Copyright Sautter 2003

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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 !

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 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!

LET’S EXPERIMENT!!

Obtain an evaporating clean, dryEvaporating dish.

Weigh the dish and set theBalance 25.00 grams ahead.

Add the hydrate to the dishUntil it just balances.

Heat the dish and contents onA hot plate for 10 minutes.Reweigh the dish and the

contents.Reheat and reweigh the dish

Until a constant weight is obtained

Hydrate Formula Data and Calculation SheetData 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

(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