Question

Molecular mass of ${{\text{K}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ is:
A. 128u
B. 112u
C. 116u
D. 122u
E. None of these

Answer 1

We know that molecular mass of a substance is the mass in atomic mass units (amu) of all atoms in the given molecule. Molecular mass is more commonly indicated as molecular weight and compressed as MW.

Complete step by step answer:
Molecular mass of potassium (K) is $39.10u$ . In the given question there are two potassium atoms. Thus molecular mass of ${{\text{K}}_{\text{2}}}$ is as follows:
${{\text{K}}_{\text{2}}}{\text{ = 39}}{\text{.10u X 2}} \\\ {\text{ = 78}}{\text{.20u}} \\\$
Molecular mass of carbon is $12.01u$ and molecular mass of O is 16.0. There are three oxygen atoms in the potassium carbonate molecule. Thus molecular mass of oxygen in potassium carbonate molecule is as follows:
${{\text{O}}_3}{\text{ = 16}}{\text{.0u X 3}} \\\ {\text{ = 48u}} \\\$
Now we add molecular mass of two potassium, one carbon and three oxygen atoms to get molecular mass of potassium carbonate $\left( {{{\text{K}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}} \right)$ is as follows:
${{\text{K}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}} = 78.20{\text{u}} + 12.01{\text{u}} + 48{\text{u}} \\\ {\text{ = 138}}{\text{.21u}} \\\$
The answer 138.21 is not given in the options. Thus option E will be the correct option for molecular weight of potassium carbonate.

Additional Information:
We know that the molecular weight of a compound is expressed as the weight of a molecule of the compound respective to a carbon atom, the atomic weight of which is supported to be exactly 12. As we know molecular weight when indicated in grams is called gram molecular weight. Molecular weight in grams is, in fact, the mass of 1 mole of molecules. Mathematical expression of molecular weight is as follows:
${\text{molecular}}\,{\text{weight = }}\dfrac{{{\text{Weight}}\,{\text{of}}\,{\text{molecules}}\,{\text{in}}\,{\text{grams}}}}{{{\text{Number}}\,{\text{of}}\,{\text{moles}}\,{\text{of}}\,{\text{molecules}}}}$

Note:
As we know, the atomic mass unit (amu) is the unit of molecular mass. Atomic mass unit is expressed as one twelfth the mass of C-12 isotope. Molecular mass is measured in Daltons (Da or u).