Acid Base Titrations Physical Science Keith Warne
ASKEDGIVEN Mole Calculations MOLES MASS VOLUME CONCENTRATION MOLAR RATIO Number Of particles Number Of particles
Titration Calculations. (COOH) 2.2H 2 O (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 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. 1.60g solute Volume is measured using a Volumetric flask. 250 cm 3 100 cm 3 200 cm 3 250 cm 3
Weighing Technique Procedure - 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 cm 3 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 cm 3
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 cm 3 Drag here
Titration Proceedure. ACID STANDARD SOLUTION BASE unknown BASE 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. zero 3.Take the zero 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 25cm 3 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 cm 3.
Titration Calculations. ACID STANDARD SOLUTION a ACID + b BASE a ACID + b BASE --> salt + water AT THE END POINT moles ACID moles BASE = ab BASE unknown BASE CaVaCaVaCbVbCbVbCaVaCaVaCbVbCbVb = ab 25cm 3 = 0.025dm 3 C(mol.dm -3 ) V (dm -3 )
CaVaCaVaCbVbCbVbCaVaCaVaCbVbCbVb=ab 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
Determination of unknown base. OXALIC ACID (KNOWN/Standard) (COOH) 2.2H 2 O (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) standard oxalic acid6.4g in 1l 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 (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 c b = 0.0038 mol.dm -3 (1)(COO) 2.2H 2 O + 2NaOH --> Na 2 (COO) 2 + 4H 2 O One mole of acid reacts with 2 moles of base..: n(acid) : n(base) 1 : 2 1 : 2 CaVaCaVaCbVbCbVbCaVaCaVaCbVbCbVb = 12
Titration Example If 22.3 cm -3 of a standard hydrochloric acid solution were required to reach end point with 25 cm -3 of an unknown sodium carbonate solution, what is the concentration of the sodium carbonate solution?
Titration Example If 22.3 cm -3 of a standard hydrochloric acid solution (0.15 M) was required to reach end point with 25 cm -3 of an unknown sodium carbonate solution, what is the concentration of the sodium carbonate solution? 2HCl + Na 2 CO 3 2NaCl + CO 2 + H 2 O 2HCl + Na 2 CO 3 2NaCl + CO 2 + H 2 O CaVaCaVaCbVbCbVbCaVaCaVaCbVbCbVb = 12 (0.15) (0.0223) C b (0.025) 12 = CbCbCbCb = = 0.268 M 2*(0.15) (0.0223) 1* (0.025)
Name of indicator Colour acid Colour base pH range Methyl Orange RedYellow... -....... -.... Bromothymol Blue YellowBlue.... -... PhenolphthaleinClearRed.... -....
Name of indicator Colour acid Colour base pH range Methyl Orange RedYellow 3 - 4 3 - 4 Bromothymol Blue YellowBlue 6 - 8 PhenolphthaleinClearRed 8 - 10
Due to the gradual change in pH the END POINT is difficult to identify. These titrations have only limited use. WEAK BASE WEAK ACID END POINT 50.00cm 3 pH at end point WEAK BASE & WEAK ACID
STRONG BASE WEAK ACID 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