Question

Question #c1600

Answer 1

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"`

Molarity is defined as moles of solute 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"`