What is the molar mass of ammonium dichromate \[{\left( {N{H_4}} \right)_2}C{r_2}{O_7}\]?
A) $252g/mol$
B) $152g/mol$
C) $234g/mol$
D) None of these

Hint: We need to remember that it is easy to find the molar mass of any compound given if you know the molar mass of all the elements present in it. We also remember that the \[Cr{O_4} - \]this is a dichromate ion whereas \[Cr{O_4} - \]is a chromate ion that has only one Chromium ion.

Complete answer:
We will go step by step to get the molar mass of ammonium dichromate, for this we need to know the molar mass of each element present in this compound. Let's start.
\[N = 14\]
\[H = 1\], since $4$ atoms of \[H\] therefore $4$
There are two unit of ammonium i.e. \[N{H_4}\] so
 molar mass of \[N{H_4}\] = \[14 + 4 = 18\]
\[18 \times 2 = 36\](as two units of ammonium)
\[Cr = 52\], since $2$ atoms of \[Cr\] therefore, $104$
\[O = 16\], since $7$ atoms of \[O\] therefore, $112$
So the molar mass of ammonium chromate is as follows:
$ = 36 + 104 + 112$
On addition we get,
$ = 252g/mol$
Option A) this is a correct option as the molar mass of ammonium dichromate is $252g/mol$ which is solved above.
Option B) this is an incorrect option as $152g/mol$ is the molar mass of ammonium chromate i.e. \[{\left( {N{H_4}} \right)_2}Cr{O_4}\].
Option C) this is an incorrect option as the correct answer is $252g/mol$.
Option D) this is an incorrect option as we got the correct option as A.

Note:
We must look at the compound given and its formula very carefully in order to find the molar mass we must look at the valency and charge. We have to remember that the molar mass can be calculated by adding the number of element’s mass together. Valence electron plays an important role in identifying the charge on a particular atom or molecule.