Outlines an experiment used to determine the formula of a common hydrate. **More good stuff available at: www.wsautter.com and http://www.youtube.com/results?search_query=wnsautter&aq=f

1. Copyright Sautter 2015

2. HOW ARE THE CHEMICAL FORMULAS FOR
COMPOUNDS DETERMINED?
• THEY CONTAIN
WATER !!
• WE ARE GOING TO CARRY
OUT AN EXPERIMENT TO
FIND THE CHEMICAL
FORMULA FOR A SUBSTANCE
KNOWN AS A “HYDRATE”
• WHAT DO YOU THINK MAKES
A COMPOUND A HYDRATE?
• HERE’S A HINT !
2

3. HERE’S AN EXAMPLE :
COPPER II SULFATE PENTAHYDRATE
• WHAT IS THE FORMULA
FOR COPPER II SULFATE?
• IT CONSISTS OF A
COPPER II ION (Cu+2) AND
A SULFATE ION (SO4
-2) SO
ITS FORUMALA IS CuSO4
• BUT WHAT ABOUT THE
WATER ?
• WHAT DOES THE PREFIX
PENTA MEAN?
• AND THE WORD
HYDRATE?
• “FIVE WATER”
• THE FORMULA IS
WRITTEN AS CuSO4
.
5 H2O
• THE “DOT” IN THE
FORMULA DOES NOT
MEAN MULTIPLY, IT
MEANS “PLUS” FIVE
WATER MOLECULES
• WHY DO YOU THINK THEY
JUST DON’T USE A “+”
INSTEAD OF THE “DOT” ?
3

4. A “+” IS USED IN A CHEMICAL EQUATION TO MEAN “REACTS
WITH” OR “IS PRODUCED ALONG WITH”.
A “ .
“ THAT THE WATER MOLECULES ARE PART OF THE
COMPOUND, NOT REACTING WITH IT.
• FOR EXAMPLE IN:
Na + H2O  NaOH + H2
THE “+” BETWEEN THE
SODIUM AND THE WATER
MEANS THEY REACT WITH
EACHOTHER TO FORM
THE PRODUCTS. THE “+”
BETWEEN THE SODIUM
HYDROXIDE AND THE
WATER MEANS THAT
WATER IS PRODUCED
ALONG WITH THE NaOH.
• IN THE CuSO4
.
5 H2O THE
WATER IS NOT REACTING
WITH THE COPPER II
SULFATE, IT IS COMBINED
WITH IT AS PART OF THE
COMPOUND.
• A DOT DOES NOT
MEAN MULTIPLY
IN A HYDRATE
FORMULA !!
4

5. HOW CAN WE FIND OUT HOW MANY WATER MOLECULES ARE
INVOLVED WITH A PARTICULAR COMPOUND?
• IT IS KNOWN THAT
BARIUM CHLORIDE IS A
HYDRATED SALT
(CONTAINS WATER
MOLECULES), BUT HOW
MANY?
• THE FORMULA FOR THE
COMPOUND IS THEN
BaCl2
.
X H2O WHERE X IS
THE NUMBER OF WATER
MOLECULES *
• * (WE KNOW THAT THE
FORMULA FOR THE NON WATER
PART OF THE SUBSTANCE IS BaCl2
BECAUSE Ba IS A +2 ION AND Cl IS
A -1 ION, THEREFORE A 1 TO 2
RATIO! THIS IS CALLED THE
ANHYDROUS PART OF THE
SUBSTANCE SINCE IT IS
“WITHOUT WATER”)
• “X” CAN BE ANY SMALL
WHOLE NUMBER BUT
CERTAINLY NEVER A
DECIMAL. IT IS
IMPOSSIBLE TO HAVE
PART OF A WATER
MOLECULE, THAT IS FOR
SURE
• HERE’S HOW WE CAN FIND
OUT!
5

6. LET’S EXPERIMENT!!
6

7. Obtain an evaporating clean, dry
Evaporating dish.
Weigh the dish and set the
Balance 25.00 grams ahead.
Add the hydrate to the dish
Until it just balances.
Heat the dish and contents on
A hot plate for 10 minutes.
Reweigh the dish and the
contents.
Reheat and reweigh the dish
Until a constant weight is obtained
7

8. 8

9. Hydrate Formula Data and Calculation Sheet
Data Entries:
(1) Mass of evaporating dish (clean and dry) ____________
(2) Mass of dish and hydrated compound ____________
(3) Mass of dish and compound after first heating ____________
(4) Mass of dish and compound after second heating ____________
(5) Mass of dish and compound after third heating ____________
(6) Name of salt - __________________________
Calculations:
(7) The correct formula for anhydrous compound is: ___________________
(8) The mass of the hydrate used in the above experiment is: (#2 - #1)
_________________
(9) The mass of water lost by the hydrate is: (#2 - #5)
_________________
(10) The mass of the anhydrous salt is: (#5 - #1)
__________________
Anhydrous means without water
9

10. (11) The molar mass of the anhydrous salt is:
___________________
(12) The moles of water lost by the hydrate: (#9/18)
__________________
(13) The moles of anhydrous salt remaining after heating are:(# 10/#11)
____________________
(14) The ratio of moles of water to moles of anhydrous salt is: (#12/#13)
_____________________
(15) The number of water molecules contained in the hydrate is: (#14 rounded off)
_____________
(16) The correct formula for the hydrated salt is:
____________ . ____H2O
(17) The correct name for this hydrate is:
___________________________
(18) The percent water found in this hydrate is:( #9/#8) x 100%
___________________________
10

11. 11

12. STEP I - OBTAIN A SAMPLE OF AN
UNKNOWN HYDRATE
• (A) WEIGH A CLEAN, DRY EVAPORATING DISH.
RECORD ITS MASS.*(Data Entry #1)
• (B) SET THE BALANCE 25.00 GRAMS AHEAD
(MORE THAN THE WEIGHT OF THE EMPTY DISH).
• (C) CAREFULLY ADD THE UNKNOWN TO THE
DISH UNTIL IT JUST BALANCES. YOU NOW HAVE
EXACTLY 25.00 GRAMS OF THE HYDRATE IN THE
DISH. RECORD THE MASS.(Data Entry #2)
• * RECORD ALL WEIGHT TO THE NEAREST .01 GRAMS
12

13. WEAR YOUR GOGGLES !!
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14. 14
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