Question

Formula mass unit of ${ Na }_{ 2 }{ CO }_{ 3 }.{ 10H }_{ 2 }{ O }$ is;
a.) ${ 290 }$
b.) ${ 286 }$
c.) ${ 180.02 }$
d.) ${ 105.97 }$

Answer 1

Hint: The formula unit mass is equal to the sum of atomic masses of all the atoms in an empirical formula of the compound. The formula unit mass depends on the molar mass of the substance.

Complete step-by-step answer:

Formula unit mass of ${ Na }_{ 2 }{ CO }_{ 3 }$
Atomic mass of Na = ${ 22.989 }$u
Atomic mass of C = ${ 12.01 }$u
Atomic mass of O = ${ 15.99 }$u
= ${ 2\times atomic\quad mass\quad of\quad Na+1\times atomic\quad mass\quad of\quad C+3\times atomic\quad mass\quad of\quad O }$
Now, put the values of atomic masses, we get
= ${ 2\times 22.989+1\times 12.01+3\times 15.99 }$
= ${ 105.97 }$u

Formula unit mass of ${ 10H }_{ 2 }{ O }$
Atomic mass of H = ${ 1.007 }$u
Atomic mass of O = ${ 15.99 }$u
= ${ 10\times atomic\quad mass\quad of\quad { H }_{ 2 } }{ O }$
Now, put the values of atomic masses, we get
= ${ 10\times 18.02u }$
= ${ 180.02u }$
Hence, the formula unit mass of ${ Na }_{ 2 }{ CO }_{ 3 }.{ 10H }_{ 2 }{ O }$;
= ${ 105.97+180.02 }$u
= ${ 285.99u }$

Additional Information:
The molecular mass of a molecule is the average mass calculated by adding together the atomic weights of the atoms in the molecular formula. Molecular mass is the sum of the atomic masses of the atoms in a molecule.
The formula mass of formula weight of a molecule is the sum of atomic weights of the atoms in its empirical formula. It is the molecular mass of the total compound containing X number of molecules of that compound.

Note: The possibility to make a mistake is that you may confuse between molecular mass and formula mass.
The formula mass and molecular mass are two different terms and should not be confused as implying the same meaning.