Lab 4C

We did a lab on Thursday and it was about the composition of the hydrate. 

The purpose of the lab was to determine the percentage of water in an unknown hydrate, to determine the moles of water present in each mole of this unknown hydrate, when given the molar mass of the anhydrous salt and to find out the empirical formula of the hydrate.

So the procedure of this lab was:

1. put on safety equipment (always the most important)
2. set up the equipment (ring stand, ring, bunsen burner, pipestem triangle, crucible, and crucible tongs)
3. turn on the bunsen burner to dry the crucible
4. remove the burner and allow the crucible to cool for about 3 minutes
5. determine the mass of the empty crucible
6. place 1/3 full of hudrate and determine the mass (record it down)
7. place the crucible and contents on the pipestem triangle and heat until the crucible is a dull red (maintain the temperature for 5 minutes)
8. turn off and allow the crucible to cool for 5 minutes 
9. record the mass 
10. reheat the crucible for another 5 minutes, cool as before, and determine the mass (the mass should be the same as the first one) 
11. add water to the content of the crucible and note any changes that occur

Analysis of Results

1) The percentage of water in the hydrate is calculated by :
  mass of water   divided by mass of hydrate x 100%

2) The number of moles the anhydrous salt left behind is calculated by :

Mass after heating – mass of crucible = mass of anhydrous salt
Moles of anhydrous salt = mass of anhydrous salt x 1 mole = moles of anhydrous salt                                                                               120.4g

3) The number of moles of water removed by the heat is calculated by :

Mass of hydrate = mass of crucible & hydrate – mass of crucible

 mass of water = mass of hydrate – mass of salt

Moles of water = mass of water x 1 mole = moles of water

                                                       18g

4) The moles of water per mole of anhydrous salt is calculated by :

Moles of water divided by moles of salt is the  ratio of water to anhydrous salt

5) The empirical formula of the hydrate is MgSO₄ · 7H₂O Magnesium Sulphate Heptahydrate.

Follow-Up Questions

1) SOURCES OF ERROR

• NOT HEATING IT TO BLUE FLAME
• HEATING IT WAY TOO LONG
• WRONG MEASUREMENT
• USING THE WRONG EQUIPMENT

2) Solution :

  Na => 16.1g x 1 mole  = 0.7 moles of Na    x3     2                           23g

C => 0.35 moles of C                              x3         1
O =>  1.05 moles of O                             x3       3 

Water => 3.5 moles of water                  x3     10

The empirical formula is Na₂Co₃ · 10H₂O

3) Solution:

Molar Mass of Na₂CO₃  x  5.82g = 106g/mol x 5.82 = 2.16g
Molar Mass of Na₂CO₃ · 10H₂O   286g/mol

4) Solution:

Moles of Na 18.53g x 1 mole = 0.806 moles of Na        1 x 2          2                                       23g
 

S = 0.806 moles of S                                            1 x 2          2
O = 1.21 moles of O                                             1.5 x 2      3  

H2O = 2                                                               2.5 x 2       5

The empirical formula is Na₂S₂O₃ · 5H₂O.

VIDEOS!

Empirical Formula : Hydrate