Question

Menthol, the substance we can smell in mentholated cough drops, is composed of C, H, and O. A 0.1005 g sample of menthol is combusted, producing 0.2829 g of carbon dioxide and 0.1159 g of water. What is the empirical formula for menthol? If mentho has a molar mass of 156 g/mol, what is its molecular formula?

Answer 1

Answer:

molecular formula = C₁₀H₂₀O

Explanation:

the carbon dioxide is obtained on combustion from an organic molecule due to reaction of carbon with oxygen and water is formed due to reaction of hydrogen with oxygen to form water.

The reactions are:

$C+O_{2}--> CO_{2}\\H_{2}+ 0.5O_{2} --->H_{2}O$

From above equations

i) 44g of carbon dioxide is formed from 12g of carbon

Therefore 0.2829g of carbon dioxide must be formed from = $\frac{12X0.2829}{44}=0.077 grams$of carbon

ii) 18g of water is formed from 2 grams of hydrogen

Therefore 0.1159 grams of water must be formed from =

$\frac{2X0.1159}{18}= 0.0128g$hydrogen

Total weight of sample taken = 0.1005

so mass of oxygen = 0.1005-(0.077+0.0129)=0.0106 g

moles of carbon = mass / atomic mass = 0.077/12=0.00641

moles of hydrogen = mass /atomic mass = 0.0128/1=0.0128

moles of oxygen = mass / atomic mass =0.0106/ 16=0.00066

Let us divide all the moles with the smallest one

moles of carbon = 0.00641 / 0.00066 = 10 (approx)

moles of hydrogen = 0.0128/0.00066 = 20 (approx)

moles of oxygen = 0.00066/0.00066 = 1

Empirical formula of menthol = C₁₀H₂₀O

Empirical mass =10X12 + 20X1 + 16= 156

Which is equal to molar mass hence the empirical formula = molecular formula