Stoichiometry Applications 2 Unit 2

Calculations with hydrated salts:

-some inorganic salts are hydrated (water molecules incorporated in their crystal lattices)

-Anhydrous salts have no water molecules, and can be produced by heating/dehydrating hydrated salt.

Hydrated inorganic salt example

CoCI2 .6H20 - cobalt (II) chloride hexahydrate.

Means 6 water molecules for every CoCI2

Nomenclature for hydrated salt

Name ionic salt, and see prefix of number of water molecules and add hydrate - see example.

How to find the empirical formula of a hydrate salt:

stoichiometry

see steps in notes

How to measure levels of dissolved salts

electrical conductivity, as salts are ionic compounds,

Gravimetric analysis - Mass

-A quantitative analysis using mass to find the AMOUNT of substance, and can also find amount of water in sample, mass of salt in water sample (requiring the formation of a precipitate and mass-mass stoic)

How to use gravimetric analysis to find the amount of water?

weigh sample, then heat, then cool in desiccator, then weigh again.

repeat until mass is constant.

Analysis by mass

-finds amount of salt by forming (weigh, dissolve in water, add an excess of reactant) and weighing a precipitate (repeated drying and weighing until mass is constant).

Volumetric analysis (volume)

-measuring volumes of solutions. Used for acid-base and redox reactions.

If one solution has accurately known concentration, the second solution concentration can be found using stoic.

Standard solution

solution with accurately known concentration.

Prepared from a primary standard (stable, high purity, highly soluble in water, high molar mass for accurate weighing compound) that is weighed accurately and made up to a fixed volume.

What 2 methods determined a solution’s concentration?

1. react it with another solution with an accurately known concentration.

2. take a substance called a primary standard and dissolve it in a known volume of water.

Primary standards must have:

easily obtainable in pure form, inexpensive, high molecular mass for accurate weighing, easily stored w/out reacting with water/gases in atmosphere, has a known chemical formula.

How to calculate the concentration of a standard solution?

1. use chemical formula of PRIMARY standard and find molar mass of compound

2. use mass and molar mass of PRIMARY standard to find amount dissolved in volumetric flask in mol. n=m/M

3. Use mol of primary standard and volume of flask to find concentration of standard solution . C = n/v

Acid-base titrations are a type of

volumetric analysis and uses v-v stoic

Volumetric equipment

Volumetric flask - prepares standard solution, accurately weighed sample dissolved in deionised water in flask

Pipette - measures volume of solution (aliquot), poured into conical flask for analysis.

Burette - standard solution placed in it, and delivers accurate volume into titre (conical flask).

Acid base titration description

solution dispensed from burette into conical flask that contains volume of another solution (precisely measured using pipette)

acid-base indicator added to conical flask, and it changes colour when stoichiometric amounts of the reactants have been added, making it possible to see when the reaction is complete.

Sources of error during titration

indicator chosen changes colour too soon - burette (overestimated), titration flask(under) - the choice of indicator is critical

uncertainties in equipment

20 ml pipettes with +- 0.03 mL

Equivalence point

point in titration where stoichiometric proportions of the reactants have been mixed.

end point

point during titration where the chosen indicator changes colour. End point close to equivalence point for accuracy.