Question

How many moles of hydrogen are contained in 55.90 g of ammonium dichromate, (NH4)2Cr2O7?

Answer 1

There are 252.07 grams of ammonium dichromate per mol of ammonium dichromate molecules. There are 8 atoms of hydrogen per molecule of ammonium dichromate.

Convert grams to mole of molecules. And then molecules to atoms.

55.90 / 252.07 = moles of molecules

Since there are 8 hydrogen per 1 molecule multiply this by 8.

55.90 * 8 / 252.07 = 1.77 moles of hydrogen

Answer 2

To calculate the molar mass of a compound, you multiply the subscripts of each element times its molar mass (atomic weight in g/mol).
(NH
4
)
2
Cr
2
O
7
When there is no written subscript, it is understood to be
1
. Also, when elements are enclosed in parentheses, the subscripts on the inside are multiplied by the number outside the parentheses.
Molar Masses of the Elements
Nitogen:
14.007 g/mol

Hydrogen:
1.00794 g/mol

Chromium:
51.9961 g/mol

Oxygen:
15.999 g/mol
Molar Mass of Ammonium Dichromate
(
2
×
14.007
)
+
(
8
×
1.00794
)
+
(
2
×
51.9961
)
+
(
7
×
15.999
)
=
252.06 g/mol