Question

Calculate the formula unit mass (u) of ZnO. $\left[ {{\text{Zn = 65u, 0 = 16u}}} \right]$
A.) 80
B.) 69
C.) 82
D.) 81

Answer 1

Hint: Formula unit mass is defined as the sum of atomic masses of the elements present in a formula unit of a given compound. Hence, it is calculated by adding the atomic masses of all atoms that are present in one formula unit.
Step by step answer:
Formula unit mass is calculated by adding the atomic masses of all atoms that are present in one formula unit.
We know, atomic mass of zinc (Zn) is 65u and the atomic mass of oxygen (O) is 16u. To calculate the formula unit of mass, we can add the atomic masses of zinc and oxygen present in the compound ZnO.

Atomic mass of Zn = 65u

Atomic mass of O = 16u

Therefore, formula unit mass of ${\text{ZnO = 65 + 16 = 81u}}$
Hence, the formula unit mass of ZnO is 81u
Therefore, the correct option is Option D.

Additional Information:
For a given isotope, the sum of the number of protons and neutrons in the nucleus of that isotope is known as the mass number. This is because each proton and neutron weighs one atomic mass unit (amu). By adding together, the number of protons and neutrons and multiplying it by 1amu, one can easily calculate the mass of the atom. We need an atomic mass unit to establish the value of Avogadro's number, that is, the number of particles in one molecule.

Note: An atomic mass unit can be defined as a mass which is equal to one twelfth of the mass of an atom of carbon-12. The mass of any isotope of any element can be expressed in relation to this carbon-12 standard.