The taste of sour milk is lactic acid.What is the molecular formula for lactic acid if the percent composition is 40.00% C,6.71% H,53.29% O,and the approximate molar mass is 90 g/mol?

Question

Quizplus · Accepted Answer

We need to determine the molecular formula for lactic acid.

Step 1: Convert the percent composition to grams:

40.00% C -> 40.00g C 
6.71% H -> 6.71g H 
53.29% O -> 53.29g O

Step 2: Convert the grams to moles:

40.00g C / 12.01 g/mol = 3.33 mol C 
6.71g H / 1.01 g/mol = 6.64 mol H 
53.29g O / 16.00 g/mol = 3.33 mol O

Step 3: Divide each mole value by the smallest mole value to get the simplest mole ratio:

C: 3.33 mol / 3.33 mol = 1 
H: 6.64 mol / 3.33 mol = 1.99 -> 2 
O: 3.33 mol / 3.33 mol = 1

Step 4: Write the empirical formula:

CH₂O

Step 5: Determine the molecular formula using the approximate molar mass:

Empirical formula mass = 30.03 g/mol 
Molecular formula mass = 90 g/mol

Molecular formula mass / empirical formula mass = 90 g/mol / 30.03 g/mol = 2.996

Round to the nearest whole number: 3

Molecular formula = (CH₂O)₃ = C₃H₆O₃

The Taste of Sour Milk Is Lactic Acid