Exp 5 Percent Water in a Hydrated Salt

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 • Ca. Cl 2. 4 H 2 O • Cu. SO 4. 5 H 2 O • Ba. Cl 2. 2 H 2 O • Na 2 HPO 4. 12 H 2 O 4 mol H 2 O/mol Ca. Cl 2 5 mol H 2 O/mol Cu. SO 4 (blue) 2 mol H 2 O/mol Ba. Cl 2 12 mol H 2 O/mol Na 2 HPO 4 – Most hydrates have simple formulas with a ratio of 1 salt: x. H 2 O, but it is not always the case • cadmium sulfate hydrate is best represented as (Cd. SO 4)3. 8 H 2 O • 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 H 2 O on crystal surface • Some salts incorporate H 2 O in crystal structure – Absorption of H 2 O – Definite amount of H 2 O 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 H 2 O gradually – yields anhydrous form – Co. Cl 2. 6 H 2 O – Co. Cl 2. 2 H 2 O – Co. Cl 2. 6 H 2 O red 6 mol H 2 O/mol Co. Cl 2 (red) red (hexahydrate) 2 mol H 2 O/mol Co. Cl 2 (violet) violet (dihydrate) no H 2 O, anhydrous (blue) blue D -4 H 2 O Co. Cl 2. 2 H 2 O violet D -2 H 2 O Co. Cl 2 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-thetouch 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): 1. Average mass of crucible+lid 2. Add about 2. 0 – 2. 5 g sample into the fired cool crucible 3. Weigh crucible + lid + hydrate sample (0. 0001 g precision) 4. Mass of hydrate sample = (crucible + lid + hydrate sample ) – (crucible+lid) Part B: Thermal decomposition product of sample 1. 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 2. 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 Mass(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 % H 2 O: • 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. 0 g for a narrow crucible • USE 2. 0 -2. 5 g for a wider-mouth crucible

Next week Due Monday July 1, 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 Experimental Procedures Safety Precautions for this lab