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

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

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Titrations Titrations Presentation Transcript

  • Acid Base Titrations Physical Science Keith Warne
  • Mole Calculations ASKED GIVEN MOLES MOLES MASS MASS VOLUME VOLUME CONCENTRATION CONCENTRATION MOLAR RATIO Number Of particles Number Of particles
  • Titration Calculations.
    • (COOH) 2 .2H 2 O
    • Unknown Base (NaOH)
    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
  • Standard Solution
    • A standard solution is one for which the ………………. is ………………….. known.
    • Since
    • c =
    • Needed:
    • The number of ………….. of solute ( …………… )
    • The ………… of solution .
    • These values must be ………………... determined .
    Mass is determined accurately using an ………………... balance . 1.60g solute
    • Volume is measured using a
    • ……………… . flask.
    • 250 cm 3
    • 100 cm 3
    • 200 cm 3
    250 cm 3
  • Standard Solution
    • A standard solution is one for which the concentration is precisely known.
    • Since
    • c = n(solute ) / v(solvent)
    • = 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 . 1.60g solute
    • Volume is measured using a
    • Volumetric flask.
    • 250 cm 3
    • 100 cm 3
    • 200 cm 3
    250 cm 3
  • Weighing Technique
    • P rocedure - Weighing by difference.
    • …… .. scales and …….. the pan.
    • Weigh the weighing …………… .
    • Add (………………………) the required amount of salt. Take care not to drop any salt onto the pan.
    • …………………… . the salt to a clean beaker.
    • …………………… the weighing container.
    • ………… . 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 ………………………………
    Results: Mass salt + container: ………… Final Mass container: ………… Mass salt transferred: 1.60g ……… ..
  • Weighing Technique
    • P rocedure - Weighing by difference.
    • Zero scales and clean the pan.
    • Weigh the weighing container .
    • Add (approximately) the required amount of salt. Take care not to drop any salt onto the pan.
    • Transfer the salt to a clean beaker.
    • Reweigh the weighing container.
    • 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
    Results: Mass salt + container: ………… Final Mass container: ………… Mass salt transferred: 2.45g (COOH) 2
  • Making a standard solution.
    • …………… .. a clean & dry 100 cm 3 ………… with a little distilled water.
    • ……………… the correctly weighed amount of salt to the beaker. Ensure NO SALT IS ……………..
    • 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.
    • Add ALL the solution to …………. ………. via funnel. Ensure glass rod and beaker are thoroughly rinsed. (Include ……………….)
    • Add enough solvent to bring the level ………………………….
    ……… ..g solute 250 cm 3
  • Making a standard solution.
    • Rinse a clean & dry 100 cm 3 beaker with a little distilled water.
    • Transfer the correctly weighed amount of salt to the beaker. Ensure NO SALT IS Lost
    • 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.
    • Add ALL the solution to a volumetric flask via funnel. Ensure glass rod and beaker are thoroughly rinsed. ( Include rinsings )
    • Add enough solvent to bring the level up to the mark.
    ……… ..g solute 250 cm 3
  • Making a standard solution.
    • Use a dropper to bring the level up to the mark.
    • The BOTTOM of the meniscus must JUST TOUCH THE LINE of the flask.
    • 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 cm 3 Drag here
  • Titration Proceedure. ACID STANDARD SOLUTION unknown BASE
    • Rinse # the burette with distilled water and then with small quantities of the STANDARD ACID solution.
    • Fill the burette with the standard ACID solution.
    • Take the zero reading. Does not have to be ZERO.
    • Rinse # a clean conical flask with DISTILLED WATER.
    • Rinse # a clean pippette with the unknown base solution.
    • Pippette 25cm 3 of the unknown base solution into the conical flask.
    • Add 3-5 drops of a suitable indicator to the conical flask.
    • Titrate the acid against the base until the FIRST PERMANENT COLOUR CHANGE.
    • Note down the volume of acid and repeat this procedure with a fresh conical flask until CONCORDANT RESULTS are obtained. ~0.1 cm 3 .
  • Titration Calculations.
    • AT THE END POINT
    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 )
  • 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
  • Determination of unknown base.
    • OXALIC ACID (KNOWN/Standard )
    • (COOH) 2 .2H 2 O
    • M r = (2(12+32+1)+2(18)=
    • 6.4g in 1l (1dm 3 )
    • Moles(ACID) = m/M r
    • = 6.4/( )
    • = [ ] in 1dm 3
    • Concentration (ACID) = n/v = [ ] M
    • Unknown Base (NaOH)
    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.
  • Determination of unknown base.
    • OXALIC ACID (KNOWN/Standard )
    • (COOH) 2 .2H 2 O
    • M r = (2(12+32+1)+2(18)= 126 g.mol -1
    • 6.4g in 1l (1dm 3 )
    • Moles(ACID) = m/M r
    • = 6.4/(126)
    • = 0.051 in 1dm 3
    • Concentration (ACID) = n/v
    • = 0.051/1
    • = 0.051M
    • Unknown Base (NaOH)
    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.
  • Titration Calculations.
    • (COO) 2 .2H 2 O
    • M r = (2(12+32+1)+2(18)= 126
    • 6.4g in 1l (1dm 3 )
    • Moles(ACID) = m/M r
    • = 6.4/(126)
    • = 0.05mol/1dm 3
    • Concentration (ACID) = 0.05 M
    • Unknown Base (NaOH)
    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
  • Different Indicators Name of indicator Colour acid Colour base pH range Methyl Orange Red Yellow ... - .... Bromothymol Blue Yellow Blue .... - ... Phenolphthalein Clear Red .... - ....
  • 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
  • Titration Curves pH 0 .... Amount of ............ added ...
  • Titration Curves pH 0 14 Amount of BASE added 7 STRONG BASE STRONG ACID
  •  
    • The end point has a pH lower than 7 because the SALT of a STRONG acid and a weak base is ACIDIC!!
    STRONG ACID WEAK BASE WEAK BASE STRONG ACID END POINT
  •  
  • WEAK ACID & STRONG BASE
    • The pH at the end point is HIGHER than 7 because the salt of a weak acid and STRONG base is BASIC.
    STRONG BASE WEAK ACID END POINT 50.00cm 3 pH at end point
  •  
    • Due to the gradual change in pH the END POINT is difficult to identify.
    • These titrations have only limited use.
    WEAK BASE & WEAK ACID WEAK BASE WEAK ACID END POINT 50.00cm 3 pH at end point
  • 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
  •