The taste of sour milk is lactic acid

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The taste of sour milk is lactic acid. What is the molecular formula for lactic acid if the percent composition is 40.00% of C, 6.71% of H, 53.29% of O and the approximate molar mass is 90 g/mol
a. CH2O
b. CHO2
c. C3H6O3
d. CHO
e. C6HO8

The correct answer and explanation is :

Answer: c. C₃H₆O₃

Explanation:

Lactic acid, responsible for the sour taste in fermented dairy products like sour milk, has the molecular formula C₃H₆O₃. To determine this, we can follow these steps:

  1. Calculate the Empirical Formula: Given the percent composition:
  • Carbon (C): 40.00%
  • Hydrogen (H): 6.71%
  • Oxygen (O): 53.29% Assuming a 100 g sample, the masses of each element are:
  • Carbon: 40.00 g
  • Hydrogen: 6.71 g
  • Oxygen: 53.29 g Convert these masses to moles:
  • Carbon: 40.00 g / 12.01 g/mol ≈ 3.33 mol
  • Hydrogen: 6.71 g / 1.008 g/mol ≈ 6.66 mol
  • Oxygen: 53.29 g / 16.00 g/mol ≈ 3.33 mol Determine the simplest whole-number ratio by dividing each by the smallest number of moles (3.33 mol):
  • Carbon: 3.33 mol / 3.33 mol = 1
  • Hydrogen: 6.66 mol / 3.33 mol ≈ 2
  • Oxygen: 3.33 mol / 3.33 mol = 1 This gives the empirical formula CH₂O.
  1. Determine the Molecular Formula: The empirical formula mass is:
  • C: 12.01 g/mol
  • H₂: 2.016 g/mol
  • O: 16.00 g/mol Total = 12.01 + 2.016 + 16.00 = 30.026 g/mol Given the approximate molar mass of lactic acid is 90 g/mol, calculate the ratio: 90 g/mol / 30.026 g/mol ≈ 3 Multiply the empirical formula by this ratio:
  • Carbon: C₃
  • Hydrogen: H₆
  • Oxygen: O₃ Thus, the molecular formula is C₃H₆O₃.

Structural Representation:

The structural formula of lactic acid is:

   HO-CH₂-CH(OH)-COOH

This structure consists of a three-carbon chain with a hydroxyl group (-OH) and a carboxyl group (-COOH) attached, characteristic of lactic acid.

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