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 • 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 -2 H2O -4 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 decimalpoints • 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 substance 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 Example: Thermal decomposition of BaCl2.2H2O - Analysis • Average mass of crucible+lid _14.3087_ (g) • Mass of crucible + lid + barium chloride hydrate _15.7175_ (g) • Mass of BaCl2.2H2O (hydrate) _1.4088 (g) • Mass of crucible + lid + anhydrous salt _15.5050_ (g) • Mass of anhydrous product _1.1963 (g) • Ratio of anhydrous product/hydrate _0.84916__
Exp 5 – Additional Information Examples of Decomposition products for Barium chloride Because the observed mass of the anhydrous product is _1.1963__ g, ratio = 1.1963/1.4088 = 0.849 the product is _BaCl2_.
Exp 5 – Additional Information Tests for Identifying hydrates • Put in 4 test tubes ~ 0.5 g each (small amount, pea sized) • Cobalt(II) chloride hexahydrate, CoCl2.6H2O • Nickel(II) chloride hexahydrate, NiCl2.6H2O • Iron(II) sulfate heptahydrate, FeSO4.7H2O • Copper(II) sulfate pentahydrate, CuSO4.5H2O • Heat gently and observe H2O vapor on side of tube • Record color changes of hydrate • Test condensation with blue litmus paper; record response If the color of the litmus paper changes, an acid is formed and released • Record color and appearance of residue • Heat 1 min in full flame; retest with moistened litmus paper • Record color observations
Exp 5 – Additional Information Part B: Identifying hydrates Observations Example: CrCl3.6H2O • Gentle heating: green solid changes to dark liquid; water vapor (condensation on side of tube) and acid (change in color of litmus paper) evolve • Intense heating: liquid changes to violet solid that increases in size; evolution of water and acid continues Tested Samples • Cobalt(II) chloride hexahydrate, CoCl2.6H2O • Copper(II) sulfate pentahydrate, CuSO4.5H2O • Nickel chloride hexahydrate, NiCl2.6H2O • Iron(II) sulfate heptahydrate, FeSO4.7H2O
Next week Due Monday March 4, 2013 For Exp 5 done today, Monday Feb 25 • Report sheets p. 83-84 • Questions #1-7 on p. 84 For next week’s Exp 2: • Prelab assignment: p.81-82 • Read and understand: • Goal of Experiment • Experimental Procedures • Safety Precautions for this lab