what is the molecular formula?

Question

Lactic acid, also known as milk acid, is a chemical compound that plays a role in several biochemical processes. It has a molecular formula mass of 90.0 amu and a percent composition by mass of 40.0% carbon, 6.71% hydrogen and 53.3% oxygen. What is the molecular formula of lactic acid?

Answer 1

There is a step-by-step procedure for solving problems of this kind.

1. Assume you have 100 grams of the compound; then, you will have 40 grams of carbon, 6.71 g of hydrogen, and 53.3 grams of oxygen.
2. Divide those weight results by the atomic weights of each element. In this case, you get C= 3.33, H = 6.71, and O = 3.33.
3. Divide each number in step 2 by 3.33 to get the smallest whole number ratio which is CH₂O.
4. Let x be the number of atoms of carbon, 2x the number of atoms of hydrogen, and x the number of atoms of oxygen. Solve the equation 12x + 2x + 16x = 90 where the numerical coefficients are the approximate atomic weight of each element; then, C_xH_2xO_x will be the molecular formula.