Period 1 - Gravimetric Estimation of Chloride Ions

8/6/2019 Period 1 - Gravimetric Estimation of Chloride Ions

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Aneeqa Haider, Ariel Tsang, Carrie Fan, Fabiha Nuzhat


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Chloride ion

Cl + e Cl

Chloride

Results from thecombination of Cl2 with

a metal (e.g. NaCl)

Reactants: NaCl,

AgNO3Chloride Ion


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Gravimetric analysis The set of procedures to determine the quantity of a

substance present in the mass of a solid

Seven steps

1. Drying and measuring the masses of samples to beanalysed

2. Dissolving the sample in distilled water.

3. Precipitating the substance by adding a reagent.

4. Separating the precipitate from the solution byfiltration.

5. Washing precipitate free of impurities.

6. Drying precipitate to obtain mass.

7. Determine the amount of the original ion based on theknown mass and composition of the precipitate.


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Objective: Determine the amount ofchloride ions present in a given solutionof NaCl using AgNO3 as a reagent.

Problem: What is the amount of chlorideions present in 0.2 g of Sodium Chloride(NaCl)?

Hypothesis: The number of chloride ionspresent in 0.2 g of NaCl is approximately2.06 x 10.


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Erlenmeyer Flask

(2)

Beaker (1)

Funnel (1)

Ashless Filter Paper

(1)

Paper Clips (4)

Balance

0.2 g of Sodium

Chloride -NaCl

3 g of Silver Nitrate -

AgNO3 (aq)

Distilled Water

Dropper

Test Tubes (2)

Bunsen Burner

Crucible and lid

Crucible tongs

Retort Stand

Ring Clamp

Clay Triangle

Safety Goggles

Spatula

Stirring Rod

Graduated Cylinder


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1. Formation of the precipitate

2. Filtration of the solution containing the

precipitate

3. Measurement of the mass of AgCl bydrying the filter paper

4. Measurement of the mass of AgCl byburning the ashless filter paper


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0.2 g of NaCl wasdissolved in DistilledWater in Erlenmeyer

Flask 3.4 g of AgNO3 (aq) was

poured into ErlenmeyerFlask containing NaCl

(aq). Solution was put into

rest until all theprecipitate formed.


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Filter paper, funnel and Erlenmeyer flask

were set up

The solution containing the precipitatewas poured through the filter paper

Washed periodically with Distilled water


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Filter paper and precipitate were completely

dried

Mass of the precipitate:

Mass of the filter paper with precipitate -

Mass of the filter paper


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Retort stand, ring clamp, clay triangle, andBunsen burner were set up

Filter paper was carefully folded with the

precipitate inside, and placed in thecrucible

Crucible was heated until no more filterpaper was left

Mass of Precipitate: Mass of crucible, lid & precipitate

Mass of crucible & lid


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General Safety

Precautions

Safety precautions

specific for this

experiment:

Avoiding contact with Silver Chloride (AgCl)

Safety precautions while using the Bunsen burner


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Formation of the Precipitate

Objects Mass (g) Calculation

Filter paper 1.04 gFilter paper with NaCl 1.24 g

NaCl 0.2 g (1.24 g - 1.0 g)

Graduated cylinder 22.48 g

Graduated cylinder with

AgNO3

27.08 g

AgNO3 3.4 g (27.48 g - 22.48 g)


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Measurement of the mass of AgCl by

drying the filter paper


Filter paper 1.04 g

Filter paper with

AgCl

1.43 g

AgCl 0.39 g (1.43 g - 1.04 g)


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Measurement of the mass of AgCl by

drying the filter paper


Crucible and lid 32.13 g

Crucible, lid and

contents (AgCl)

32.64 g

AgCl 0.51 g (32.64 g - 32.13 g)


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Mass used of Sodium Chloride (NaCl):

0.2 g

Molar mass of NaCl: 35.45 g/mol

Percentage composition by mass of Silver

Chloride (AgCl):

Silver (Ag) = 75%

Chloride (Cl) = 25%


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Mass of filter paper:

1.04g

Mass of filter paper + AgCl:

1.43g

AgCl:

(1.43g 1.04g) = 0.39g

Mass of chloride ions present:

0.25 x 0.39g = 0.0975g


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Number of moles of chloride ions:

= Mass of ClMolar Mass of Cl

= 0.0975g

35.45g= 0.00275 mol


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Mole = Avogadro's number 6.022 x 1023

Number of chloride ions:

= (# of moles) x (Avogadro's number)

= (0.00275) x (6.022 x 1023)

= 1.656 x 1021 chloride ions present in 0.2g of NaCl by drying filter paper


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Mass of crucible + lid + filter paper:

32.13g

Mass of crucible + lid + filter paper + AgCl 32.64g

AgCl

(32.64g 32.13g) = 0.51g Mass of chloride ions present:

0.25 x 0.51g = 0.1275g


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Number of moles of chloride ions:

= Mass of ClMolar Mass of Cl

= 0.1275g

35.45g= 0.00360 mol


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Mole = Avogadro's number: 6.022 x 1023

Number of chloride ions: (# of moles) x (Avogadro's number)

(0.00360) x (6.022 x 1023)

2.167 x 1021 chloride ions present in 0.2 g of NaClby burning filter paper


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Law of Conservation of Mass

Mass of the reactants = Mass of the products

Mass of Cl ions in NaCl (reactant)

= Mass of Cl ion is AgCl (product)


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Double displacement reaction:

AB + CD AD + CB

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)


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Hydration

provides greater

stability than

lattice energy Hydration shell

Water dipole moment


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More stable as a solid precipitate than

separate ions

Hydration energy provided is less thanlattice energy released when compound

forms


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White crystalline solid

Light sensitive

Purple black

Change colour when AgCl Ag + Cl

Dry powder, doesnt draw moisture from

the air

Very low solubility

MP: 455C BP: 1550C


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AgNO3 is the excess reagent

Amount of chloride ions = product In this double displacement reaction all of the

NaCl must be used up

LR = NaCl : limits amount of AgNO3 used

limits amount of productsAn excess of AgNO3 will not react since all the

NaCl is used up already


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Otherwise, mass of the impurities would

be included in the mass of the AgCl

precipitate

Causes an inaccurate measurement of

mass of AgCl

Causes inaccurate determination of thenumber of Cl ions


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Contents: AgCl and ashless filter paper

Ashless filter paper turned into CO2

Remaining content: AgCl

AgCl is a white coloured powder atSATP

Upon heating, AgCl undergoesdecomposition to yield Ag and Cl


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Percentage Yield

= Actual Yield x 100

Theoretical Yield

= 80%

Percentage Error

= (Theoretical Yield Actual Yield) x 100

Theoretical Yield

= 20%


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By drying filter paper:

Number of chloride ions in 0.2g of NaCl is

1.656 x 1021

By burning ashless filter paper: Number of chloride ions in 0.2g of NaCl is

2.167 x 1021

Amount of ions present in NaCl =

amount of ions present in AgCl

Law of Conservation of Mass


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The reading on the electrical balance wasobserved to change constantly

Due to slight air currents

Contents being weighted were extremely light

Measured multiple times

Small amounts of AgCl were stuck in theflask after attempts to remove it

Caused alterations in final mass

Inaccurate percentage yield


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Ashless filter paper not burned away

completely

final mass greater than expected

Crucible was placed on counter to allow

for cooling

picked up unwanted particles on the counter


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Use of better quality ashless filter paper

that will completely burn away without

leaving any unwanted residue

Use of clean crucible and lid

Minimum transfer of the samples from

container to container


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Period 1 - Gravimetric Estimation of Chloride Ions

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