Elements in group 1-2, 13-18, the number of valence electrons is related to the group number. For example, in the first group, the alkali metals there is one valence electron, however in group 13, there are 3 valence electrons. Valence electrons are also known as the outershell electrons.
Answer:

Explanation:
Assuming that all caculations are at normal pressure and -1.72°C :

Where
is the number of moles of hydrogen
is the mass of hydrogen
is the density of hydrogen
Hey there!
H₃PO₄
Find molar mass.
H: 3 x 1.008 = 3.024
P: 1 x 30.97 = 30.97
O: 4 x 16 = 64
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97.994 grams
The mass of 1 mole of H₃PO₄ is 97.994 grams.
We have 4.5 moles.
97.994 x 4.5 = 440
The mass of 4.5 moles of H₃PO₄ is 440 grams.
Hope this helps!