Question

Formula unit mass for ${(N{H_4})_2}S{O_4}$ is:
A. 142
B. 129
C. 137
D. 132

Answer 1

Hint: The formula unit mass of a chemical compound is defined as the sum of average atomic masses of all the constituents/elements of the compound, represented in atomic mass units.Thus, if we know the atomic masses of each element, we can compute the formula unit mass easily.

Complete step by step answer:
As we know, the formula unit mass of a chemical compound is calculated by adding up the average atomic masses of each element present in that compound. By looking at the periodic table of elements, we see that:
Atomic mass of nitrogen, $N = 14amu$
Atomic mass of hydrogen, $H = 1amu$
Atomic mass of sulphur, $S = 32amu$
Atomic mass of oxygen, $O = 16amu$
As we can see from the chemical formula of the compound given to us, we have one nitrogen atom, eight hydrogen atoms, one sulphur atom and four oxygen atoms. While calculating formula unit mass, we have to add the mass of each atom. Hence, by multiplying the number of atoms of each type with their respective average atomic masses, we get:
Formula unit mass of ${(N{H_4})_2}S{O_4} = (14 \times 2) + (1 \times 8) + (32 \times 1) + (16 \times 4)$
Simplifying this, we get:
Formula unit mass of \[{(N{H_4})_2}S{O_4} = 28 + 8 + 32 + 64\]
On adding this, we get:
Formula unit mass of \[{(N{H_4})_2}S{O_4} = 132amu\]

So, the correct answer is Option D.

Note: Noted that the formula unit mass is different from the molecular mass of the compound. Molecular mass is the mass of one mole of the compound, and is expressed in grams, while the formula unit mass is expressed in the atomic mass units ( $amu$ ). $1amu$ is the mass of a single hydrogen atom. Note that the values of formula unit mass and molecular mass of a compound are numerically equal, but they differ in the units in which they are expressed.