Empirical Formula of a Hydrate



Empirical Formula of a Hydrate

Questions: What is the percentage of water in a hydrated salt? What is the empirical formula of a hydrated salt?

Introduction:

Many inorganic compounds take up definite proportions of water as an integral part of their crystal structures. This water of crystallization may be driven off by the application of heat. This will allow you to determine the percentage of water that the hydrated salt contained. Since the law of definite composition holds true for all compounds, the number of moles driven off is some simple, whole number. If the formula of the anhydrous compound is known, you can then determine the formula of the hydrate.

Equipment/Materials:

Top loading balances Hydrated salt compounds

Hot plates Heat resistant pads

400 mL beakers Beaker tongs

Safety Considerations:

Wear safety goggles and apron. Handle hot beakers with beaker tongs. Remember that hot glass does not look hot!

Procedure:

1. Record the name of the hydrated salt to be used by your group in the data table.

2. Determine the mass of a clean, dry beaker and record in the Data Table.

3. Add enough of a hydrated inorganic salt compound to thinly cover the bottom of the beaker. Record the mass of the beaker and hydrate in the Data Table.

4. Using beaker tongs place the beaker and hydrate on the hot plate. Turn the hot plate on low heat (no more than 2 or ¼ of the way). Heat until condensation formation at the top of the beaker stops.

5. Using beaker tongs remove the beaker from the hot plate and allow it to cool on the pad for 5 minutes.

6. Record the mass of the beaker and salt.

7. Place the beaker and hydrate back on the hot plate (remember, DO NOT touch the beaker). Heat on low until no more condensation forms on top of the beaker.

8. Remove the beaker from the hot plate and allow it to cool for 2 minutes.

9. Repeat steps 6 and 7 until two consecutive mass readings agree within 0.01 g. Once the mass readings agree within 0.01 g, you have reached constant mass and all water has been removed from the hydrate.

10. Once all readings have been taken, return the anhydrous compound to your teacher for proper disposal. Clean all apparatus and make sure your lab bench is clean. Be sure to wash your hands before leaving the laboratory.

Data Table:

|Name of Hydrated Salt: | |

|Mass of beaker | g |

|Mass of beaker and hydrate | g |

|Mass of beaker and salt after 1st heating | g |

|Mass of beaker and salt after 2nd heating | g |

|Mass of beaker and salt after 3rd heating (if | g |

|necessary) | |

Calculations. Show all work.

1. What was the original mass of the hydrated salt?

2. What is the mass of water that was driven from the salt?

3. What percentage of the hydrated salt was water?

4. A salt that contains water is a hydrate. A salt that has had the water driven off is called anhydrous. What is the mass of anhydrous salt?

5. What is the formula of the anhydrous salt? What is the molar mass?

6. Convert the mass of the anhydrous salt to moles.

7. What is the molar mass of water?

8. Calculate the number of moles of water driven off.

9. The following is an example of the empirical formula of a hydrated crystal:

BaCl2 ( 2 H20

In this compound, for every one mole of barium chloride, there are two moles of water. It is the hydrated salt “Barium Chloride Dihydrate”. This is the SIMPLEST WHOLE NUMBER RATIO OF MOLES TO MOLES. The formulas are written so that it is apparent that for every one mole of anhydrous salt, there are a given number of moles of water of crystallization. Using your information, calculate the ratio of the moles of anhydrous salt to moles of water driven off. The ratio must be in whole numbers. Divide BOTH numbers by the smallest of the two so one of the whole numbers will be “1”, and round the other to the closest whole number. Show your work below.

10. After determining the ratio, write the formula as in the example above and then determine the name of your salt. The IUPAC prefixes used to write formulas are:

[pic]

What is the formula of your hydrated salt (use the example in question 9)?

What is the name of your hydrated salt?

Extended Questions:

11. If you stopped your experiment short, and did not drive off all of the water, how would that affect the empirical formula that you determined?

12. What procedure did you perform in this experiment to try to ensure all the hydrate was evaporated?

13. Calculate molar mass of your hydrate.

14. If you did not get the correct empirical formula, discuss possible sources of error.

15. If you were told that your hydrate had a formula of CoCl2(6H2O and that you had 1.80 moles of CoCl2, how many moles of H2O would be present?

16. What is the name of CoCl2(6H2O?

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