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A set of slides created to teach Titrations to learners at Bishops Diocesan College in Cape Town.

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### Titrations

1. 1. Acid Base Titrations Physical Science Keith Warne
2. 2. Mole Calculations ASKED GIVEN MOLES MOLES MASS MASS VOLUME VOLUME CONCENTRATION CONCENTRATION MOLAR RATIO Number Of particles Number Of particles
3. 3. Titration Calculations. <ul><li>(COOH) 2 .2H 2 O </li></ul><ul><li>Unknown Base (NaOH) </li></ul>OXALIC ACID STANDARD SOLUTION c b = ………….. mol.dm -3 (COOH) 2 .2H 2 O + 2NaOH --> Na 2 (COO) 2 + 4H 2 O ? 250 cm 3 ……… ..g
4. 4. Standard Solution <ul><li>A standard solution is one for which the ………………. is ………………….. known. </li></ul><ul><li>Since </li></ul><ul><li>c = </li></ul><ul><li>Needed: </li></ul><ul><li>The number of ………….. of solute ( …………… ) </li></ul><ul><li>The ………… of solution . </li></ul><ul><li>These values must be ………………... determined . </li></ul>Mass is determined accurately using an ………………... balance . 1.60g solute <ul><li>Volume is measured using a </li></ul><ul><li>……………… . flask. </li></ul><ul><li>250 cm 3 </li></ul><ul><li>100 cm 3 </li></ul><ul><li>200 cm 3 </li></ul>250 cm 3
5. 5. Standard Solution <ul><li>A standard solution is one for which the concentration is precisely known. </li></ul><ul><li>Since </li></ul><ul><li>c = n(solute ) / v(solvent) </li></ul><ul><li> = m / Mr V </li></ul><ul><li>Needed: </li></ul><ul><li>The number of moles of solute ( Mass ) </li></ul><ul><li>The volume of solution . </li></ul><ul><li>These values must be accurately determined . </li></ul>Mass is determined accurately using an electronic balance . 1.60g solute <ul><li>Volume is measured using a </li></ul><ul><li>Volumetric flask. </li></ul><ul><li>250 cm 3 </li></ul><ul><li>100 cm 3 </li></ul><ul><li>200 cm 3 </li></ul>250 cm 3
6. 6. Weighing Technique <ul><li>P rocedure - Weighing by difference. </li></ul><ul><li>…… .. scales and …….. the pan. </li></ul><ul><li>Weigh the weighing …………… . </li></ul><ul><li>Add (………………………) the required amount of salt. Take care not to drop any salt onto the pan. </li></ul><ul><li>…………………… . the salt to a clean beaker. </li></ul><ul><li>…………………… the weighing container. </li></ul><ul><li>………… . the final mass of the container from the mass of salt and container to give the mass of salt transferred to the beaker. </li></ul><ul><li>Mass is determined accurately using an balance (electronic or triple beam) . </li></ul><ul><li>Possible accuracies of ……………………………… </li></ul>Results: Mass salt + container: ………… Final Mass container: ………… Mass salt transferred: 1.60g ……… ..
7. 7. Weighing Technique <ul><li>P rocedure - Weighing by difference. </li></ul><ul><li>Zero scales and clean the pan. </li></ul><ul><li>Weigh the weighing container . </li></ul><ul><li>Add (approximately) the required amount of salt. Take care not to drop any salt onto the pan. </li></ul><ul><li>Transfer the salt to a clean beaker. </li></ul><ul><li>Reweigh the weighing container. </li></ul><ul><li>Subtract the final mass of the container from the mass of salt and container to give the mass of salt transferred to the beaker. </li></ul><ul><li>Mass is determined accurately using an balance (electronic or triple beam) . </li></ul><ul><li>Possible accuracies of 0.1 - 0.0001g </li></ul>Results: Mass salt + container: ………… Final Mass container: ………… Mass salt transferred: 2.45g (COOH) 2
8. 8. Making a standard solution. <ul><li>…………… .. a clean & dry 100 cm 3 ………… with a little distilled water. </li></ul><ul><li>……………… the correctly weighed amount of salt to the beaker. Ensure NO SALT IS …………….. </li></ul><ul><li>Add …………………. the salt and stir gently with a glass rod until all salt is dissolved. DO NOT REMOVE THE ROD FROM THE SOLUTION NOR ALLOW ………… ……….. OF SOLUTION TO ESCAPE. </li></ul><ul><li>Add ALL the solution to …………. ………. via funnel. Ensure glass rod and beaker are thoroughly rinsed. (Include ……………….) </li></ul><ul><li>Add enough solvent to bring the level …………………………. </li></ul>……… ..g solute 250 cm 3
9. 9. Making a standard solution. <ul><li>Rinse a clean & dry 100 cm 3 beaker with a little distilled water. </li></ul><ul><li>Transfer the correctly weighed amount of salt to the beaker. Ensure NO SALT IS Lost </li></ul><ul><li>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. </li></ul><ul><li>Add ALL the solution to a volumetric flask via funnel. Ensure glass rod and beaker are thoroughly rinsed. ( Include rinsings ) </li></ul><ul><li>Add enough solvent to bring the level up to the mark. </li></ul>……… ..g solute 250 cm 3
10. 10. Making a standard solution. <ul><li>Use a dropper to bring the level up to the mark. </li></ul><ul><li>The BOTTOM of the meniscus must JUST TOUCH THE LINE of the flask. </li></ul><ul><li>The flask should then be inverted at least 10 times to ensure thorough mixing. </li></ul>The bottom of the meniscus must JUST touch the line!!! 250 cm 3 Drag here
11. 11. Titration Proceedure. ACID STANDARD SOLUTION unknown BASE <ul><li>Rinse # the burette with distilled water and then with small quantities of the STANDARD ACID solution. </li></ul><ul><li>Fill the burette with the standard ACID solution. </li></ul><ul><li>Take the zero reading. Does not have to be ZERO. </li></ul><ul><li>Rinse # a clean conical flask with DISTILLED WATER. </li></ul><ul><li>Rinse # a clean pippette with the unknown base solution. </li></ul><ul><li>Pippette 25cm 3 of the unknown base solution into the conical flask. </li></ul><ul><li>Add 3-5 drops of a suitable indicator to the conical flask. </li></ul><ul><li>Titrate the acid against the base until the FIRST PERMANENT COLOUR CHANGE. </li></ul><ul><li>Note down the volume of acid and repeat this procedure with a fresh conical flask until CONCORDANT RESULTS are obtained. ~0.1 cm 3 . </li></ul>
12. 12. Titration Calculations. <ul><li>AT THE END POINT </li></ul>ACID STANDARD SOLUTION a ACID + b BASE --> salt + water moles ACID moles BASE = a b unknown BASE C a V a C b V b = a b 25cm 3 = 0.025dm 3 C(mol.dm -3 ) V (dm -3 )
13. 13. 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)Na 2 CO 3 + 2 HCl  2NaCl + H 2 O + CO 2 Conc of base = your standard soln. Worked out. Vol. of base= pippette (25cm 3 ) Solve for x Titration Calculations C a V a C b V b = a b
14. 14. Determination of unknown base. <ul><li>OXALIC ACID (KNOWN/Standard ) </li></ul><ul><li>(COOH) 2 .2H 2 O </li></ul><ul><li>M r = (2(12+32+1)+2(18)= </li></ul><ul><li>6.4g in 1l (1dm 3 ) </li></ul><ul><li>Moles(ACID) = m/M r </li></ul><ul><li>= 6.4/( ) </li></ul><ul><li>= [ ] in 1dm 3 </li></ul><ul><li>Concentration (ACID) = n/v = [ ] M </li></ul><ul><li>Unknown Base (NaOH) </li></ul>A standard oxalic acid solution is made up using 6.4g in 1l . 25cm 3 of an unknown sodium hydroxide solution required 22,4 cm 3 of the standard acid to reach end point. Calculate the concentration of the unknown base.
15. 15. Determination of unknown base. <ul><li>OXALIC ACID (KNOWN/Standard ) </li></ul><ul><li>(COOH) 2 .2H 2 O </li></ul><ul><li>M r = (2(12+32+1)+2(18)= 126 g.mol -1 </li></ul><ul><li>6.4g in 1l (1dm 3 ) </li></ul><ul><li>Moles(ACID) = m/M r </li></ul><ul><li>= 6.4/(126) </li></ul><ul><li>= 0.051 in 1dm 3 </li></ul><ul><li>Concentration (ACID) = n/v </li></ul><ul><li>= 0.051/1 </li></ul><ul><li>= 0.051M </li></ul><ul><li>Unknown Base (NaOH) </li></ul>A standard oxalic acid solution is made up using 6.4g in 1l . 25cm 3 of an unknown sodium hydroxide solution required 22,4 cm 3 of the standard acid to reach end point. Calculate the concentration of the unknown base.
16. 16. Titration Calculations. <ul><li>(COO) 2 .2H 2 O </li></ul><ul><li>M r = (2(12+32+1)+2(18)= 126 </li></ul><ul><li>6.4g in 1l (1dm 3 ) </li></ul><ul><li>Moles(ACID) = m/M r </li></ul><ul><li>= 6.4/(126) </li></ul><ul><li>= 0.05mol/1dm 3 </li></ul><ul><li>Concentration (ACID) = 0.05 M </li></ul><ul><li>Unknown Base (NaOH) </li></ul>OXALIC ACID STANDARD SOLUTION AT THE END POINT 2 x n(Acid) = n(Base) 2xc a v a = c b V b 2 x (0.054) (22.4x10 -3 )=c b (25x10 -3 ) c b = 0.0038 mol.dm -3 (1) (COO) 2 .2H 2 O + 2 NaOH --> Na 2 (COO) 2 + 4H 2 O One mole of acid reacts with 2 moles of base. .: n(acid) : n(base) 1 : 2 C a V a C b V b = 1 2
17. 17. Different Indicators Name of indicator Colour acid Colour base pH range Methyl Orange Red Yellow ... - .... Bromothymol Blue Yellow Blue .... - ... Phenolphthalein Clear Red .... - ....
18. 18. Different Indicators Name of indicator Colour acid Colour base pH range Methyl Orange Red Yellow 3 - 4 Bromothymol Blue Yellow Blue 6 - 8 Phenolphthalein Clear Red 8 - 10
19. 19. Titration Curves pH 0 .... Amount of ............ added ...
20. 20. Titration Curves pH 0 14 Amount of BASE added 7 STRONG BASE STRONG ACID
21. 22. <ul><li>The end point has a pH lower than 7 because the SALT of a STRONG acid and a weak base is ACIDIC!! </li></ul>STRONG ACID WEAK BASE WEAK BASE STRONG ACID END POINT
22. 24. WEAK ACID & STRONG BASE <ul><li>The pH at the end point is HIGHER than 7 because the salt of a weak acid and STRONG base is BASIC. </li></ul>STRONG BASE WEAK ACID END POINT 50.00cm 3 pH at end point
23. 26. <ul><li>Due to the gradual change in pH the END POINT is difficult to identify. </li></ul><ul><li>These titrations have only limited use. </li></ul>WEAK BASE & WEAK ACID WEAK BASE WEAK ACID END POINT 50.00cm 3 pH at end point
24. 27. STRONG BASE WEAK ACID Which Indicator to use? Strong acid/strong base Bromothymol blue Strong acid/weak base Methyl orange Weak acid/strong base Phenolphthalein pH 0 14 Moles of base added 7 Strong acid Weak acid Weak base Strong base Blue Bromothymol blue Yellow Methyl Orange Red Pink Phenolphthalien Colourless STRONG BASE STRONG ACID WEAK BASE STRONG ACID
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