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Experiment 7- Gravimetric Analysis of a Chloride Salt

front 1

Purpose of Experiment 7?

back 1

to illustrate typical techniques used in gravimetric analysis

by quantitatively determining

the amount of chloride in an unknown

front 2

What is quantitative analysis?

back 2

aspect of analytical chemistry concerned with determining

how much of one or more constituents is

present in a sample of material

front 3

Two common quantitative methods used in analytical chemistry

back 3

Gravimetric analysis

Volumetric analysis

front 4

Gravimetric analysis

back 4

derives name from the fact that

the constituent being determined can be in isolated

in some weighable form

front 5

Volumetric Analysis

back 5

derives name from the fact that

the method used to determine amount of constituent involves

measuring the volume of a reagent

front 6

What are the steps involved in gravimetric analysis (7)?

(These are not steps, they are not are in order)

back 6

1. Dry and weigh samples of material

2. Dissolve the samples

3. Precipitate the constituent (substance you are trying to attain) by adding a suitable reagent

4. Isolate the precipitate by filtration

5. Wash precipitate to free it of contaminants

6. Dry the precipitate (to obtain weighable form)

7. Calculate the percentage of the desired consituent (from masses of the sample and precipitate)

front 7

Example of precipitation reaction

back 7

a chloride ion may be precipitated by adding the silver ion to make AgCl

- AgCl is very insoluble, so adding Ag to Cl precipitates AgCl quantitatively

(the precipitate can be collected, dried, and weighed)

From the amount of AgCl obtained, amount of Cl can be calculated

front 8

Stoichiometry

back 8

the determination of the proportions in which elements combine

and the mass relations in any chemical reaction

front 9

gravimetric factor

back 9

- converts gram of a compound into grams of a single element

- used repeatedly in analytical chemistry and are tabulated in books

front 10

Example of Gravimetric Analysis

back 10

front 11

Which acid was used in this procedure?

Which silver compound was used?

back 11

HNO3 - nitric acid

AgNO3- silver nitrate (.25M instead of .5 M)

front 12

How was the filter paper placed into the funnel?

back 12

open the paper so that it has one piece of paper against one side of the funnel and three pieces of paper against the other

- wet the paper w/ distilled water to hold it in place

front 13

Which compound was highly flammable and needed to be kept away from the flames?

back 13

acetone

front 14

What were the instructions for storing the AgCl precipitate? Why?

back 14

Keep it out of bright light

because it is photosensitive and slowly decomposes in the presence of light

(hv= symbol for electromagnetic radiation

front 15

What is the purpose of standard deviation?

back 15

to estimate the precision of your results

The smaller the deviation, the more precise the measurements

front 16

accuracy

back 16

correctness of measurement,

closeness to true result

front 17

precision

back 17

internal consistency among one's own results (reproducibility)

front 18

error

back 18

difference btwn the true result and the determined result

front 19

Determinate errors

back 19

errors in method or performance that can be discovered and eliminated

*these errors are known and are controllable (e.g.instrumental errors, human errors)

front 20

Indeterminate errors

back 20

random errors, which are incapable of discovery but which can be treated by statistics

*these errors are unknown and beyond analysts' control (e.g. room temperature

front 21

mean

back 21

arithmetic mean or overage (μ)

front 22

median

back 22

midpoint of results (if odd number of results)

average of two middle results (if even number of results)

front 23

How is relative deviation calculated?

back 23

by dividing the average deviation from mean by the mean

front 24

What is the best measure of precision

back 24

standard deviation

front 25

What is the rule for retaining or discarding a standard deviation figure?

back 25

discard any result that is more than two standard deviations away from the mean