Volume is the amount of space the matter takes up, density is mass OF the volume
The answers are low concentrated (dilute) and high concentrated respectively.
As the low concentrated salt solution has a higher water potential than that of the high concentrated salt solution, water molecules will flow from the region of higher water potential to the region of lower water potential, thus from the dilute salt solution to the high concentrated salt solution. This is due to the movement called osmosis. Note that osmosis also requires water to flow through a differentially permeable membrane, which means the membrane can allow certain substances (not all) to go in or out. If the differentially permeable membrane is not present, the movement of water molecules may be regarded as diffusion.
Therefore, the answers for the blanks are low concentrated and high concentrated.
We shall find the molar mass first.
Ca5(PO4)3(OH) = (40 * 5) + 3 (31 + 4(16)) + 16 + 1 = 200 + 285 + 17 = 485 + 17 = 502.
Percent of Calcium = 200/ 502 * 100 = 39.8%
Percent of Phosphorus = 91/502 * 100 = 18.1%
Percent of Hydrogen = 1/502 * 100 = 0.19%
Percent of Oxygen = 100 - (39.8 + 18.1 + 0.19) = 41.91%

(i) 1 mole of Ar =6.022×10^23
atoms of Ar
52 mol of Ar=52×6.022×10 ^23
atoms of Ar =3.131×10 ^25 atoms of Ar
(ii) Atomic mass of He = 4amu
Or
4amu is the mass of He atoms = 1
Therefore 52 amu is the mass of He atoms=0.25×52=13 atoms of He
(iii) Gram atomic mass of He = 4g
Or
4g of He contains =6.022×10 ^23
atoms
Therefore 52 g of He contains=
6.022×10 ^23/4×52 =7.83×10 ^24 atoms