Chemistry
posted by Sam .
What is the formula mass of ammonium phosphide, (NHsubscript4)subscript3P?
85.0 u
49.0 u
48.0 u
57.0 u
I thought I was to look these up in the Periodic Table, but they aren't there, so I'm confused on what to do. Any help would be appreciated.

You DO (REALLY DO) look them up on the periodic table. The periodic table lists elements, not compounds, so you look up the elements and add them.
(NH4)3P. Here are the approximate values but you need to confirm them and make them more exact. I've just used whole numbers.
N = 14 x 3 of them = 42
H = 1 x 12 (that's 4 x 3) of them = 12
P = 31 x 1 of them = 31
Now add 42 + 4 + 31 = formula mass (NH4)3P