There are many ways of expressing the concentration of a solution. One of them is to write it in terms of the number of moles of the solute that are present in one liter of the solution. This is called the molar concentration or the molarity.

The volume of the solution given to us is 9 mL and it has 2 g of iron (III) hydroxide. The molar mass of iron (III) hydroxide is 88.85 g/ mole. 2 grams of iron (III) hydroxide constitute 2 / 88.85 = 0.0225 moles.

As 9 mL of the solution has 0.0225 moles of iron (III) hydroxide, 1 liter of the solution will have 0.0225*(1000/9) = 2.501 moles.

This gives the concentration of a solution with a volume of 9 mL that contains 2 grams of iron(III) hydroxide as 2.501 M