5.85g of NaCl are dissolved in 500cm3 of water.Calculate the formality of the solution.?

The formality of a solution is defined as the number of formula mass of any solute dissolved in 1 litre of solution. Therefore, there is scarcely any difference between formality and molarity of a solution.

Formula (NaCl) mass of sodium chloride = 23 + 35.5 = 58.5 .

58.5 g NaCl when dissolved in 1000 cm^3 solution becomes 1 formal solution ;

5.85 g NaCl when dissolved in 1000 cm^3 solution becomes (5.85)/(58.5) formal solution or 0.1 formal solution .

5.85 g NaCl when dissolved in 500 cm^3 solution becomes (2 X 0.1) or 0.2 formal solution .

Here one thing I must mention that, the problem creates confusion by saying 5.85 g NaCl is dissolved in 500 cm^3 water. Actually, solid NaCl has some volume, whatever smaller it is, as compared to water used for dissolution. According to definition of formality as well as of molarity, one formal or one molar solution is that solution which contains one formula mass/molar mass in 1000 cm^3/ 1 L of its volume. It never says that 1 mole of solute is dissolved in 1000 cm^3/ 1 L water.

If the problem is correct on its own ground, then you will have no way other than to calculate the total volume of the solution from some other data (here that is not provided) given in the question.