A30.7 g sample of strontium nitrate, sr(no3)2•nh2o, is heated to a constant mass of 22.9 g. calculate the hydration number.

The difference of the sample mass and the heated sample mass is the amount of water which evaporated by heating process, then we can calculate:  

mass of H₂O = 30.7 -  22.9 = 7.8 g

then calculate the number of moles:  

number of moles of H₂O =  mass / molecular mass

number of moles of H₂O = 7.8 / 18 = 0.43 moles

So the hydration number is 0.43.

Sr(NO₃)₂ • 0.43 H₂O

i believe it is 40–40–20%

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