# Question #c1600

Feb 1, 2015

You'd need $\text{3.1 g}$ of sodium thiosulfate pentahydrate.

The molecular formula for sodium thiosulfate pentahydrate is $N {a}_{2} {S}_{2} {O}_{3} \cdot 5 {H}_{2} O$, which means that the molar mass of the compound will include the 5 water molecules.

$\text{molar mass" = 2*23 + 2*32.1 + 3* 16 + 5*18 = "248.2 g/mol}$

Molarity is defined as moles of solute per liter of solution; in this case, the solute includes the 5 water molecules per molecule of $N {a}_{2} {S}_{2} {O}_{3}$. The number of moles of thiosulfate pentahydrate you'll need is

$C = \frac{n}{V} \implies n = C \cdot V$

$n = \text{0.025 mol/L" * 500*10^(-3)"L" = 12.5*10^(-3)"moles}$

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

$12.5 \cdot {10}^{- 3} \text{moles" * ("248.2 g")/("1 mole") = "3.1 g}$