Sodium thiosulfate is a hydrated compound containing five water molecules. calculate the amount of hydrated sodium thiosulfate necessary to make 500ml of 0.025M solution (remember that the water molecules are included in the molecular weight)?

You'd need ##"3.1 g"## of sodium thiosulfate pentahydrate.

The molecular formula for sodium thiosulfate pentahydrate is ##Na_2S_2O_3*5H_2O##, which means that the molar mass of the compound will include the 5 water molecules.

##"molar mass" = 2*23 + 2*32.1 + 3* 16 + 5*18 = "248.2 g/mol"##

is defined as moles of per liter of solution; in this case, the solute includes the 5 water molecules per molecule of ##Na_2S_2O_3##. The number of moles of thiosulfate pentahydrate you'll need is

##C = n/V => n = C * V##

##n = "0.025 mol/L" * 500*10^(-3)"L" = 12.5*10^(-3)"moles"##

Use the calculated molar weight of the hydrate to determine how much of it you need in grams

##12.5 * 10^(-3)"moles" * ("248.2 g")/("1 mole") = "3.1 g"##