Question

Menthol (molar mass = 156.3 g/mol), the strong-smelling substance in many cough drops, is a compound of carbon, hydrogen and oxygen. When 0.1595g of menthol was burned in a combustion apparatus, 0.449g of CO2 and 0.184g of H2O formed. What is menthol’s molecular formula?

Answer 1

Answer: The molecular formula for the menthol is
`C_(10)H_(20)O`

Step-by-step explanation:

The chemical equation for the combustion of hydrocarbon having carbon, hydrogen and oxygen follows:

`C_xH_yO_z+O_2\rightarrow CO_2+H_2O`

where, 'x', 'y' and 'z' are the subscripts of Carbon, hydrogen and oxygen respectively.

We are given:

Mass of
`CO_2=0.449g`

Mass of
`H_2O=0.184g`

We know that:

Molar mass of carbon dioxide = 44 g/mol

Molar mass of water = 18 g/mol

• For calculating the mass of carbon:

In 44g of carbon dioxide, 12 g of carbon is contained.

So, in 0.449 g of carbon dioxide,
`(12)/(44)* 0.449=0.122g` of carbon will be contained.

• For calculating the mass of hydrogen:

In 18g of water, 2 g of hydrogen is contained.

So, in 0.184 g of water,
`(2)/(18)* 0.184=0.0204g` of hydrogen will be contained.

• Mass of oxygen in the compound = (0.1595) - (0.122 + 0.0204) = 0.0171 g

To formulate the empirical formula, we need to follow some steps:

• Step 1: Converting the given masses into moles.

Moles of Carbon =
`\frac{\text{Given mass of Carbon}}{\text{Molar mass of Carbon}}=(0.122g)/(12g/mole)=0.0102moles`

Moles of Hydrogen =
`\frac{\text{Given mass of Hydrogen}}{\text{Molar mass of Hydrogen}}=(0.0204g)/(1g/mole)=0.0204moles`

Moles of Oxygen =
`\frac{\text{Given mass of oxygen}}{\text{Molar mass of oxygen}}=(0.0171g)/(16g/mole)=0.00107moles`

• Step 2: Calculating the mole ratio of the given elements.

For the mole ratio, we divide each value of the moles by the smallest number of moles calculated which is 0.00107 moles.

For Carbon =
`(0.0102)/(0.00107)=9.53\approx 10`

For Hydrogen =
`(0.0204)/(0.00107)=19.54\approx 20`

For Oxygen =
`(0.00107)/(0.00107)=1`

• Step 3: Taking the mole ratio as their subscripts.

The ratio of C : H : O = 10 : 20 : 1

Hence, the empirical formula for the given compound is
`C_(10)H_(20)O_1=C_(10)H_(20)O`

For determining the molecular formula, we need to determine the valency which is multiplied by each element to get the molecular formula.

The equation used to calculate the valency is :

`n=\frac{\text{molecular mass}}{\text{empirical mass}}`

We are given:

Mass of molecular formula = 156.3 g/mol

Mass of empirical formula = 156 g/mol

Putting values in above equation, we get:

`n=(156.3g/mol)/(156g/mol)=1`

Multiplying this valency by the subscript of every element of empirical formula, we get:

`C_((1* 10))H_((1* 20))O_((1* 1))=C_(10)H_(20)O`

Thus, the molecular formula for the menthol is
`C_(10)H_(20)O`