The Formula of a Hydrate Mole Ratio of Salt to Water Std 3


Purpose: A hydrate is a salt which contains water. When heated, most or all of the

water can be driven off. The mass of water driven off can be converted to

moles and compared to the moles of dry salt (anhydrate). This molar ratio

gives the empirical formula (simplest mole ratio of water to the dry salt).

Materials: Ring stand, clay triangle, evaporating dish, burner, tongs, an unknown hydrate

salt, balance


1. Inspect your evaporating dish and be sure they are dry. Heat if necessary and

allow them to cool.

2. Determine the weight of your evaporating dish.

3. Obtain a sample of magnesium sulfate ( MgSO4 xH2O ) or other hydrated salt.

4. Fill the evaporating dish about 1 spoon full of the hydrate crystals.

5. Weigh the evaporating dish + hydrate.

6. Begin heating the evaporating dish gently to avoid spattering.

7. Continue heating for about 10 minutes or until the bottom is a dull red.

8. Allow the evaporating dish to cool about 5 minutes or until you can comfortably

touch it.

9. Weigh the evaporating dish and dry salt.

10. Repeat steps 6,7,8 and 9.

11. At this point you should have two consistent weights (both weights are the same) of

anhydrous salt. If not repeat steps 6, 7, 8 and 9.


Data/Calculations : ALL MASSES ACCURATE TO 3 DECIMALS ! ! !

Mass of evaporating dish __________ g

Mass of evaporating dish + hydrate ( MgSO4 xH2O ) __________ g

Mass of hydrate __________ g


Mass of evaporating dish + dry salt __________ g 1st heating

Mass of evaporating dish + dry salt __________ g 2nd heating

Mass of dry salt __________ g

Molar mass of dry salt ( MgSO4 ) __________ g/mole

Moles of dry salt = mass dry salt/molar mass dry salt mole dry salt __________ mole dry salt


Mass of water = mass hydrate - mass dry salt __________ g

Molar mass of water ( H2O) __________ g/mole

Moles of water = mass water/molar mass water __________ mole water

Final Calculations

Moles of water per 1 mole dry salt: __moles water / __moles dry salt = __: 1 (ratio)

Mole ratio of dry salt to water: 1 mole dry salt : __mole water (ratio)

Empirical formula of magnesium sulfate hydrate: MgSO4 ___H20 (calculated from results)



You are to write three paragraphs, one of each of the following topics:

        What you determined to be the purpose of this lab

        Sources of error (at least 2 possible sources)

        What you learned