Unit 2 Equilibrium - Part 4

Question 1

Gravimetric methods

Answer 1

analytical determinations based on the mass of a solid. A method of obtaining solid analyte is via precipitation

Question 2

Steps of precipitation gravimetry

Answer 2

1) precipitate analyte
2) wash precipitate
3) dry precipitate
4) weight precipitate
5) calculate amount of analyte

Question 3

Argentometric

Answer 3

type of titration involving the silver(I) ion. It’s used for halide and halide-like anions

the precipitant is silver nitrate solution in buret

the equivalence point is the amount of silver added is equal to the amount of the analyte

endpoint is the observable change that approximately signals the equivalence point, where an indicator dye or electrochemical methods can be used

smaller Ksp value means less silver, meaning higher pAg

Question 4

Fluorescein

Answer 4

indicator for Argentometric titration. Used with titration of halides

before the eq. pt., colloidal AgX has a negatively charged surface
after the eq. pt., colloidal AgX has a positively charged surface

Question 5

Volhard method

Answer 5

indicator for Argentometric titration. Used with titration of halide solution with an excess of silver nitrate. We add Fe3+ as an indicator

we back-titrate with thiocyanate solution

forms red Fe(SCN)2+ at first excess of thiocyanate

Question 6

EDTA titrations

Answer 6

complexing agent: buret filled with EDTA solution at basic pH

auxiliary complexing agent: many cations form hydroxide precipitates at basic pH. Can use ammonia to complex cations and maintain solubility at basic pH

eq. pt.: when the amount of EDTA added is equal to the amount of metal analyte
endpoint: observable change that approximately signals the equivalence point

Question 7

At what conditions are EDTA titrations usually done at?

Answer 7

they are typically done at pH 10 and higher to maximize the quantity of Y4- (aq). A problem of this is that many metal analytes form insoluble or sparingly soluble metal hydroxides. We can avoid the formation of metal hydroxide precipitates by using an auxiliary complexing agent

Question 8

What is an example of a common auxiliary complexing agent?

Answer 8

ammonia

Question 9

Disadvantage of using auxiliary complexing agents

Answer 9

it decreases the sharpness (how steep the jump is) of endpoints in EDTA titrations. This is because of the repeated binding of the metal ion. A minimum amount of auxiliary complexing agents should be used

Question 10

When is a back titration used and what are its steps?

Answer 10

used if A and B react too slowly, or it may be difficult to indicate an endpoint

1) first A is reacted with a known and excess amount of B
2) the remaining B is titrated with C, which provides a rapid reaction and an endpoint that can be indicated
3) the difference between the amount of B measured by titration and the amount originally added permits determination of the amount of A

Question 11

Indirect-titration

Answer 11

used when the titrations reaction is too slow, a suitable indicator is not available, or if there is no useful direct titration reaction

a back titration is an example of an indirect-titration