Question

What is the formula mass unit of HCl?
[A] 36.5
[B] 35.5
[C] 32.5
[D] 34.5

Answer 1

Hint: To find the solution to this question, firstly determine the number of atoms of each element present in the given compound. Then write the atomic weights of the two atoms present in HCl. You can get the formula mass unit by adding the atomic weights of the atoms present in the formula of the compound.

Complete Step by Step Solution: Before answering the question, let us understand the meaning of formula mass unit.
We know that a compound can be made up from atoms of different elements and also a molecule can contain more than one atom. The empirical formula of a compound gives us the idea about the atoms and the numbers of atoms present in the compound and these atoms have different atomic weights.
We define the formula weight of a substance as the sum of the atomic masses of each atom which are present in the chemical formula of the compound. We express the formula mass in terms of atomic mass units.
Now, let us calculate the formula mass of HCl.
As we can see form the formula that hydrochloric acid contains 1 atom of hydrogen and 1 atom of chlorine.
We must know that the atomic weight of hydrogen is 1 and that of chlorine is 35.5.
Therefore, as we have seen from the above definition that formula mass is the sum of the atomic masses of the atoms present in the compound.
So, for HCl it is atomic weight of hydrogen + atomic weight of chlorine i.e. 1 + 35.5 = 36.5.
We can understand from the above discussion that the formula mass unit of HCl is 36.5.

Therefore, the correct answer is option [A] 36.5

Note: We must remember that formula mass and molar mass are not the same. The formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula. The molar mass is the mass of 1 mol of a substance in grams. However, the formula mass of a covalent compound is called the molecular mass of the compound.