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 (redred) (hexahydrate)– CoCl2.2H2O 2 mol H2O/mol CoCl2 (violetviolet) (dihydrate)– CoCl2 no H2O, anhydrous (blueblue)

CoCl2.6H2O CoCl2.2H2O CoCl2 -4 H2O -2 H2Ored

violetblue

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):

1. Average mass of crucible+lid Mass(g)

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

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

SubstanceFormula Weight

Moles Mass (g) Ratio

BaCl2.2H2O 244.27 0.0057674 1.4088

BaCl2. H2O226.25 0.0057674 1.3048 0.926

BaCl2208.24 0.0057674 1.2010 0.852

BaO153.33 0.0057674 0.8843 0.628

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 hydrates1. Put in 4 test tubes ~ 0.5 g each (small amount, pea sized)

a) Cobalt(II) chloride hexahydrate, CoCl2.6H2O

b) Nickel(II) chloride hexahydrate, NiCl2.6H2O

c) Iron(II) sulfate heptahydrate, FeSO4.7H2O

d) Copper(II) sulfate pentahydrate, CuSO4.5H2O

1. Heat gently and observe H2O vapor on side of tube2. Record color changes of hydrate3. Test condensation with blue litmus paper; record response

If the color of the litmus paper changes, an acid is formed and released5. Record color and appearance of residue6. Heat 1 min in full flame; retest with moistened litmus paper7. Record color observations

Exp 5 – Additional Information

Part B: Identifying hydrates

Observations

Example: CrCl3.6H2O

a) Gentle heating: green solid changes to dark liquid; water vapor (condensation on side of tube) and acid (change in color of litmus paper) evolve

b) Intense heating: liquid changes to violet solid that increases in size; evolution of water and acid continues

Tested Samplesa) Cobalt(II) chloride hexahydrate, CoCl2.6H2O

b) Copper(II) sulfate pentahydrate, CuSO4.5H2O

c) Nickel chloride hexahydrate, NiCl2.6H2O

d) Iron(II) sulfate heptahydrate, FeSO4.7H2O

Next week

Due Monday March 4, 2013For 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