Chapter 8 (chemical analysis)

Question 1

what is qualitative analysis?

Answer 1

It allows for identification of a specific substance based on descriptors and observations (eg. colour, smell, sight, state)

Question 2

What is quantitative analysis?

Answer 2

It determines the quantity of a substance based on numerical values and measurements.

Question 3

What are the 3 main methods of analytical technologies? Briefly explain each.

Answer 3

• colourimetry (analysis by colour: uses the light absorbed, emitted, or transmitted by the chemical
• gravimetric analysis: uses stoichiometric calculations based on a fixed mass of a reagent
• titration analysis: uses stoichiometric calculations based on the measured solution volume of a reagent

Question 4

Is colourimetry quantitative or qualitative?

Answer 4

Qualitative

Question 5

Describe colourimetry (3 points). What tool can be used in the process?

Answer 5

• most aq solutions are colourless
• solutions with monatomic and polyatomic transition ions have a colour
• the colour is due to the ion’s ability to absorb specific wavelengths of light
• a spectrometer can be used

Question 6

What is a spectrometer?

Answer 6

It is a tool that measures ion concentration and the quantity of light/

Question 7

Describe flame tests (4 points). Name a tool that can be used

Answer 7

• used to identify metal ions in unknown compounds
• usually group 1 and 2 ions
• the metal ions are heated and release certain wavelengths of visible radiation
• colours are dependent on the energy of electrons in an ion
• a spectrophotometer can be used

Question 8

What is a spectrophotometer?

Answer 8

It measures different wavelengths.

Question 9

___________________ is an important part of quantitative analysis.

Answer 9

Precipitation

Question 10

When does precipitation occur? Why is is not possible to predict the amount of excess required?

Answer 10

Precipitation occurs when a reaction forms a slightly soluble product. It is not possible to predict the amount of excess required because the amount of limited reagent is not known.

Question 11

What are 4 assumptions we make about a reaction to use stoichiometry?

Answer 11

• spontaneity
• quick
• quantitative
• stoichiometric

Question 12

________ and ________ is used to ensure precipitation completeness. Explain the process.

Answer 12

trial and error;
- mix equal volumes of each reagent
- allow precipitate to form and settle
- slowly add small amounts of excess with a dropper and watch for a precipitate (cloudiness)
- if cloudiness forms, ass more excess because the reaction is not complete
- no cloudiness means that the limiting reagent is done reacting and the reaction is complete

Question 13

What is a precipitation reaction?

Answer 13

When a solid forms from mixing liquids

Question 14

Why is there a limiting reagent?

Answer 14

• a reaction rarely has exactly the right amount of each reactant
• there’s usually left over (excess)

Question 15

What is the limiting reagent?

Answer 15

The reactant the limits the amount of product created.

Question 16

What is the excess reagent?

Answer 16

The reactant not completely used up

Question 17

Which reagent is the controlling one?

Answer 17

The limiting

Question 18

A reasonable reagent excess to ensure completion of a reaction is _____%

Answer 18

10%

Question 19

If you are given two amounts of reactants and must determine which is limiting, you should use the _______ _________.

Answer 19

Mole ratio

Question 20

What is volumetric analysis?

Answer 20

A technique used to find an amount of a certain substance by doing a titration.

Question 21

Explain a titration. Use the terms titrant, sample, burette, and Erlenmeyer flask.

Answer 21

A titration is adding a certain volume of solution substance with a known concentration (titrant) from a burette into a fixed volume of solution being analyzed (sample), usually in an Erlenmeyer flask.

Question 22

What is an equivalence point?

Answer 22

When the exact theoretical amount of titrant has been added (the reaction stops and is at its completion)

Question 23

What is an end point?

Answer 23

An experimental amount that causes an observable change; the point in a titration when a sudden colour change is observed is the end point (titration stopped, volume of titrant determined)

Question 24

What are some requirements/facts about titrations?

Answer 24

• reaction between titrant and sample
• concentration of at least one reactant must be known
• minimum of 3 trials (the results are averaged for stoichiometry calculations)
• average volume is the endpoint (should coincide with the equivalence point)

Question 25

What is a standard solution?

Answer 25

The concentration of the solution is accurately known in a standard solution.

Question 26

What is a primary standard? What state of solute cannot be a primary standard?

Answer 26

Chemical used that can be obtained with high purity and no contaminants (mass can be measured to high accuracy). They are very pure and stable. A gas solute cannot be a primary standard (because the gas will escape when a container the opened).

Question 27

What is standardizing a solution?

Answer 27

Finding the concentration of a solution after it is prepared, by reacting it with another another solution that has been prepared from a primary standard.

Question 28

describe a net ionic equation for strong monoprotic acid--strong monoprotic base reactions

Answer 28

- ions other than hydrogen and hydroxide ions are spectators - OH-(aq) + H+(aq) ---> H20(l)

Question 29

What type of titrations have a pH of 7 at their equivalence points?

Answer 29

Strong acid-base titrations

Question 30

Describe a titration curve/pH curve

Answer 30

Plotting of the pH of the sample as a function of the amount of titrant added

Question 31

When the ________ is near, one final drop of the titrant should cause a permanent ______ change if an indicator is used.

Answer 31

end point; colour

Question 32

what is the inflection point on a pH curve?

Answer 32

The point where the direction of the curvature changes

Question 33

True or false: when a titration is done, the pH curve does not end at the endpoint, and excess titrant is added to ensure completion

Answer 33

True

Question 34

Acid-base titrations are found as ________ and __________ curves.

Answer 34

monoprotic; polyprotic