Given Information:
- The acid contains C, H, and O.
- The acid contains 26.7% carbon (C) and 2.2% hydrogen (H).
- The vapor density of its ethyl ester is 73.

Approach:
To determine the molecular formula of the acid, we can follow these steps:
1. Calculate the empirical formula of the acid.
2. Calculate the empirical formula mass.
3. Calculate the molecular formula mass.
4. Determine the molecular formula using the molecular formula mass and empirical formula mass.

Calculating the Empirical Formula:
1. Assume we have 100g of the acid.
2. Determine the mass of carbon (C) in the acid.
- Mass of C = 26.7% of 100g = 26.7g
3. Determine the moles of carbon (C).
- Moles of C = Mass of C / Atomic mass of C = 26.7g / 12.01g/mol ≈ 2.22 mol
4. Determine the mass of hydrogen (H) in the acid.
- Mass of H = 2.2% of 100g = 2.2g
5. Determine the moles of hydrogen (H).
- Moles of H = Mass of H / Atomic mass of H = 2.2g / 1.01g/mol ≈ 2.18 mol
6. Determine the moles of oxygen (O) by difference.
- Moles of O = Total moles - Moles of C - Moles of H
= 1 mol - 2.22 mol - 2.18 mol ≈ -3.4 mol
Since the moles of oxygen are negative, it indicates an error in the given data.

Calculating the Empirical Formula Mass:
1. Determine the molar mass of carbon (C).
- Molar mass of C = Atomic mass of C = 12.01g/mol
2. Determine the molar mass of hydrogen (H).
- Molar mass of H = Atomic mass of H = 1.01g/mol
3. Determine the molar mass of oxygen (O).
- Molar mass of O = Atomic mass of O = 16.00g/mol
4. Calculate the empirical formula mass.
- Empirical formula mass = (Moles of C × Molar mass of C) + (Moles of H × Molar mass of H) + (Moles of O × Molar mass of O)

Calculating the Molecular Formula:
1. Determine the vapor density of the ethyl ester.
2. Calculate the empirical formula mass of the ethyl ester.
- Empirical formula mass of ethyl ester = Vapor density × (Molar mass of H2 / Molar mass of ethyl ester)
3. Calculate the molecular formula mass of the ethyl ester.
- Molecular formula mass of ethyl ester = Empirical formula mass of ethyl ester × n (where n is an integer)
4. Determine the molecular formula by comparing the molecular formula mass and empirical formula mass.

Conclusion:
Since the given data is incomplete, with negative moles of oxygen, it is not