Question

State the atomic mass of \[{{H}_{2}}O\].
A. 16u
B. 17u
C. 18u
D. 15u

Answer 1

Hint: Calculate the sum of individual atomic masses of the atoms present in the compound. Do not confuse between atomic number and atomic mass.

Complete step by step answer:
“The atomic mass of an element is the average mass of the atoms of an element measured in the atomic mass unit – amu – represented by U. The atomic mass is a weighted average of all of the isotopes of that element, in which the mass of each isotope is multiplied by the abundance of that particular isotope. (Atomic mass is also referred to as atomic weight, but the term "mass" is more accurate.)”
The atomic mass of hydrogen and oxygen is 1U and 16U respectively. Also, \[{{H}_{2}}O\] is the chemical formula for water.
Since \[{{H}_{2}}O\] contains two atoms of hydrogen and 1 atom of oxygen, therefore –
Atomic mass of \[{{H}_{2}}O\] = (2 x atomic mass of hydrogen) + (atomic mass of oxygen)
= (2 x 1) + (16) U
= 18 U
Therefore, the answer is – option (c)– the atomic mass of \[{{H}_{2}}O\] is 18U.

Additional information:
Atomic mass is also represented by the term ‘Dalton’.

Note: The modern periodic table is arranged according to increasing atomic number instead of atomic mass. The original periodic table (Mendeleev’s periodic table) of the elements, published in 1869 by Dimitri Mendeleev arranged the elements that were known at the time in order of increasing atomic weight (because this was before the discovery of the nucleus and the interior structure of the atom).