In lab we heated copper sulfate pentahydrate (CuSO_4 middot 5H_2O) to prove that
ID: 1058853 • Letter: I
Question
In lab we heated copper sulfate pentahydrate (CuSO_4 middot 5H_2O) to prove that there are 5 moles of water for every mole of CuSO_4. For this problem, I want you to perform the same experiment (on paper only) for Epsom salt, MgSO_4 middot xH2O. A 3.50 g sample of Epsom salt was heated for five minutes and the resulting solid, MgSO_4, had a mass of 1.85 g. From this data determine the number of moles of water lost and the amount of MgSO_4 that remains. Using your data, what is the value of x in the formula for Epsom salt?Explanation / Answer
Ans. Given,
Mass of Epsom salt sample taken (Initial mass) = 3.50 g
Dry mass of Epsom salt (Final mass, moisture free) = 1.85 g
Mass of water lost during heating salt sample = Initial mass – final mass
= 3.50 g – 1.85 g = 1.65 g
Number of moles of water in 1.65 g = (Mass / molar mass) of water
= 1.65 g / 18.015 g mol-1
= 0.09159 moles
Number of moles of moisture free MgSO4 in 1.85 g = (Mass / molar mass) of MgSO4
= 1.85 g / 120.36 g mol-1
= 0.01537 moles
Now,
0.01537 moles MgSO4 has 0.09159 moles water molecules
Or, 1 mol - - (0.09159 / 0.01537) moles water
= 5.9588 moles = 6 moles (approx)
That is, 1 mol MgSO4 is associated (hydrated) with approximately 6 moles H2O.
Therefore,
X = 6 in Epsom salt
Or, Epsom salt has formula – MgSO4.6H2O
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