Phosphorous has 5 valence electrons and so gaining 3 electrons will make it isoelectronic with the stable noble gas argon, and will produce the P3- ion
The process in which the concentration of the solution is lessened by the addition of water is said to be dilution and equation of dilution relates the initial concentration and volume of stock solution with the final concentration and volume of the solution.
Formula is given by:
(1)
where,
is the initial concentration
is the initial volume
is the final concentration
is the final volume
Now,
= 0.850 M
= 4.12 L
=?
= 10.00 L
Substitute the give values in formula (1),


= 
Thus, the final concentration of the
solution = 
Density is defined as mass/volume, and the volume is 36.2-25 mL
so just substitute into the equation to get the answer.
Hope this helps.