How many grams of the hydrate must the student weigh out in order to get the desired amount of the anhydrous compound?

Question

A student is running an experiment in which 54.1 grams of KF is needed, but the only jar of reagent in the lab is labelled potassium fluoride dihydrate. How many grams of the hydrate must the student weigh out in order to get the desired amount of the anhydrous compound?

Answer 1

Potassium fluoride dihydrate = KF•2H₂O

molar mass KF = 58.1 g/mol

molar mass KF•2H₂O = 94.1 g/mol

54.1 g KF x 1 mol KF / 58.1 g = 0.931 mols KF needed

0.931 mol KF x 1 mol KF•2H₂O / mol KF x 94.1 g KF•2H₂O/ mol = 87.6 g KF•2H₂O

Done another way using % composition...

% KF by mass = 58.1 g / 94.1 g (x100) = 61.7%

mass of dihydrate required = 54.1 g / 0.617 = 87.6 g KF•2H₂O