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

Procedure:

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)

Questions:

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