[mandy9008]

problem:
calculate the total mass of the protons and electrons in ¹⁹₉F.
use 1.007825 amu as the mass of ¹₁H (mass of p⁺ and e^-)

my work:
19-9=10
9(1.007825)+10(?)=18.998
i do not know what to put where the ? is!!

[ARGOS++]

Dear mandy9008;

Again:  What is # of nucleons minus # of Protons  =   # of what ??

As soon as you answer this question, then you know the missing value from your former question!
(Its value never change! - But the question don't ask for it!!)

Good Luck!
                    ARGOS⁺⁺

[mandy9008]

# of neutrons?

[ARGOS++]

Dear mandy9008;

Correct!:  -  the # of neutrons!
(And you know the mass of a neutron: http://www.chemicalforums.com/index.php?topic=32784.0 )

But:

problem:  calculate the total mass of the protons and electrons in ¹⁹₉F. 

  -  IMHO: The question is not asking for anything about neutrons, or their mass.

Good Luck!
                    ARGOS⁺⁺