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Hydrated Ionic Compounds. and how they fit into the molecular formula thing. Many ionic compounds crystallize from aqueous solution with one or more water molecules incorporated into their crystal structure. These are called hydrated ionic compounds, or “hydrates” to their close friends.

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hydrated ionic compounds

Hydrated Ionic Compounds

and how they fit into the

molecular formula thing

slide2
Many ionic compounds crystallize from aqueous solution with one or more water molecules incorporated into their crystal structure.

These are called hydrated ionic compounds, or “hydrates” to their close friends.

slide3
Hydrates have a specific number of water molecules associated with each formula unit (fu) of the compound.

eg. copper II sulfate·x hydrate—blue

The water molecules are weakly bonded, and can be removed by

heating.

CuSO4·xH2O(s) + heat CuSO4(s) + xH2O(g)

mgso 4 7h 2 o s heat mgso 4 s 7h 2 o g magnesium sulfate anhydrous heptahydrate magnesium sulfate
MgSO4·7H2O(s) + heat  MgSO4(s) + 7H2O(g)magnesium sulfate anhydrous heptahydrate magnesium sulfate

Note:

  • The dot in the formula of an ionic hydrate does not mean multiplication. It denotes a loose association of the water molecules.
  • eg. MgSO4—without any waters of hydration—is known as anhydrous magnesium sulfate.

3. Usually the number of waters of hydration is a whole number.

sample problem
sample problem

Let’s say you want to determine the number of waters of hydration in hydrated barium hydroxide, Ba(OH)2·xH2O.

How would do this? What measurements would you take?

slide7
mass the hydrate (be more specific . . .)
  • weigh empty test tube; add hydrate; reweigh
  • heat test tube over bunsen burner flame to drive off water
  • re-weigh test tube with anhydrous Ba(OH)2
here are some data
here are some data . . .

6.00 g of barium hydroxide hydrate is heated over a bunsen burner flame to drive off the waters of hydration.

After heating, 3.26 g of anhydrous Ba(OH)2 remains. Determine the number of waters of hydration in hydrated barium hydroxide.

Calculate the formula of Ba(OH)2·xH2O.

slide9
set up a ratio:

mm Ba(OH)2 : mass Ba(OH)2 obtained

mm Ba(OH)2·xH2Omass hydrate heated

171.4 g : 3.26 g

y6.00 g

slide10
cross-multiply to solve for y:

y = 315.5 g  which is

the molar mass of the hydrate.

mass of water in hydrate =

315.5 g – 171.4 g = 144.1 g which is

mass of H2O/mol hydrate. Convert to mol H2O

144.1 g/18.02 g/mol = 8 mol H2O. And so...

Ba(OH)2·8H2O is the formula of hydrate.

homework
Homework

p 277 13 – 18

p 278 52, 54, 57 – 60

We’ll do a version of Inv 6-C on p 286

Section Review and Chapter Review Questions are all good. Knock yourself out . . .

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