Question

Determine the percentage composition of calcium hydroxide, $Ca{(OH)_2}$. $(1mol = 74g)$
A) ${\text{40% }}$
B) ${\text{43% }}$
C) ${\text{54% }}$
D) ${\text{69% }}$
E) ${\text{74% }}$

Answer 1

Hint:The percent composition of an element in a compound is the percent value of the total mass of that element with respect to the total mass of the compound. One can also use the formula and relate the value of the mass of calcium for the percent calculation.

Complete answer:
1) First of all we will analyze the values given in the question and try to find out the percent composition.
The total mass of compound i.e. $Ca{(OH)_2}$ $ = 74g/mol$
The atomic mass of component calcium i.e. $Ca = 40g/mol$
As we know the atomic number of calcium is twenty grams per mole and the mass number will be twice that which is forty grams per mole.
2) Now we can relate this information to find out the percent value of the calcium as below,
${\text{74g/mol}}$ of a compound i.e. $Ca{(OH)_2}$ ${\text{ = }}$ Contains ${\text{40 g/mol}}$ calcium
${\text{100 g/mol}}$ of a compound i.e. $Ca{(OH)_2}$ ${\text{ = }}$ Contains ${\text{X}}$ calcium
As we need to find out the percent composition we need to relate the values with respect to the term ${\text{100}}$ which will lead us to the percent composition.
3) We can write this in mathematical form by doing the cross multiplication method as below,
${\text{X}}$ calcium i.e. percent of calcium $ = \dfrac{{40 \times 100}}{{74}}$
Now let us do the calculation part where we get,
${\text{X}}$ calcium i.e. percent of calcium $ = \dfrac{{4000}}{{74}}$
By doing the calculation of division we get,
${\text{X}}$ calcium i.e. percent of calcium ${\text{ = 54}} \cdot {\text{05% }}$
4) Therefore, the percent composition of calcium in the compound $Ca{(OH)_2}$ is ${\text{54% }}$
Which shows the option C as the correct choice of the answer.

Note:
Percent composition gives the idea of an element’s presence in a compound and this data can be related to form a chemical reaction and finding out the products. The percent composition gives the ratio of each individual component or element present in a compound.