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Acid Base Acid Base Titrations Titrations Physical Science Physical Science Keith Warne Keith Warne

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Acid Base TitrationsAcid Base TitrationsPhysical SciencePhysical Science

Keith WarneKeith Warne

Mole Calculations

MOLES MOLES

MASS MASS

VOLUME VOLUME

CONCENTRATIONCONCENTRATION

MOLARRATIO

Number Of

particles

Number Of

particles

Titration Calculations.Titration Calculations.

(COOH)(COOH)22.2H.2H22O O Unknown Unknown BaseBase (NaOH) (NaOH)OXALIC ACIDOXALIC ACID STANDARD SOLUTION STANDARD SOLUTION

ccb b = ………….. mol.dm= ………….. mol.dm-3-3

(COOH)(COOH)22.2H.2H22O + 2NaOH --> NaO + 2NaOH --> Na22(COO)(COO)2 2 + 4H + 4H22OO

250 cm3

………..g

?

Standard Solution

A standard solution is one for which the concentration is precisely known.

Since

c = n(solute)/v(solution)

= m/Mr V

Needed:• The number of moles of solute

(Mass)• The volume of solution.These values must be accurately

determined.

Mass is determined accurately using an electronic balance.

1.60gsolute

Volume is measured using a

• 100 cm3 • 200 cm3

250 cm3

Weighing TechniqueProcedure - Weighing by difference.

1. Zero scales and clean the pan.

2. Weigh the weighing container.

3. Add (approximately) the required amount of salt. Take care not to drop any salt onto the pan.

4. Transfer the salt to a clean beaker.

5. Reweigh the weighing container.

6. Subtract the final mass of the container from the mass of salt and container to give the mass of salt transferred to the beaker.

Mass is determined accurately using an balance (electronic or triple beam).

• Possible accuracies of 0.1 - 0.0001g

2.45g

(COOH)2

Results: Mass salt + container: ………… Final Mass container: …………

Mass salt transferred:

Making a standard solution. 1. Rinse a clean & dry 100 cm3 beaker

with a little distilled water.

2. Transfer the correctly weighed amount of salt to the beaker. Ensure NO SALT IS Lost

3. Add distilled water to the salt and stir gently with a glass rod until all salt is dissolved. DO NOT REMOVE THE ROD FROM THE SOLUTION NOR ALLOW ANY DROPS OF SOLUTION TO ESCAPE.

4. Add ALL the solution to a volumetric flask via funnel. Ensure glass rod and beaker are thoroughly rinsed. (Include rinsings)

5. Add enough solvent to bring the level up to the mark.

………..g

solute

250 cm3

Making a standard solution. 1. Use a dropper to bring

the level up to the mark.

2. The BOTTOM of the meniscus must JUST TOUCH THE LINE of the flask.

3. The flask should then be inverted at least 10 times to ensure thorough mixing.

The bottom of the meniscus must JUST touch the line!!!

250 cm3

Drag here

Titration Proceedure.Titration Proceedure.

ACIDACID STANDARD STANDARD SOLUTIONSOLUTION

unknown BASEBASE

1. Rinse# the burette with distilled water and then with small quantities of the STANDARD ACID solution.

2. Fill the burette with the standard ACID solution.

3. Take the zerozero reading. Does not have to be ZERO.

4. Rinse # a clean conical flask with DISTILLED WATER.

5. Rinse # a clean pippette with the unknown base solution.

6. Pippette 25cm3 of the unknown base solution into the conical flask.

7. Add 3-5 drops of a suitable indicator to the conical flask.

8. Titrate the acid against the base until the FIRST PERMANENT COLOUR CHANGE.

9. Note down the volume of acid and repeat this procedure with a fresh conical flask until CONCORDANT RESULTS are obtained. ~0.1 cm3.

Titration Calculations.Titration Calculations.

ACIDACID STANDARD SOLUTIONSTANDARD SOLUTION

a ACIDa ACID + + b BASEb BASE --> salt + water

AT THE END POINTAT THE END POINT

molesmoles ACID ACIDmoles moles BASEBASE

== aabb

unknown BASEBASE

CCaaVVaa

CCbbVVbb

== aabb

25cm3

= 0.025dm3

C(mol.dm-3)V (dm-3)

CCaaVVaa

CCbbVVbb

==aabb

Conc of acid = ? x (trying to find)

Volume of acid – burette = average titre

Molar ratio from balanced reaction a = acid coefficient (2) b = base coef. (1)

(1)Na2 CO3 + 2 HCl 2NaCl + H2O + CO2

Conc of base= your standard soln. Worked out. Vol. of base=

pippette (25cm3)

Solve for x

Titration Calculations

Determination of Determination of unknown base.unknown base.

OXALIC ACID (KNOWN/StandardOXALIC ACID (KNOWN/Standard))

(COOH)(COOH)22.2H.2H22O O

MMrr = (2(12+32+1)+2(18)= 126 g.mol = (2(12+32+1)+2(18)= 126 g.mol-1-1

6.4g in 1l6.4g in 1l (1dm (1dm33))

Moles(ACID) = m/MMoles(ACID) = m/Mrr

= 6.4/(126) = 6.4/(126)

= 0.051 in 1dm= 0.051 in 1dm33

Concentration (ACID) = n/v Concentration (ACID) = n/v = 0.051/1 = 0.051/1 = 0.051M= 0.051M

Unknown Base (NaOH)Unknown Base (NaOH)

A standard oxalic acidstandard oxalic acid solution is made up using 6.4g in 1l6.4g in 1l. 25cm3 of an unknown sodium hydroxide solution required 22,4 cm3 of the standard acid to reach end point. Calculate the concentration of the unknown base.

Titration Calculations.Titration Calculations.

(COO)(COO)22.2H.2H22O O

MMrr = (2(12+32+1)+2(18)= 126 = (2(12+32+1)+2(18)= 126

6.4g in 1l (1dm6.4g in 1l (1dm33))

Moles(ACID) = m/MMoles(ACID) = m/Mrr

= 6.4/(126) = 6.4/(126)

= 0.05mol/1dm= 0.05mol/1dm33

Concentration (ACID) = 0.05 MConcentration (ACID) = 0.05 M

Unknown Base (NaOH)Unknown Base (NaOH)OXALIC ACID OXALIC ACID STANDARD SOLUTIONSTANDARD SOLUTION

AT THE END POINTAT THE END POINT

2 x n(Acid) = n(Base)2 x n(Acid) = n(Base)

2xc2xcaavva a = c= cbbVVbb

2 x (0.054) (22.4x102 x (0.054) (22.4x10-3-3)=c)=cbb(25x10(25x10-3-3))

ccb b = 0.0038 = 0.0038 mol.dmmol.dm-3-3

(1)(1)(COO)(COO)22.2H.2H22O + O + 22NaOH --> NaNaOH --> Na22(COO)(COO)2 2 + 4H + 4H22OO

One mole of acid One mole of acid reacts withreacts with 2 moles of base. 2 moles of base..: n(acid) .: n(acid) : : n(base) n(base) 11 : : 2 2

CCaaVVaa

CCbbVVbb

==1122

Titration ExampleTitration Example If 22.3 cmIf 22.3 cm-3 -3 of a standard hydrochloric acid solution were of a standard hydrochloric acid solution were

required to reach end point with 25 cmrequired to reach end point with 25 cm-3-3 of an unknown of an unknown sodium carbonate solution, what is the concentration of sodium carbonate solution, what is the concentration of the sodium carbonate solution?the sodium carbonate solution?

Titration ExampleTitration Example If 22.3 cmIf 22.3 cm-3 -3 of a standard hydrochloric acid solution (0.15 of a standard hydrochloric acid solution (0.15

M) was required to reach end point with 25 cmM) was required to reach end point with 25 cm-3-3 of an of an unknown sodium carbonate solution, what is the unknown sodium carbonate solution, what is the concentration of the sodium carbonate solution?concentration of the sodium carbonate solution?

2HCl + Na2HCl + Na22COCO3 3 2NaCl + CO 2NaCl + CO2 2 + H+ H22OO

CCaaVVaa

CCbbVVbb

== 1122

(0.15) (0.0223)(0.15) (0.0223)CCbb (0.025) (0.025)

1122

==

CCbb = = = = 0.268 M0.268 M2*(0.15) (0.0223)2*(0.15) (0.0223)1* (0.025)1* (0.025)

NameName

of indicator of indicator

Colour Colour acidacid

Colour Colour basebase

pH pH rangerange

Methyl OrangeMethyl Orange RedRed YellowYellow ... - ....... - ....

Bromothymol BlueBromothymol Blue YellowYellow BlueBlue .... - ....... - ...

PhenolphthaleinPhenolphthalein ClearClear RedRed .... - ........ - ....

NameName

of indicator of indicator

Colour Colour acidacid

Colour Colour basebase

pH pH rangerange

Methyl OrangeMethyl Orange RedRed YellowYellow 3 - 43 - 4

Bromothymol BlueBromothymol Blue YellowYellow BlueBlue 6 - 86 - 8

PhenolphthaleinPhenolphthalein ClearClear RedRed 8 - 108 - 10

pH

0

....

...

pH

0

14

7

STRONG BASE

STRONG ACID

The end The end point has a point has a pH lower pH lower than 7 than 7 because because the SALT the SALT of a of a STRONG STRONG acid and a acid and a weak base weak base is is ACIDIC!!ACIDIC!!

WEAK BASE

STRONG ACID

END POINT

STRONG ACID WEAK BASESTRONG ACID WEAK BASE

WEAK ACID & STRONG BASEWEAK ACID & STRONG BASE

The pH at The pH at the end the end point is point is HIGHER HIGHER than 7 than 7 because because the salt of a the salt of a weak acid weak acid and and STRONG STRONG base is base is BASIC.BASIC.

STRONG BASE

WEAK ACID

END POINT

50.00cm3

pH at end point

Due to the Due to the gradual gradual change in pH change in pH the END the END POINT is POINT is difficult to difficult to identify.identify.

These These titrations titrations have only have only limited use.limited use.

WEAK BASE

WEAK ACID

END POINT

50.00cm3

pH at end point

WEAK BASE & WEAK ACIDWEAK BASE & WEAK ACID

STRONG BASE WEAK ACID

Strong acid/strong baseStrong acid/strong base Bromothymol blueBromothymol blue

Strong acid/weak baseStrong acid/weak base Methyl orangeMethyl orange

Weak acid/strong baseWeak acid/strong base PhenolphthaleinPhenolphthalein

pH

0

14

7

Strong acid

Weak acid

Weak base

Strong base

Blue

Bromothymol blue

Yellow

Methyl OrangeRed

Pink

PhenolphthalienColourless

STRONG BASE STRONG ACID

WEAK BASE STRONG ACID