LABORATORY PROCEDURE
SAFETY WARNINGS:
 HOT OBJECTS –
High temperatures from hot plates and laboratory burners can cause severe burns.
Use proper care when handling hot objects and when near hot plates or burners. Only
handle hot glassware with tongs or approved devices.
Always allow an object to cool before weighing it on the balance. If it is cool to the
touch, then it can be weighed. You should test to see if objects are hot without
touching them. Bring the back of your hand near the evaporating dish, and feel for
any heat. If you feel heat, it is still too hot to handle.
DETERMINATION OF THE FORMULA OF EPSOM SALT (MgSO4.nH2O)
1. Using tongs, obtain a clean, dry evaporating dish from the oven. Dishes are kept
in the oven so that they will not absorb water from the atmosphere. Allow dish to
cool. (The dish need only be cool enough so that you can touch it. Trying to weigh
a hot dish will damage the balance and cause an erroneous reading.) Zero the
balance and place the clean dry evaporating dish on the balance. Record the weight
of the dish.
2. Place approximately accurately 1.2 grams of epsom salt into the dish and record
the weight of the dish and compound.
3. Place the dish on a hot plate (or on a clay triangle above a laboratory burner),
cover with a clean ribbed watch glass, and heat. Heat gently (set the hot plate to
“5”). As the water is driven out of the solid particles some of it will condense on
the watch glass. The crystals of epsom salt will probably split and spatter as the
water escapes, jumping out of the dish (if no cover is used) or they may stick to the
wet watch glass, both of which causes an error.
4. When the water appears to have been removed (the watch glass is dry), increase
the setting on the hotplate to “8” and heat for 4 or 5 minutes (if using a burner
increase the size of the flame).
5. Using tongs, remove the watch glass cover. If there are any crystals of epsom
salt on the underside of the watch glass brush them off into the evaporating dish
with a glass rod, and place the watch glass on a clay triangle to cool. Remove the
evaporating dish from the hot plate and sit it on another clay triangle on the bench to
cool. Then weigh it and record the weight. Don’t handle the evaporating dish while
cooling.
6. Place the evaporating dish (the watch glass cover is not needed after the first
heating) on the hot plate (or burner), and heat again for 4 or 5 minutes. Continue to
heat, cool and weigh until a constant weight is obtained (±.003g). Note: If you
delay too long before weighing the sample, it will start to pick up moisture from the
air.
7. Decrease the hot plate setting to “5”. Place the spent sample of epsom salts in the
waste container. Wipe out the inside of the evaporating dish with a dry paper towel.
(Do not wash with water.)
8. Using a new sample of epsom salt, repeat the experiment (steps 2 to 6) at least
two more times (minimum of 3 runs).
9. Using the data obtained, calculate the number of water molecules per “molecule”
of MgSO4 in epsom salts for each run separately. Write the formula and name of
the hydrate.
10. Show me your calculations and results before you clean up.