Name: Avogadro's Law and Its Uses
Uploaded: 2019-07-19
Duration: 10 min 28 s
Description: Avogadro's law is an experimental gas law relating the volume of a gas to the amount of substance of gas present. Let us learn more through this video.

Question

Caffeine contains $49.5 % C, 28.9 % H, 16.5 % O and 5.20 % H .$ Calculate the molecular formula. Given that $0.2$ moles of caffeine weigh $38.04 g$ .

Accepted Answer

Calculation of empirical formula of caffeine:

Element Atomic mass $%$ Relative no. of moles Simplest mole ratio Whole number ratio

$C$ $12.01$ $49.5$ $\frac{49.5}{12.01} = 4.12$ $\frac{4.12}{1.03} = 4.00$ $4$

$H$ $1.008$ $5.20$ $\frac{5.20}{1.008} = 5.16$ $\frac{5.16}{1.03} = 5.01$ $5$

$N$ $14.01$ $28.9$ $\frac{28.9}{14.01} = 2.06$ $0.2$ $2$

$O$ $16.0$ $16.5$ $\frac{16.5}{16.0} = 1.03$ $\frac{1.03}{1.03} = 1.00$ $1$

Therefore, the empirical formula of caffeine $= C_{4} N_{2} {OH}_{5}$

The calculation of molecular formula:

Empirical formula mass of caffeine $C_{4} N_{2} {OH}_{5} = (12.01 \times 4) + (2 \times 14.01) + (16.0) + (5 \times 1.008) = 97.1$

$0.2$ moles of caffeine $= 38.84 g$

Therefore, $1$ mole of caffeine $= \frac{38.84}{0.2} \times 1 = 194.2$

$n = \frac{Molecular mass}{Empirical formula mass} = \frac{194.2}{97.1} = 2$

Hence, the molecular formula of caffeine $= 2 \times C_{4} N_{2} {OH}_{5} = C_{8} N_{4} {OH}_{10}$

Caffeine contains $49.5 % C, 28.9 % H, 16.5 % O and 5.20 % H .$ Calculate the molecular formula. Given that $0.2$ moles of caffeine weigh $38.04 g$ .

Chapter-Wise 3D Chemistry Videos from Dr. H.C. Srivastava ISC Nootan Chemistry Volume 1 for Class XI

Avogadro's Law and Its Uses

Determination of Relative Molecular Masses

Chemical Equivalent

Learn and Prepare for any exam you want !

Caffeine contains 49.5% C, 28.9% H, 16.5% O and 5.20% H. Calculate the molecular formula. Given that 0.2 moles of caffeine weigh 38.04 g.

Chapter-Wise 3D Chemistry Videos from Dr. H.C. Srivastava ISC Nootan Chemistry Volume 1 for Class XI

Avogadro's Law and Its Uses

Determination of Relative Molecular Masses

Chemical Equivalent

Learn and Prepare for any exam you want !

Caffeine contains $49.5 % C, 28.9 % H, 16.5 % O and 5.20 % H .$ Calculate the molecular formula. Given that $0.2$ moles of caffeine weigh $38.04 g$ .