Question

How do you find the molecular formula of a compound?

Answer 1

Molecular formula:

1. A molecular formula is a chemical notation that indicates the number and kind of components present in a single molecule of a substance.
2. The simplest form of the molecular formula is the empirical formula. It can figure out what amounts each constituent in the molecule should be in.
3. The Molecular formula empirical$=$n$\times$empirical formula, where n is an integer multiple.

Example:

1. For calculating molecular formula let us take a compound having $49.48\%$carbon, $5.19\%$hydrogen,$16.48\%$ oxygen, and $28.85\%$ nitrogen. The molecular weight of the compound$=194.19\mathrm{g}{\mathrm{mol}}^{-1}$.

Step 1: Calculating the weight of the particular atom

Multiplying percent composition by weight we get,

Carbon$=194.19\times 0.4948\Rightarrow 96.0852$

Hydrogen$=194.19\times 0.0519\Rightarrow 10.0784$

Oxygen$=194.19\times 0.1648\Rightarrow 32.0025$

Nitrogen$=194.19\times 0.2285\Rightarrow 56.0238$

Step 2: Dividing each value by its atomic weight

Carbon$=\frac{96.0852}{12.011}\Rightarrow 7.9997$

Hydrogen$=\frac{10.0784}{1.008}\Rightarrow 9.998$

Oxygen $=\frac{32.0025}{15.9994}\Rightarrow 2.000$

Nitrogen$=\frac{56.0238}{14.0067}\Rightarrow 3.9997$

Step 3: Rounding off to the closest whole number:

Carbon$=7.997\approx 8$

Hydrogen$=9.998\approx 10$

Oxygen$=2$

Nitrogen$=3.997\approx 4$

Step 4: Writing molecular formula:

Write each value of the whole number along with its atom.

We get the molecular formula as${\mathrm{C}}_8{\mathrm{H}}_{10}{\mathrm{O}}_2{\mathrm{N}}_4$.