Question

What is the weight of bromine needed for the reaction with ${\mathbf{21gm}}$ of ${C_3}{H_6}$(atomic weight of $Br = 80)$
A.$40{\text{g}}$
B.$80{\text{g}}$
C.$160{\text{g}}$
D.$120{\text{g}}$

Answer 1

To answer this question, you should recall the concept of bromination of alkenes. Apply the mole concept in the reaction to find the answer to this question. Equal moles of bromine will react with the alkene.
Formula Used: ${\text{moles}} = \dfrac{{{\text{mass}}}}{{{\text{molar mass}}}}$

Complete step by step answer:
The reaction involving bromination of the aforementioned alkene can be represented by the equation: ${C_3}{H_6} + B{r_2} \to {C_3}{H_6}B{r_2}$.
The molar mass of ${C_3}{H_6} = 42{\text{gm}}$ and $Br = 160{\text{ gm}}$
Now from the reaction, we can conclude that $42{\text{gm}}$ of ${C_3}{H_6}$ ​ needs $160{\text{ gm}}$of bromine.
Simply applying the unitary method: $1{\text{gm}}$ of ${C_3}{H_6}$would need $160/42{\text{g}}$ of bromine.
$\therefore$$$21{\text{ gm}}$of${C_3}{H_6}$​ would require$\dfrac{{160}}{{42}} \times 21 = 80{\text{gm}};$$of bromine.

Thus, the correct option is B.

Note:
The concept that a mole of any substance contains the same number of particles was formed out of research which was conducted by Italian physicist Amedeo Avogadro. Avogadro constant can be defined as the number of molecules, atoms, or ions in one mole of a substance: $6.022 \times {10^{23}}$ per mol.
It is derived from the number of atoms of the pure isotope $^{{\text{12}}}{\text{C}}$ in 12 grams of that substance and is the reciprocal of atomic mass in grams. Now the mole concept can be applied to ions and formula units.
Illustration 1: 1 mole of ${O_2}$ means Avogadro's number of oxygen molecules and it will be equal to 2 times Avogadro's number of oxygen atoms.
Illustration 2: 1 g-molecule of ${O_2}$ is the same as one mole of the oxygen molecule and contains Avogadro's number of nitrogen molecules and twice this number of atoms.
Illustration 3: 1 g-atom of Nitrogen means 1 mole of nitrogen atoms and contains Avogadro's number of nitrogen atoms. The mass of 1 mole of any species is equal to its molar mass.
The formulae for the mole concept can be summarized as:
${\text{No}}{\text{. of moles = }}\dfrac{{{\text{Mass of the Substance in grams}}}}{{{\text{Molar mass of a Substance}}}} = \dfrac{{{\text{Number of Atoms or Molecules}}}}{{6.022 \times {{10}^{23}}}}.