Question

What is the mass percent of oxygen in $\text{A}{{\text{l}}_{\text{2}}}{{\left( \text{S}{{\text{O}}_{\text{4}}} \right)}_{\text{3}}}\text{.18}{{\text{H}}_{\text{2}}}\text{O}$ . The molar mass of the substance is $666.43$ $\text{g/mol}$ .
(A) $9.60$
(B) $128.8$
(C) $143.2$
(D) 72

Answer 1

Mass percent is a way of expressing a concentration or describing the component in a particular mixture. The mass percent of an element in a compound is equal to the amount of the element present with respect to the weight of the compound.

Complete Step by Step Answer
To find out the mass percent of oxygen in $\text{A}{{\text{l}}_{\text{2}}}{{\left( \text{S}{{\text{O}}_{\text{4}}} \right)}_{\text{3}}}\text{.18}{{\text{H}}_{\text{2}}}\text{O}$ , let us find out the number of oxygen atoms present in the compound.
In the aluminium sulphate, there are 12 atoms of oxygen and from the water molecules we get 18 atoms of oxygen. In total there will be $12+18=30$ atoms of oxygen in the additional product.
The mass of each oxygen atom is equal to 16 atomic mass units.
Therefore, the mass of 30 oxygen atoms is equal to $30\times 16=480$
The molar mass of the substance is $666.43$ $\text{g/mol}$
Therefore the mass percent of oxygen in the compound is $\dfrac{480}{666.43}\times 100=72$
So the correct answer is option D.

Note
One mole of a substance is defined as the amount of mass of a substance that is equal to the mass of $6.023\times {{10}^{23}}$ particles, where the word “particles” refer to ions, molecules, atoms, and subatomic particles. The number $6.023\times {{10}^{23}}$ , which is also known as Avogadro's number.
The term “gram-molecule” was used for one mol of molecules and “gram-atom” for one mole of atoms.
The mole is a convenient way to express the amounts of the reactants and the products that are involved in a chemical reaction.