Question

The percentage of hydrogen in water is.
A) $44.45$
B) $5.55$
C) $88.89$
D) $11.11$

Answer 1

We know that mass percent of any element in a compound is $100$ times the mass of the element divided by the total mass.
The percentage composition of an element can be calculated using the formula,
$Mass\,percentage\,of\,element = \dfrac{{Mass\,of\,element}}{{Mass\,of\,compound}} \times 100$

Complete step by step answer:
We know that,
The molecular weight of water is $18\,g/mol.$
The mass of hydrogen is $2\,g/mol.$
The percentage of hydrogen in the water can be calculated as,
The percentage of hydrogen in the water {\text{ = }}\dfrac{{2\,{\text{g/mol}}}}{{18\,{\text{g/mol}}}}{{ \times 100 = 11}}{\text{.11}}\,{\text{% }}
The percentage of hydrogen in the water is {\text{11}}{\text{.11% }}{\text{.}}
Therefore, the option D is correct.
Now we calculate the percentage of oxygen in water molecules.
We know that,
The molecular weight of water is $18\,g/mol.$
The mass of oxygen is $16\,g/mol.$
The percentage of oxygen in the water can be calculated as,
The percentage of oxygen in the water {\text{ = }}\dfrac{{16\,{\text{g/mol}}}}{{18\,{\text{g/mol}}}}{{ \times 100 = 88}}{\text{.88}}\,{\text{% }}
The percentage of oxygen in the water is {\text{88}}{\text{.88% }}{\text{.}}

So, the correct answer is Option D .

Note:
If the percentage composition of the elements is given one calculate the empirical formula of the compound. Now, we see empirical formula,
Empirical Formula:
The simplest whole number ratio of each type of atom in a compound is called empirical formula. It can be the same as the compound’s molecular formula but not always. It can be calculated from information about the mass of each element in a compound or from the percentage composition.
The steps for determining the empirical formula of a compound as follows:
Obtain the mass of each element present in grams.
Determine the number of moles of each atom present.
Divide the number of moles of each element by the smallest number of moles.
Convert the numbers to whole numbers. The set of whole numbers are the subscripts in the empirical formula.