Question #c1600

1 Answer
Stefan V.
Feb 1, 2015

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"#

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