1. A 5.25g sample of magnesium sulfate hydrate is heated until the masses stabil

Question

1. A 5.25g sample of magnesium sulfate hydrate is heated until the masses stabilize. If the residue has a mass of 2.56g, answer the following. Show your formula and then how your data is substituted into the formula.

? What is the mass of the original hydrate?

? What is the mass of the anhydrous salt?

? What mass of water was removed?

? What percentage of the compound was water?

? How many moles of water were removed?

? How many moles of salt were present?

? What is the mole ratio of water to salt?

? What is the empirical formula of the hydrate?

Suppose that some of the material did spatter out of the crucible.

a. Which numbers in your data would change, and how would they change?

b. How would your formula change? Explain.

Explanation / Answer

Magnesium sulfate hydrate is , MgSO4.7H2O

Heating to 200oC will drive of all the water to leave the MgSO4

MgSO4.7H2O ---------------> MgSO4 + 7H2O

Further heating causes it to decompose to magnesium oxide and sulfurtrioxide

MgSO4 ----------> MgO + SO3

a ) Mass of MgSO4 = 120.4 g/mol

Mass of water i.e 7H2O is , = 126 g/mol

Mass of original hydrate is , = 120.4 + 126 = 246.4 g/mol

b ) Mass of Anhydrous salt is ,

      mass of Mg = 24.4 , mass of s = 32 , mass of Oxygen =16

     = [24 + 32 + (4 * 16)] =120.4

Mass of Anhydrous salt is , = 120.4 g/mol

c ) mass of H2O = 18 g/mol

here we have Hepta hydrated salt , i.e 7H2O

     = 7 * 18 =126 g/mol

Mass of water removed = 126 g/mol

d ) Mass of water = 126

    Total mass of hepta hydrate = 120.4 + 126 = 246.4

Percent water in hydrate = 126 / 246.4 * 100 = 51.1%

e ) Mass of water = 5.25 - 2.56 = 2.69 gr

     Moles of water = 2.69g / 18.01 g/mole

    moles of water were removed = 0.149mol

f ) Moles of salt = Wt / Mwt = 5.25 / 120.4

   Moles of salt were present = 0.044 mol

g ) Mole ratio = moles of water : moles of salt

                    = 0.149 : 0.044 = 3 : 1 ( i.e 3 moles of water & 1 mole of MgSO4)

                        OR

Mole ratio      = 3.39

h ) Empirical formula of the hydrate , = MgSO4.3H2O

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