Question

Find the present composition of calcium (Ca), oxygen (O) and hydrogen (H) in calcium hydroxide $Ca(OH)_{2}$.

Answer 1

The percentage "composition" of "each element" is equal to the "mass of that element" “divided” by the present “total mass” (the mass of the formula) multiplied by 100%.

The given compound: $Ca(OH{ ) }_{ 2 }$

$Formula\quad mass\quad =\quad one\quad calcium\quad \frac { a }{ mu } \quad +\quad 2\times \quad Hydrogen\frac { a }{ mu } \quad +\quad 2\times Oxygen\frac { a }{ mu }$

Substitute the molar mass of each element

Molar mass of Ca = 40

Molar mass of O = 16

Molar mass of Hydrogen = 1

$Formula\quad mass\quad of\quad Ca(OH{ ) }_{ 2 }\quad =\quad 40\quad +\quad (2\quad \times \quad 16)\quad +\quad (2\quad \times \quad 1)\quad =\quad 72$

Let's calculate the %composition

Percentage of  $Ca =\quad \frac { 40 }{ 72 }\quad \times \quad 100\quad =\quad 54$%