To find this answer it is necessary to first find the chemical
formula of aluminum sulfate utilizing the valence charges from the
periodic table. Aluminum Sulfate is a molecular formed by the ionic
bonding of aluminum to to the polyatomic ion of sulfate

Al = +3

SO4 is a polyatomic ion with a charge of -2

Therefore after balancing the equation, you get the chemical
formula

Al2(SO4)3

Next, in order to find osmolarity, you must first find molarity.
This is done by converting grams to moles.

Moles are calculated in grams/Liter and is equal to the
molecular weight. What is the molecular weight of aluminum sulfate.
Looking at the periodic table, you sum the atomic weights of each
element, shown in AMUs.

Al = 27 (x2) = 54

S = 32 (x3) =96

O = 16 (x12) = 192

We add these up to determine the molecular weight of the
substance, which can be used to determine the molarity of this
solution.

MW = 342g, which means that one mole of aluminum sulfate is 342
grams per liter of water. We can now use the information presented
in the above problem to determine molarity

10 grams dissolved in 200 ml of water.

Since measurements are in grams per liter, we can assume that if
10 grams were dissolved in 200 ml of water, then 50 grams will
dissolve in 1000 ml, or 50 grams per liter (multiply both factors
by five).

We must use our grams of solute to convert grams into our
molarity

50 g x 1 mole/342 grams of aluminum sulfate to determine the
molarity.

= .15 M

At this point, to determine osmolarity, you must be aware that
osmolarity is the measure of PARTICLES within a solution per liter.
Realizing that aluminum sulfate is held together by an ionic bond,
it will freely dissolve in water and produce Al ions and SO4
ions.

So we sum the particles of each:

2 Al ions per molecule

3 SO4 ions per molecule

Therefore, multiplying the molarity of the aluminum sulfate
solution times 5 (total 5 particles from each molecule) will give
you the osmolarity.

= 0.73 Osm per liter of aqueous solution.