Exp 5 – Percent Water in a Hydrated Salt Background In chemistry, a neutral ionic compound is typically referred to as a “salt” Hydrates are inorganic salts that contain a specific number of water molecules crystallized with the salt (“water of crystallization” or “water of hydration“) Examples are CaCl2.4H2O 4 mol H2O/mol CaCl2 CuSO4.5H2O 5 mol H2O/mol CuSO4 (blue) BaCl2.2H2O 2 mol H2O/mol BaCl2 Na2HPO4.12 H2O 12 mol H2O/mol Na2HPO4 Most hydrates have simple formulas with a ratio of 1 salt: xH2O,but it is not always the case cadmium sulfate hydrate is best represented as (CdSO4)3.8H2O When determining the formula of a hydrate you must not assume that it is one with a simple formula.

Exp 5 – Percent Water in a Hydrated Salt Adsorption and Absorption Salts attract water from air and gain (some) weight Adsorption of H2O on crystal surface Some salts incorporate H2O in crystal structure Absorption of H2O Definite amount of H2O as part of crystal structure Hydrates or Hydrate Salts

Exp 5 – Percent Water in a Hydrated Salt Hydrates can normally be decomposed into the anhydrous (without water) salt and water by gentle heating. From mass data collected the number of molecules of hydrated water can be determined per molecule of anhydrous salt. Heating removes H2O gradually yields anhydrous form CoCl2.6H2O 6 mol H2O/mol CoCl2 (red) (hexahydrate) CoCl2.2H2O 2 mol H2O/mol CoCl2 (violet) (dihydrate) CoCl2 no H2O, anhydrous (blue) CoCl2.6H2O CoCl2.2H2O CoCl2 D D violet -4 H2O -2 H2O red blue

Exp 5 – Percent Water in a Hydrated Salt Some salts spontaneously lose water molecules to the atmosphere: efflorescent Some salts spontaneously absorb water from the atmosphere: deliquescent Determinations carried out by measurements of mass throughout the experiment are referred to as gravimetric analysis Thus, the determination of % by mass of water in a hydrated salt uses gravimetric analysis.

Exp 5 – Percent Water in a Hydrated Salt Part A: Sample Preparation Heat a crucible + lid on a clay triangle in a the flame of a lab burner for about 5 minutes (as you did for Exp. 7) Let the crucible+lid cool to warm-to-the-touch and weigh. Record in 4 decimal points Reheat the crucible+lid to make sure the crucible is really dry and repeat the weighing as above Repeat until two consecutive weightings are within 0.001 g of each other Report the weights on the lab report sheet and determine the average of your weighings

Exp 5 – Percent Water in a Hydrated Salt Part A (continued): Average mass of crucible+lid Mass(g) Add about 2.0 –2.5 g sample into the fired cool crucible Weigh crucible + lid + hydrate sample (0.0001 g precision) Mass of hydrate sample = (crucible + lid + hydrate sample ) – (crucible+lid) Part B: Thermal decomposition product of sample Heat the crucible gently for 2-3 min with lid slightly ajar Continue heating at full flame for 10 min Cool crucible to “warm to touch” and weigh Repeat heating for 2 min, and cool crucible (“warm to touch”) Weigh crucible + lid + residue (0.0001 g precision) Record as Final mass if the difference from previous weighing is not more than + or - 0.010 g

Exp 5 – Percent Water in a Hydrated Salt Calculations: #4. Percent by Mass of volatile water % by mass= mass water x 100% mass hydrated salt #6. Standard Deviation* of % H2O: Measures the spread of your results (how far they are from each other) #7. Relative Standard Deviation* (%RSD): Measures the spread of the individual results compared to the average result

Exp 5 – Additional Information Why is it important to apply heat only as instructed? The salt itself could decompose into other compounds Can correct % calculations still be obtained? Yes but you must know the formulas of the possible decomposition products and do extra calculations _

CORRECTION: USE an amount of sample ~ 1.5 to 2.0g for a narrow crucible USE 2.0-2.5 g for a wider-mouth crucible

Next week Due Monday July1, 2013 Exp 5 done today Report sheets p. 83-84 Questions #1-7 on p. 84 Next week’s Exp 2 p.53: Prelab assignment: answer Read and understand: Goal of Experiment Experimental Procedures Safety Precautions for this lab