How To Find The Formula Of A Hydrate

Determining the Formula of a Hydrate

Background: Forms of Matter
Ionic Hydrates
Procedure

Objectives

Introduction to quantitative analysis: determination of proportions of subunits in a chemical sample
Laboratory tool: Bunsen burner
Introduction to forms of affair with example of ionic hydrates

Background: Forms of Matter

Elements: Matter composed of but one type of atom (eastward.grand. Na, He, O₂, P₄)
Compounds: Matter composed of more than one type of atom. E.thou.
- Na₂SO_iv (ionic chemical compound; Na₂SO_iv is the formula unit)
- H_iiO (covalent; exists as split up H₂O molecules)
Molecules: Matter composed of more than one atom; the atoms are joined past covalent bonds (e.g., O₂, P_iv, C₆H₁₂O₆, TiCl_four).

Ionic Hydrates

Ionic compounds (salts) in which 1 or more water molecules are bound in the crystalline structure of the salt:

CoCl_ii	^.	6H₂O
(anhydrous salt)		(waters of hydration)

BaCl_two ^.2H₂O

CaCO₃ ^.two 1/ii H_twoO

CaSO₄ ^.5H_twoO

There is a unique ratio of water molecules to formula units in the crystalline structure.

Interactions in an Ionic Hydrate

BaCl₂ ^.3H₂O

Process (Overview)

Decide the mass of a dry crucible.
Add a known mass of ionic hydrate.
Heat to vaporize waters of hydration.

Heat the anhydrous production to a constant mass ., due east.g.,

	estrus
ZnSO₄ ^.xH₂O (s)	--->	ZnSO_iv (s)	+	xH₂O
(hydrated table salt)		(anhydrous		(water
		salt)		lost)

From the masses of vaporized h2o and of the anhydrous ionic salt, determine the formula of the hydrate.

Procedure (Details)

Each educatee will work individually.

Thoroughly wipe a crucible and cover with a clean cloth towel to remove dirt and other particulate thing. Then decide the mass of the crucible (and cover) to +/- 0.01 g.
Obtain almost 1 thousand of a hydrate sample and transfer the sample to the crucible.
1. Samples will be found on the reagent counter; exist sure to write down the identification code.
2. Examination tubes containing one thou of hydrate samples will also be found on the reagent counter. To quickly obtain about 1 g of hydrate sample, accept a test tube from your locker and obtain about the same quantity of hydrate, from a reagent bottle, as you lot find in i of the reference samples. After you obtain your sample, quickly replce the cap to the reagent canteen and tighten securely.
3. Transfer the hydrate sample to your crucible.
Determine the mass of the crucible, hydrate sample, and crucible cover.
Using your crucible tongs, place the crucible and contents back on the clay triangle. Partially cover the opening of the crucible with the cover.
Heat your crucible and its contents with a low flame for 5 min. Increase the flame temperature and heat with a medium flame for 5 min. Further increase the flame and heat the sample for an boosted 10 min. Do non permit the crucible to turn cerise. (Overheating may pb to decomposition of your sample!)
Using crucible tongs, remove the crucible (with cover in place) from the clay triangle and identify on wire gauze on the lab bench.
Permit the crucible to absurd to room temperature. (Agree your manus about 1 cm to a higher place the crucible to exam.) Then determine the mass of the crucible and contents (and cover) to +/- 0.01 g.
Heat your sample to constant mass.
- Reheat your crucible, with the cover ajar as before, for 10 min. using a hot flame. Permit the crucible to cool to room temperature and reweigh.
- The difference between the masses of the crucible plus sample, after the kickoff and 2d heating, should be no greater than 0.03 g.
- If the difference is greater than 0.03 g, repeat the heating and cooling procedure until the departure between consecutive heatings is less than this limit.
- You will and so take heated your sample to a "constant mass".
Subsequently yous have heated your crucible and contents to constant mass, transfer the crucible'southward contents to the disposal container.
Y'all may wish, if time permits, to perform a second determination.

Determine the formula of the hydrate

Samples i, iii, and v are hydrates of magnesium sulfate, MgSO₄ ^.xH₂O
Samples ii and four are hydrates of zinc sulfate, ZnSO_four ^.xH₂O

To make up one's mind the formula, you must make up one's mind the following

	# mols H_iiO		# mols H_iiO
ten =	^{______________}	; or y =	^{______________}
	# mols MgSO₄		# mols ZnSO₄

To make up one's mind the number of moles of anhydrous salt and of H_iiO:

			1 mol MgSO₄
# mols MgSO₄ =	g MgSO₄	x	^{______________}
	(measured mass of		g MgSO₄
	anhydrous salt)		(1/tooth mass)

			i mol H_twoO
# mols H₂O =	thou H₂O	x	^{______________}
	(mass of		thousand H_iiO
	water lost)		(one/tooth mass)