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
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
9. Weigh the evaporating dish and dry salt.
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
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
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