/mol) + (2 × 1.01 g/mol) + (16.00 g/mol) = 30.03 g/mol

6. Divide the formula weight by the empirical formula mass to obtain the whole-number ratio.

- Molecular formula mass = 90.0 g/mol / 30.03 g/mol = 2.996 (approximately 3)

7. Multiply the empirical formula by the whole-number ratio calculated in step 6 to get the molecular formula.

- Molecular formula = (C₁H₂O₁) × 3 = C₃H₆O₃

Therefore, the empirical formula of the unknown compound is C₁H₂O₁, and the molecular formula is C₃H₆O₃.

To find the empirical and molecular formulas for the unknown compound, we need to follow a step-by-step process:

Step 1: Determine the empirical formula.
The empirical formula gives the simplest whole-number ratio of atoms in a compound. To find the empirical formula, we need to convert the mass percentages of elements into moles.

Given:
Mass percentage of carbon (C) = 40.00%
Mass percentage of hydrogen (H) = 6.67%
Mass percentage of oxygen (O) = 53.33%

Assume we have a 100g sample of the compound. We can find the mass of each element in grams using the given percentages:
Mass of carbon (C) = 40.00g
Mass of hydrogen (H) = 6.67g
Mass of oxygen (O) = 53.33g

Next, we need to calculate the number of moles for each element. We use their atomic masses to convert grams to moles:
Molar mass of carbon (C) = 12.01 g/mol
Molar mass of hydrogen (H) = 1.01 g/mol
Molar mass of oxygen (O) = 16.00 g/mol

Number of moles of carbon (C) = Mass / Molar mass = 40.00g / 12.01 g/mol = 3.332 moles
Number of moles of hydrogen (H) = Mass / Molar mass = 6.67g / 1.01 g/mol = 6.604 moles
Number of moles of oxygen (O) = Mass / Molar mass = 53.33g / 16.00 g/mol = 3.333 moles

To get the simplest whole-number ratio, divide each element's mole value by the smallest mole value:
Carbon (C): 3.332 moles / 3.332 moles = 1
Hydrogen (H): 6.604 moles / 3.332 moles = 1.982 ≈ 2
Oxygen (O): 3.333 moles / 3.332 moles = 1

Therefore, the empirical formula of the unknown compound is CH2O.

Step 2: Determine the molecular formula.
The empirical formula provides the ratio of atoms, but not the actual number of atoms in a molecule. To find the molecular formula, we need additional information—the molar mass of the unknown compound.

Given:
Formula weight of the unknown compound = 90.0 g/mol

The formula weight is equal to the sum of the molar masses of the atoms in the molecular formula. Since the empirical formula weight is CH2O (12.01 g/mol + 1.01 g/mol x 2 + 16.00 g/mol = 30.03 g/mol), we can find the ratio of the empirical formula weight to the actual formula weight:
Ratio = Formula weight of the unknown compound / Empirical formula weight
Ratio = 90.0 g/mol / 30.03 g/mol ≈ 2.997

Since the ratio is approximately 3, we can multiply the empirical formula by this ratio to determine the molecular formula:
Molecular formula = (CH2O)3 = C3H6O3

Therefore, the empirical formula for the unknown compound is CH2O, and the molecular formula is C3H6O3.