2nd Exam- GRAVIMETRIC

Question 1

the analyte is converted to a sparingly soluble precipitate which then filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment, and weighed.

Answer 1

Precipitation methods

Question 2

analyte is separated from other constituents by conversion to a gas of known chemical composition. The volatile product is then collected and weighed or alternatively the mass of the product is determined indirectly from the loss of mass in the sample.

Answer 2

Volatilization Methods

Question 3

Precipitation Methods STEPS WMCFWDW

Answer 3

Steps:
1. Weighing the sample
2. Moisture determination
3. Conversion of the analyte to
a sparingly soluble salt (precipitate)
4. Filtration of the precipitate
5. Washing the precipitate
6. Drying or ignition of the precipitate (conversion to a form suitable for weighing)
7. Weighing of the converted form of the precipitate

Question 4

is formed when an aggregate of atoms, ions or molecules is greater than 10–4 cm.

Answer 4

precipitate

Question 5

ions in soln < colloidal particles < precipitate

Answer 5

10^-8 < 10^-7 to 10^-4 < 10^-4 cm

Question 6

are electrically charged and resist combination to form larger particles which will settle down the solution.

Answer 6

colloidal particles

Question 7

gravimetric precipitating agent should react specifically or if not selectively with the analyte, and would give a product that is:

Answer 7

1. readilyfilteredandwashedfreewithcontaminants.
2. of sufficiently low solubility so that no significant loss of the analyte occurs during filtration and washing.
3. unreactivewiththeconstituentsoftheatmosphere.
4. of known composition after it is dried, or if necessary ignited.

Question 8

Factors that Affect the Particle Size of a Precipitate

Answer 8

colloidal suspension
crystalline suspension

Question 9

are tiny particles that are invisible to the naked eye (10-7 to 10-4 cm in diameter); shows no tendency to settle from the solution nor are they easily filtered.

Answer 9

colloidal suspension

Question 10

the temporary dispersion of such particles in the liquid phase, dimensions are in order of tenths of a millimeter or greater.

Answer 10

crystalline suspension

Question 11

The particle size of a precipitate is influenced by such experimental variables such as

Answer 11

precipitate solubility, temperature, reactant concentration, and rate at which reactants are mixed.

Question 12

The particle size of a precipitate is influenced by such experimental variables such as precipitate solubility, temperature, reactant concentration, and rate at which reactants are mixed. The net effect of these variables can be accounted by assuming that the particle size is related to a single property of a system called

Answer 12

relative supersaturation or Von Weimarn’s ratio.

= Q-S / S

Q is conc of a species at any instant

S is its equilibrium solubility

Question 13

an unstable solution that contains a higher solute concentration than a saturated solution. With time, _______ is relieved by precipitation of the excess solute.

Answer 13

Supersaturated solution; supersaturaturation

Question 14

when relative supersaturation is large, precipitate tends to be .

Answer 14

colloidal

Question 15

when relative supersaturation is small (Q is low and S is high), a _________ is more likely to form.

Answer 15

crystalline solid

Question 16

Precipitates form by two different pathways

Answer 16

1. Nucleation 2. Particle Growth

Question 17

a few ions, atoms or molecules come together to form a stable solid. Often, these nuclei form on the surface of suspended solid contaminants such as dust particles. Further precipitation then involves a competition between additional nucleation and growth on existing nuclei (particle growth).

Answer 17

In nucleation

Question 18

If _________ predominates, a precipitate containing a large number of small particles results; if __________ predominates, a smaller number of small particles results

Answer 18

nucleation, growth

Question 19

To increase the particle size of a precipitate, minimize the __________ during the precipitation formation.

Answer 19

relative supersaturation

Question 20

The
process of converting the colloidal suspension into
filterable solid is called

Answer 20

coagulation or
agglomeration or flocculation

Question 21

explains the reason why the ions
that are preferentially adsorbed on the surface of
the crystal lattice are those which are common to
the lattice and are in excess

Answer 21

Paneth-Fajans-Hahn Rule

Question 22

Paneth-Fajans-Hahn Rule layers

Answer 22

counter layer (counter-ion)
primary adsorptive layer
crystal

Question 23

refers to the process by which a coagulated colloid
reverts to its original dispersed state.

when a coagulated colloid is washed, some of the
electrolyte is leached from the internal liquid in contact
with the solid particles.

Answer 23

peptization of colloids

Question 24

Colloids are best precipitated from _________ (3) to ensure coagulation

Answer 24

hot,
stirred solutions
containing sufficient electrolyte

Question 25

– the process in which a precipitate is heated for an hour or more in the solution from which it was formed (the mother liquor). - weakly bound water appears to be lost from precipitate; the result is a denser mass that is easier to filter.

Answer 25

digestion

Question 26

are generally more easily filtered and purified than coagulated colloids, and the size of individual crystalline particles, and their filterability can be controlled to a degree.

Answer 26

Crystalline Precipitates

Question 27

(without stirring) for some time after formation frequently yields a purer, more filterable product resulting from the dissolution and recrystallization that occur continuously and at a enhanced rate at elevated temperature

Answer 27

Digestion of crystalline precipitates (without stirring)

Question 28

Methods of Improving Particle Size and Filterablity for crystalline solids

Answer 28

minimizing Q and/or maximizing S

Question 29

Minimizing Q

Answer 29

- using dilute solution - adding the precipitating reagent slowly - good mixing

Question 30

Maximizing S

Answer 30

- precipitating from hot solution - adjusting the pH of the precipitation medium

Question 31

a phenomenon in which otherwise soluble compounds are removed from solution during precipitate formation.

Answer 31

coprecipitation

Question 32

4 types of coprecipitation

Answer 32

surface adsorption mixed-crystal formation occlusion mechanical entrapment

Question 33

Methods for Minimizing Adsorbed Impurities on Colloids

Answer 33

digestion washing reprecipitation

Question 34

during this process, water is expelled from the solid to give a denser mass that has a smaller specific surface area for adsorption.

Answer 34

digestion

Question 35

a coagulated colloid with a solution that contains volatile electrolyte – exchange between existing counter-ions and ions in the wash liquid. or example, in the determination of silver by precipitation with chloride ion, the primary adsorbed species is chloride. ______ with an acidic solution converts the counter-ion layer largely to H+ so that both Cl- and H+ are retained by the solid. Volatile HCl is then given off when the precipitate is dried.

Answer 35

washing

Question 36

a drastic but effective way to minimize the effects of adsorption also known as double precipitaton. The filtered solid is redissolved and reprecipitated.

Answer 36

reprecipitation

Question 37

can cause significant contamination of precipitates with large specific surface areas, that is coagulated colloids. The net effect is therefore the carrying down of an otherwise soluble compound as a surface contaminant.

Answer 37

surface adsorption

Question 38

here, one of the ions in the crystal lattice of a solid is replaced by an ion of another element. For exchange to occur, the two ions must have the same charge and their sizes differ no more than about 5 %. The two salts must belong to the same crystal class. Examples include PbSO4 in BaSO4, MgKPO4 in MgNH4PO4 , SrSO4 in BaSO4 and MnS in CdS

Answer 38

mixed-crystal formation

Question 39

when a crystal is growing rapidly during precipitate formation, and foreign ions in the counter-ion layer may become trapped or occluded with the growing crystal.

Answer 39

occlusion

Question 40

occurs when crystals lie close together during growth, here, several crystals grow together and in so doing trap a portion of the solution in a tiny pocket.

Answer 40

mechanical entrapment

Question 41

At low supersaturation (rate of precipitation formation is low), both occlusion and mechanical entrapment are at a ______ (minimum/maximum) and ______ is helpful in reducing these types of coprecipitation.

Answer 41

minimum and digestion

Question 42

Surface adsorption and mixed crystal formation are _________ whereas occlusion and mechanical entrapment arise from the ______

Answer 42

equilibrium processes; kinetics ofcrystal growth.

Question 43

Coprecipitation error is observed whenever colloidal silver chloride adsorbs silver nitrate during a chloride analysis.

Answer 43

positive error

Question 44

Coprecipitation error When the contaminant does not contain the ion being determined, __________ may be observed.

Answer 44

either positive or negative errors

Question 45

technique in which a precipitation agent is generated in a solution of the analyte by a slow chemical reaction. The relative supersaturation is kept low during the entire precipitation because the precipitating reagent appears gradually and homogenously throughout the solution and reacts immediately with the analyte.

Answer 45

Precipitation from Homogeneous solution

Question 46

_________formed precipitates, both colloidal and crystalline are better suited to analysis that are solids formed by direct addition of a precipitating reagent.

Answer 46

homogeneously

Question 47

removes the solvent and any volatile species carried down with the precipitate

Answer 47

Heating (until its mass becomes constant)

Question 48

some precipitates are also _____ to decompose the solids and form a compound of known composition

Answer 48

ignited

Question 49

COLLOIDAL Relative supersaturation: _____ Mechanism: _____ Conditions: _____

Answer 49

High nucleation high Q, low S

Question 50

CRYSTALLINE Relative supersaturation: _____ Mechanism: _____ Conditions: _____

Answer 50

low particle growth low Q, high S

Question 51

SI UNITS Mass Length Time Temp Amount of Substance Electric Current Luminous Intensity

Answer 51

kg m s K mol A cd (candela)

Question 52

the quantitative relationship between among reacting chemical species

Answer 52

stoichiometry

Question 53

gives the simplest whole number ratio of atoms in a chemical compound

Answer 53

emperical formula

Question 54

specifies the number of atoms in a molecule

Answer 54

molecular formula

Question 55

Volatile Methods Steps

Answer 55

1) Weighing the sample 2) Moisture Determination 3) Analyte or its decomposition products are volatilized at a suitable temperature. 4) Volatile product is then collected and weighed or alternatively the mass of the product is determined directly from the loss of the sample.

Question 56

qualitatively eliminated from many inorganic samples by ignition. In direct determination, it is collected on any of the several solid desiccants and its mass is determined from mass gain of the desiccant. The indirect method determines the amount of water from the loss of mass of the sample during heating (less satisfactory because it must be assumed that water is the only component volatilized.

Answer 56

water

Question 57

are ordinarily decomposed by acids to give carbon dioxide, which is readily evolved from solution by heat. Direct determination of the mass of carbon dioxide is established from the increase in the mass of a solid adsorbent.

Answer 57

carbonates

Question 58

can also be determined by volatilization the hydrogen sulfide or sulfur dioxide evolved from the sample after treatment with acid is collected in a suitable absorbent.

Answer 58

sulfides and sulfites

Question 59

The classical method for the determination of _____ and ______ in organic compounds is a gravimetric procedure in which the combustion products (H2O and CO2) are collected selectively and weighed the increase in mass serves as the analytical parameter.

Answer 59

carbon and hydrogen