Question

Calculate the relative molecular mass of the each of the following:
\[
  {\text{(i) Ammonium dichromate [(N}}{{\text{H}}_{\text{4}}}{{\text{)}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}{\text{]}} \\
  {\text{(ii)}}\;{\text{Hydrated Copper Sulphate [CuS}}{{\text{O}}_{\text{4}}}{\text{.5}}{{\text{H}}_{\text{2}}}{\text{O]}} \\
  {\text{(H = 1,N = 14,O = 16,S = 32,Cr = 52,Cu = 64)}} \\
\]

Answer 1

Hint: A molecular formula is a chemical formula which indicates the type of atoms and the number of atoms present in a molecule of a compound. We can find out the relative molecular mass of a molecule if we have the molecular formula.

Complete step by step answer:
Relative molecular mass is the ratio of the sum of masses of each atom of a compound to the unified mass of the compound. It can be calculated simply by the addition of individual masses of the atoms present in a compound.
(i) First, we are taking the case of \[{\text{Ammonium dichromate [(N}}{{\text{H}}_{\text{4}}}{{\text{)}}_{\text{2}}}{\text{C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}{\text{]}}\]
We are given the atomic masses of the atoms and we will add it up.
$ {{[2 \times (14 + 4 \times 4)] + 2 \times }}\;{{52 + 7 \times 16}} \\
{{ = [2 \times (14 + 16)] + 104 + 112}} \\
{{ = 60 + 104 + 112}} \\
{{ = 276}}$
The answer is \[{\text{276}}\;{\text{u}}\].
(ii) Next, we are taking the case of \[{\text{Hydrated Copper Sulphate [CuS}}{{\text{O}}_{\text{4}}}{\text{.5}}{{\text{H}}_{\text{2}}}{\text{O]}}\]
As we have done above, we have to add the individual atomic masses like this;
${{64 + 32 + 4 \times 16 + [5 \times (2 \times 1) + 16]}} \\
{{ = 64 + 32 + 64 + 26}} \\
{{ = 186}}\;{\text{u}} $
Therefore, the answer is \[{\text{186}}\;{\text{u}}\].

Additional Information:
Atomic mass is defined on the basis of carbon atoms. One atomic mass unit is defined as the one-twelfth of the mass of a carbon atom. Carbon atom is taken as the base because it is found abundantly and is present even in the fossils as isotopes etc.

Note: Molecular formula gives us the basic information about a molecule or compound. It provides information about the kind of atoms present in it and the number of atoms used in the formation of the molecule. It also helps us to calculate the molecular mass of the compound.