Analytical Chemistry Chapter 4

Question 1

Are based on mass measurement.

Are quantitative methods that are based on
determining the mass of a pure compound to which the analyte is chemically related.

Are based on mass
measurements with an analytical balance, an instrument that yields highly accurate and precise data.

Answer 1

Gravimetric methods

Question 2

Two major types of gravimetric methods

Answer 2

Precipitation methods and Volatilization methods

Question 3

The analyte is converted to a soluble precipitate, then filtered, wash and converted to a product of known composition by suitable heat treatment that can be weighed.

Answer 3

Precipitation Method

Question 4

The analyte or its decomposition products are volatilized at a suitable temperature. The volatile product is then collected and weighed, or the mass of the product is determined indirectly from the loss in mass of the sample.

Answer 4

Volatilization Method

Question 5

Developed or refined many of the techniques of gravimetric analysis involving silver and chlorine.

The techniques were used to determine the atomic weights of 25 of the elements

Became the first American to receive Nobel prize in chemistry.

Answer 5

Theodore W. Richards

(and his students at Harvard University)

Question 6

Ideally, a gravimetric precipitating agent should react ______ or at least _____ with the analyte.

Answer 6

specifically

selectively

Question 7

_____, which are rare, react only with a single chemical species.

_____, which are more common, react with a limited number of
species.

Answer 7

Specific reagents

Selective reagents

Question 8

In addition to specificity and selectivity, the ideal precipitating reagent would react with the analyte to give a product that is:

Answer 8

1. Filtered and washed free of contaminants;
2. Of sufficiently low solubility that no significant loss of the analyte occurs during
   filtration and washing;
3. Unreactive with constituents of the atmosphere;
4. of known composition after it is dried or, if necessary, ignited

Question 9

Precipitates consisting of large particles are generally desirable for gravimetric
work. Why?

Answer 9

Because these particles are easy to filter and wash free of impurities. In addition, precipitates of this type are usually purer than are precipitates made up
of fine particles.

Question 10

The particle size of solids formed by precipitation varies enormously. At one
extreme are ________, whose tiny particles are invisible to the naked eye.

Colloidal particles show no tendency to settle from solution and are difficult to filter.

Because particles of colloidal dimensions scatter visible radiation, the path of the beam through the solution can be seen by the eye.
This phenomenon is called the _____. At the other extreme
are particles with dimensions on the order of tenths of a millimeter or greater.

The temporary dispersion of such particles in the liquid phase is called a _________. The particles of a crystalline suspension tend to settle spontaneously and
are easily filtered.

The particle size of a precipitate is influenced by precipitate solubility, temperature,
reactant concentrations, and the rate at which reactants are mixed. The net effect of these variables can be accounted for, at least qualitatively, by assuming
that the particle size is related to a single property of the system called_______, where:

RS = (Q-S)/S

This equation is also known as: __________

(Q-S)/S is large the precipitate is?

(Q-S)/S is small the precipitate is?

Answer 10

colloidal suspensions

tyndall effect

crystalline
suspension

relative
supersaturation

Q = concentration of the solute at any instant

S = equilibrium solubility

Von Weimarn Equation

Colloidal

Crystalline Solid

Question 11

A ______ consists of solid particles with diameters that are less than
10^-4 cm.

In _____, colloidal
suspensions may be perfectly ____ and appear to contain _____. The
presence of the second phase can be
detected, however, by shining the
________ into the solution.

Answer 11

colloid

diffuse light

clear and no solid

beam of flashlight

Question 12

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

Answer 12

Supersaturated solution

Question 13

The effect of relative supersaturation on particle size can be explained if we assume
that precipitates form in two ways: by _______

Answer 13

nucleation
particle growth

Question 14

Is a process in which a minimum number of atoms, ions,
or molecules join together to give a stable solid.

Precipitates form by nucleation and by particle growth. If nucleation predominates, a _____
number of very fine particles is produced. If particle growth predominates, a _____ number of larger particles is obtained.

Answer 14

Nucleation

large

smaller

Question 15

Precipitates that have very low solubilities, such as many sulfides and hydrous oxides, generally form as ______.

Answer 15

colloids

Question 16

Individual colloidal particles are so small that they are not retained by ordinary filters. Moreover, _____ prevents their settling out of solution under the influence of gravity.

Answer 16

Brownian motion

Question 17

Coagulation can be hastened by:

Answer 17

heating
stirring
adding an electrolyte

Question 18

is a process in which a substance (gas, liquid, or solid) is held on the surface of a solid.

is retention of a substance within the pores of a solid.

Answer 18

Adsorption

Question 19

The charge on a colloidal
particle formed in a gravimetric analysis is determined by the
_____________ that is in excess when the precipitation is complete.

Answer 19

charge of the lattice ion

Question 20

Attached directly to the solid surface is the ________, which consists mainly of adsorbed silver ions.

Surrounding the charged particle is a layer of solution, called the _______, which
contains sufficient excess of negative ions principally nitrate) to just balance the charge on the surface of the particle.

The primarily adsorbed silver ions and the negative counter-ion layer constitute an _______ that imparts stability to the colloidal suspension.

As colloidal particles approach one another,
this double layer exerts an electrostatic repulsive force that prevents particles from
colliding and adhering.

Answer 20

primary adsorption layer

counter-ion layer

electric double layer

Question 21

Is a process by which
a coagulated colloid returns to its dispersed state.

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

Answer 21

Peptization

Question 22

Colloids are best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation. The filterability of a coagulated colloid often improves if it is allowed to stand for an hour or more in contact with the hot solution from which it was formed. During this process, which is known as ________, weakly
bound water appears to be lost from the precipitate. The result is a denser mass that
is easier to filter.

Answer 22

digestion

Question 23

Is a process in which a
precipitate is heated in the solution from which it was formed (the
mother liquor) and allowed to stand
in contact with the solution.

Answer 23

Digestion

Question 24

are generally more easily filtered and purified than are coagulated colloids.

Answer 24

Crystalline precipitates

Question 24

The value of Q can often be minimized by using ____________, with good mixing. Often, S is increased by ____________.

Answer 24

dilute solutions and adding the precipitating reagent slowly precipitating from hot solution or by adjusting the pH of the precipitation medium

Question 25

Is the solution from which a precipitate was formed.

Answer 25

Mother liquor

Question 26

Improves the purity and filterability of both colloidal and crystalline precipitates.

Answer 26

Digestion

Question 27

is a process in which normally soluble compounds are carried out of solution by a precipitate

Answer 27

Coprecipitation

Question 28

When otherwise soluble compounds are removed from solution during precipitate formation, we refer to the process as ________. Contamination of a precipitate by a second substance whose solubility product has been exceeded is _________.

Answer 28

coprecipitation not coprecipitation

Question 29

There are four types of coprecipitation: _______ formation are equilibrium processes, and _______ arise from the kinetics of crystal growth.

Answer 29

surface adsorption mixed-crystal formation occlusion mechanical entrapment Surface adsorption and mixed-crystal occlusion and mechanical entrapment

Question 30

Is a common source of coprecipitation and is likely to cause significant contamination of precipitates with large specific surface areas, that is, coagulated colloids.

Answer 30

Adsorption

Question 31

Is often the major source of contamination in coagulated colloids but of no significance in crystalline precipitates. In adsorption, a normally soluble compound is carried out of solution on the surface of a coagulated colloid. This compound consists of the ______ and an ______ from the counter-ion layer.

Answer 31

Adsorption primarily adsorbed ion ion of opposite charge

Question 32

_______ is defined as the surface area per unit mass of solid and usually has the units of square centimeters per gram.

Answer 32

Specific surface area

Question 33

A drastic but effective way to minimize the effects of adsorption is _______. In this process, the filtered solid is redissolved and reprecipitated. The first precipitate usually carries down only a fraction of the contaminant present in the original solvent.

Answer 33

reprecipitation or double precipitation

Question 34

is a type of coprecipitation in which a contaminant ion replaces an ion in the lattice of a crystal.

Answer 34

Mixed-crystal formation

Question 35

Is a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth.

Answer 35

Occlusion

Question 36

Can cause either negative or positive errors.

Answer 36

Coprecipitation

Question 37

Is a process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution.

Answer 37

Homogeneous precipitation

Question 38

After filtration, a ________ is heated until its mass becomes constant. ______ removes the solvent and any volatile species carried down with the precipitate. Some precipitates are also ignited to decompose the solid and form a compound of known composition. This new compound is often called the_____.

Answer 38

gravimetric precipitate Heating weighing form.

Question 39

The temperature required to dehydrate a precipitate completely may be as low as ____ or as high as _____.

Answer 39

100°C 1000°C

Question 40

Recording thermal decomposition curves is called ____ The mass versus temperature curves are termed______.

Answer 40

thermogravimetry or thermal gravimetric analysis thermograms

Question 41

There are two types of organic reagents: (1) one forms slightly soluble nonionic products called ________, (2) forms products in which the bonding between the______.

Answer 41

coordination compounds inorganic species and the reagent is largely ionic

Question 42

Approximately two dozen cations form sparingly soluble chelates with 8-hydroxyquinoline. Is an organic precipitating agent of unparalleled specificity. Only nickel(II) is precipitated from a weakly alkaline solution. Is an important example of an organic precipitating reagent that forms salt-like precipitates.

Answer 42

Hydroxyquinoline Dimethylglyoxime Sodium Tetraphenylborate

Question 42

Organic reagents that yield sparingly soluble coordination compounds typically contain at least two functional groups. Each of these groups is capable of bonding with a cation by donating a pair of electrons. The functional groups are located in the molecule such that a five- or six-membered ring results from the reaction. Reagents that form compounds of this type are called ______, and their products are called ______. Are cyclical metal-organic compounds in which the metal is a part of one or more five- or six-membered rings. Ex: Heme

Answer 42

chelating agents chelates

Question 43

The two most common gravimetric methods based on volatilization are those for determining: _____is quantitatively distilled from many materials by heating. In _____, water vapor is collected on any of several solid desiccants, and its mass is determined from the mass gain of the desiccant. The _______ in which the amount of water is determined by the loss of mass of the sample during heating is less satisfactory because it must be assumed that water is the only component that is volatilized. Widely used to determine water in items of commerce.

Answer 43

water and carbon dioxide Water direct determination indirect method

Question 44

classical method for the determination of carbon and hydrogen in organic compounds is a _____ in which the combustion product H2O and CO2 are collected selectively on weighed absorbents. The ______serves as the analytical parameter.

Answer 44

Gravimetric Procedure increase in mass