The compound we are dealing with is sodium sulfate, #Na_2SO_4#.
To find the total number of protons, we have to add all the protons in the atoms together.
Each sodium #(Na)# atom has #11# protons, so #Na_2# would have #11*2=22# protons.
A sulfate #(SO_4^(2-))# molecule has one sulfur atom, which has #16# protons, and four oxygen atoms, which each have #8# protons, and therefore have a total of #4*8=32# protons.
So, a sulfate molecule would have a total of #16+32=48# protons.
Therefore, the total number of protons in one molecule of sodium sulfate would be #48+22=70# protons.
Since this molecule has a neutral or #0# charge, then the number of protons would be equal to the number of electrons. Since there are #70# protons in the molecule, there will be also #70# electrons.
We know that for a compound, its mass number is represented as the total number of protons and neutrons.
If we look at the periodic table, each sodium atom has a mass of #23 \ "amu"#, each sulfur atom has a mass of #32 \ "amu"#, each and oxygen atom has a mass of #16 \ "amu"#. So, we can figure out the mass of each compound.
Since each #Na# atom has a mass of #23 \ "amu"#, then #Na_2# would have a mass of #23*2=46 \ "amu"#.
In #(SO_4^(2-))#, there are four oxygen atoms and one sulfur atom, so the total mass would be
#4*16 \ "amu"+32 \ "amu"=96 \ "amu"#
So, a sodium sulfate molecule #(Na_2SO_4)# will have a mass of #46 \ "amu" + 96 \ "amu" =142 \ "amu"#
As each proton has a mass of #1 \ "amu"#, the total mass of the protons in sodium sulfate would have a mass of #70 \ "amu"#. That means that the total mass of neutrons is #142 \ "amu" - 70 \ "amu"=72 \ "amu"#.
Each neutron also weighs around #1 \ "amu"#, so there will be a total of #72/1=72# neutrons in the molecule.
