The Formula of a Hydrate

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Outlines an experiment used to determine the formula of a common hydrate.
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The Formula of a Hydrate

  1. 1. Copyright Sautter 2015
  2. 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. 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. 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. 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. 6. LET’S EXPERIMENT!! 6
  7. 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. 8
  9. 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. 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. 11
  12. 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. 13. WEAR YOUR GOGGLES !! 13
  14. 14. 14 Click Here

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