Copyright Sautter 2003
HOW ARE THE CHEMICAL FORMULAS FOR
COMPOUNDS DETERMINED?
•
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 !
• THEY CONTAIN
WATER !!
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” ?
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 !!
HOW CAN WE FIND OUT HOW MANY WATER MOLECULES ARE
INVOLVED WITH A PARTICULAR COMPOUND?
• IT IS KNOWN THAT
• “X” CAN BE ANY SMALL
BARIUM CHLORIDE IS A
WHOLE NUMBER BUT
HYDRATED SALT
CERTAINLY NEVER A
(CONTAINS WATER
DECIMAL. IT IS
MOLECULES), BUT HOW
IMPOSSIBLE TO HAVE
MANY?
PART OF A WATER
• THE FORMULA FOR THE
MOLECULE, THAT IS FOR
COMPOUND IS THEN BaCl2
.
SURE
X H2O WHERE X IS THE
• HERE’S HOW WE CAN FIND
NUMBER OF WATER
OUT!
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”)
LET’S EXPERIMENT!!
Add
thedish
hydrate
to
theset
dish
Heat
Reweigh
Weigh
the
the
the
and
dish
dish
contents
and
and
the
the
on dry
Obtain
Reheatan
and
evaporating
reweigh the
clean,
dish
Until
itcontents.
just
balances.
A
Balance
hot
plate
25.00
for
10
grams
minutes.
Until a constant
Evaporating
weight
dish.
is ahead.
obtained
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: ___________________
Anhydrous means without water
(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)
__________________
(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%
___________________________
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
WEAR YOUR GOGGLES !!
STEP II - HEAT AND REWEIGH THE DISH AND
CONTENTS
• (A) PLACE THE DISH CONTAINING THE HYDRATE ON A HOT
PLATE FOR ABOUT TEN MINUTES.*
• (B) REMOVE THE DISH USING CRUCIBLE TONGS AND LET
IT COOL SO YOU CAN TOUCH IT.
• (C) REWEIGH THE DISH AND CONTENTS (IT SHOULD BE
LIGHTER SINCE THE WATER IS BEING EVAPORATED FROM
THE HYDRATE BY THE HEATING) (Data Entry #3)
• (D) PLACE THE DISH BACK ON THE HOT PLATE FOR AN
ADDITIONAL FIVE MINUTES
• (E) REMOVE THE DISH, LET IT COOL AND REWEIGH IT. IF IT
IS WITHIN A GRAM OF THE WEIGHT FOUND IN PART (C)
YOU ARE DONE. IF NOT REPEAT PARTS (D) AND (E). **
WHEN THE WEIGHT IS CONSTANT, RECORD THE
WEIGHT.(Data Entry #4 and #5)
• *HEAT CAREFULLY. DO NOT LET THE CONTENTS SPATTER.
• **THIS PROCEDURE IS CALLED HEATING TO CONSTANT WEIGHT. IF THE
DISH WEIGHES THE SAME TWICE IT MEANS NO MORE WATER IS PRESENT
IN THE SAMPLE. IT IS COMPLETELY DRIED OUT
CALCULATING THE FORMULA
• (A) ASK YOUR INSTRUCTOR FOR THE NAME OF THE ANHYDROUS
SALT THAT YOU USED. (Enter on Calc. Sheet #6).
•
WRITE THE CORRECT FORMULA FOR THE SALT. (Enter on CALC.
SHEET #7)
• (B) FIND THE MASS OF WATER EVAPORATED BY SUBTRACTING
THE FINAL MASS OF THE DISH WITH THE DRIED SALT (Data Entry
#5) FROM THE ORIGINAL MASS OF THE DISH WITH THE HYDRATE
(Data Entry #2). (Enter on CALC. SHEET #9)
• (C) FIND THE WEIGHT OF THE DRIED SALT (ANHYDROUS) BY
SUBTRACTING THE MASS OF THE DISH (Data Entry # 1) FROM THE
MASS OF THE DISH WITH THE DRIED SALT (Data Entry #5). (Enter on
CALC. SHEET #10)
• (D) USING THE PERIODIC TABLE AND THE FROMULA GIVEN BY
YOUR INSTRUCTOR, FIND THE MASS OF ONE MOLE OF THE
ANHYDROUS SALT. (Enter on CALC. SHEET #11)
• (E) USING THE MASS OF WATER EVAPORATED, CALCULATE
THE MOLES OF WATER (GRAMS OF WATER LOST (CALC SHEET
# 9) DIVIDED BY 18 GRAMS PER MOLE)
• (F) USING THE MASS OF ANHYDROUS SALT, CALCULATE THE
MOLES OF SALT (GRAMS OF DRY SALT (CALC. SHEET #10)
DIVIDED BY THE MASS OF ONE MOLE FROM THE PERIODIC
TABLE (CALC. SHEET #11) (Enter on CALC. SHEET #13))
• (G) DIVIDE THE MOLES OF WATER FOUND (CALC. SHEET #12)
BY THE MOLES OF ANHYDROUS SALT FOUND (CALC. SHEET
#13) (Enter the result on CALC. SHEET #14)
• THE RATIO FOUND IN STEP (G) , MOLE OF WATER TO MOLES OF
SALT IS THE VALUE OF “X” IN THE HYDRATE FORMULA! (Enter
on CALC. SHEET #14)
• NOW LET’S USE SOME EXAMPLE NUMBERS !! WE’LL USE
COPPERII SULFATE PENTAHYDRATE IN OUR EXAMPLE.
EXAMPLE DATA FOR COPPER II SULFATE
PENTAHYDRATE
• SAMPLE DATA:
• WEIGHT OF DISH = 100.00 GRAMS
• WEIGHT OF DISH WITH HYDRATE = 125.00 GRAMS
• WEIGHT OF DISH AND SAMPLE AFTER FIRST HEATING =
118.20 GRAMS
• WEIGHT OF DISH AND SAMPLE AFTER SECOND HEATING =
116.00 GRAMS
• WEIGHT OF DISH AND SAMPLE AFTER THIRD HEATING =
115.98 GRAMS
EXAMPLE CALCULATIONS FOR COPPER II
SULFATE PENTAHYDRATE
• (A) MASS OF WATER LOST = 125.00 GRAMS - 115.98 GRAMS =
9.02 GRAMS (MASS OF DISH AND HYDRATE - MASS OF
FINAL DRIED SAMPLE)
• (B) MASS OF ANHYDROUS SALT = 115.98 GRAMS - 100.00
GRAMS = 15.98 GRAMS (MASS OF DISH AND DRIED SAMPLE
– MASS OF DISH)
• (C) MOLES OF WATER LOST = 9.02 GRAMS / 18 GRAMS PER
MOLE = .501 MOLES (MASS OF WATER/ MOLAR MASS OF
WATER)
• (D) MOLES OF ANHYDROUS SALT = 15.98 GRAMS / 159.5
GRAMS PER MOLE = .100 MOLES (WT. OF DRIED SALT /
MOLAR MASS OF ANHYDROUS SALT FROM PERIODIC
TABLE)
• (E) RATIO OF MOLES WATER TO MOLES OF ANHYDROUS
SALT = .501 MOLES WATER / .100 MOLES OF ANHYDROUS
SALT = 5.01 ROUNDED OF NEAREST WHOLE NUMBER = 5
• THE FORMULA FOR COPPER II SULFATE PENTAHYDRATE IS
FOUND TO BE CuSO4. 5 H2O
ADDITIONAL CALCULATIONS REGARDING
HYDRATED COMPOUNDS
• PERCENT COMPOSITION CALCULATIONS CAN
ALSO BE APPLIED TO HYDRATES.
• WHAT DOES PERCENT MEAN?
• IT MEANS PARTS OUT OF 100. FOR EXAMPLE A 6%
INTEREST RATE AT A BANK MEANS THAT WE GET
$6 INTEREST FOR EVERY $100 DEPOSITED!
• NOTICE THAT THE PERCENT SIGN (%) EVEN
LOOKS A BIT LIKE THE NUMBER 100!
• LET’S CALCULATE THE PERCENT WATER IN OUR
COPPER II SULFATE PENTAHYDRATE SALT.
PERCENT WATER IN A HYDRATE
• THE FORMULA USED TO CALCULATE PERCENT IS:
% = (PARTS) X 100%
(WHOLE)
IN OUR CASE WE WILL USE GRAMS OF WATER
DIVIDED BY GRAMS OF THE HYDRATE
• % WATER = 9.02 GRAMS WATER
X 100%
25.00 GRAMS OF HYDRATE
% WATER = 36.08 % IN COPPER II SULFATE
PENTAHYDRATE
ANOTHER METHOD OF CALCULATING
THE % WATER IN A HYDRATE IF THE
CORRECT FORMULA IS KNOWN
• SINCE WE KNOW THE FORMULA FOR COPPER II
SULFATE PENTAHYDRATE WE WILL USE IT AS AN
EXAMPLE.
• CuSO4. 5 H2O CONTAINS 5 WATER MOLECULES
EACH WITH A MOLAR MASS OF 18 GRAMS FOR A
TOTAL OF 5 X 18.0 = 90.0 GRAMS.
• THE HYDRATED SALT (CuSO4. 5 H2O ) HAS A
MOLAR MASS OF 249.5 GRAMS.
• THEREFORE THE % WATER CAN BE CALCULATED
BY:
% WATER = 90.0 GRAMS X 100 % = 36.08 %
249.5 GRAMS
THINGS YOU SHOULD HAVE LEARNED
DURING THIS EXCERISE.
•
•
•
•
•
(1) WHAT COMPOSES A HYDRATED SALT
(2) WHAT ANHYDROUS MEANS
(3) WHAT DRYING TO “CONSTANT WEIGHT” MEANS
(4) HOW THE FORMULA FOR A HYDRATE IS WRITTEN
(5) HOW TO DETERMINE THE FORMULA OF A HYDRATE
USING EXPERIMENTAL METHODS AND DATA OBTAINED
FROM THE EXPERIMENT
• (6) HOW TO CALCULATE THE PERCENT WATER IN A
HYDRATE FROM EXPERIMENTAL DATA
• (7) HOW TO CALCULATE THE PERCENT WATER IN A
HYDRATE FROM A KNOWN FORMULA