GRAVIMETRIC DETERMINATION OFTHE FORMULA OF A HYDRATED COMPOUND 1) Upon heating,

Question

GRAVIMETRIC DETERMINATION OFTHE FORMULA OF A HYDRATED COMPOUND 1) Upon heating, a 4,250 gram sample of Na25O. 20 sample loses 2.388 grams upon heating. What is n for this hydrate? 2) Would your experimental n be higher or lower than the true value if you made the following errors in this laboratory? You are in a hurry to leave the laboratory and you do not achieve a constant mass with heating. a. The crucible is tilted too much during heating and some of the sample falls out. b. The lab instructor accidently contaminates your hydrate sample with NacI before giving the sample to you.

Explanation / Answer

Ans1:

Mass lost by the sample is equal to the water evaporated from the sample.

Mass of water lost m = 2.388 g

Mass of the sample M = 4.250 g

mass of Na2SO4 = 4.250 - 2.388 = 1.862 g

Molecular weight of Na2SO4 = 142.04 g/mol

Molecular weight of water = 18.02 g/mol

mass ratio in the sample = n* 18.02 / 142.04 = 2.388 / 1.862

=> n = 10.109 (approx 10)

Therefore the compound is  Na2SO4.10H2O

Ans 2:

a) n value will be lower because, we left more water in the sample which we read as the mass of the compound.

b) A higher value of n because we lost some of the dried sample.

c) A lower value of n because the dried sample mass will be more than it should be.

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