SATHEE: Shortcut Methods

Shortcut Methods and Tricks to Solve Alcohol Numericals

Alcohols

JEE Numerical’s

Calculating the molecular weight of an alcohol: Use the atomic masses of carbon (12.01 g/mol), hydrogen (1.008 g/mol), and oxygen (16.00 g/mol) to find the total atomic mass of the alcohol. For example, the molecular weight of ethanol (CH3CH2OH) is:

$$(3 \times 12.01 \text{ g/mol}) + (7 \times 1.008 \text{ g/mol}) + (1 \times 16.00 \text{ g/mol}) = 46.07 \text{ g/mol}$$

$$Boiling \space Point = 78.5 + 1.9(number\space of \space carbon\space atoms)$$

For example, the boiling point of ethanol is:

$$78.5 + 1.9(2) = 83.3°\text{C}$$

$$Solubility = \frac{100}{1 + 0.04(number\space of\space carbon\space atoms)}$$

For example, the solubility of ethanol in water is:

$$\frac{100}{1 + 0.04(2)} = \frac{100}{1.08} = 92.6 \text{%}$$

$$pKa = 15.5 - \log(K_a)$$

where (K_a) is the acid dissociation constant of the alcohol. For example, the (pK_a) of ethanol is:

$$pKa = 15.5 - \log(1.3 \times 10^{-16}) = 15.5 + 15.89 = 31.39$$

CBSE Board Exam Numericals

Identifying the functional group of an alcohol: Look for the -OH group in the structural formula. If the molecule has a -OH group, it is an alcohol.
Writing the chemical formula of an alcohol: Determine the number of carbon, hydrogen, and oxygen atoms in the alcohol molecule and write the chemical formula accordingly. For example, ethanol has 2 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom, so its chemical formula is (C_2H_6O).
Drawing the structural formula of an alcohol: Draw a chain of carbon atoms and attach hydrogen atoms to each carbon atom. Then, attach the -OH group to one of the carbon atoms. For example, the structural formula of ethanol is:

CH3-CH2-OH

Naming an alcohol: Use the IUPAC nomenclature rules to name the alcohol. The root name of the alcohol is based on the number of carbon atoms in the longest carbon chain. The suffix “-ol” is added to the root name to indicate that the molecule is an alcohol. For example, the IUPAC name of ethanol is “ethanol”.