Question

What is the percentage composition of $CaB{r}_2$? (Atomic weight: Ca = 40, Br = 80)
(A) $10$
(B) $20$
(C) $25$
(D) $33$
(E) $50$

Answer 1

Hint: To calculate percentage composition, we must first find the mass/ weight of each atom present and then total weight of the molecules in the given compound. Apply these values in the percentage composition formula:
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Complete step by step solution:
The percentage composition of a compound is the ratio of the amount of each element to the total amount of all the elements present in the compound multiplied by 100.
The percentage composition of given element is expressed using the following formula
 $$

- Here $$ gives the percentage composition of the given element E, which is to be calculated.
$g_E$, gives the total amount of the element that is present in the given compound.
$g_T$, gives the total amount of all the elements present in the compound.
This ${\displaystyle \frac{g_E}{g_T}}$ is further multiplied by 100.

- The given compound is $CaB{r}_2$.
Relative atomic mass of $Ca$ in $CaB{r}_2$= 40
Relative atomic mass of $Br$ in $CaB{r}_2$ = 80
Total molecular mass of the $CaB{r}_2$ compound = $40\times 1+80\times 2$ $=200g$
% composition of $Ca$$={\displaystyle \frac{40}{200}}\times 100=20$
% composition of $Br$$={\displaystyle \frac{80}{200}}\times 100=80$
So, the correct answer is “Option B”.

Additional Information:
- The nutrition label found on the container of every processed food sold in the market employs this idea of percentage composition.
- The purity of the compounds can be checked by comparing the % composition obtained theoretically using the formula, with the values that are determined experimentally.
- % composition has been part of our day-today life, though we are unaware of it.

Note: - Determining the % composition formula will help you to determine mole: mole ratio of the elements in the compound. This ratio can be further used to determine the empirical formula of the compound.