CCEA A Level Chemistry 1.9 Acid-Base Titrations Exam Questions

About CCEA A Level Chemistry 1.9 Acid-Base Titrations

Section 1.9 of Unit AS 1 combines theory with one of the most important practical skills in the course. You need to distinguish strong and weak acids and bases by their degree of dissociation, carry out titrations of strong and weak acids and bases, and choose between phenolphthalein and methyl orange. CCEA names specific analyses such as finding the degree of hydration of sodium carbonate and the concentration of ethanoic acid in vinegar. You must prepare a standard solution, calculate titre uncertainty, select concordant results, and calculate concentrations in mol dm-3 and g dm-3. These skills return in A2 volumetric analysis and acid-base equilibria.

What you need to know

• Strong and weak Strong acids (HCl, H2SO4, HNO3) are fully dissociated in water; weak acids (ethanoic acid) are only partly dissociated. Strong bases such as NaOH are fully dissociated; ammonia is a weak base.
• Indicators Phenolphthalein is colourless in acid and pink in alkali, used where a strong base is involved. Methyl orange is red in acid and yellow in alkali (orange at the end point), used where a strong acid is involved.
• Standard solutions Weigh the solid accurately, dissolve in deionised water in a beaker, transfer with washings to a volumetric flask, make up to the mark with the bottom of the meniscus on the line, then invert to mix.
• Titration technique Rinse the burette with the solution it will contain and the pipette with its solution. Add dropwise near the end point, swirl, and repeat until concordant titres (within 0.10 cm3) are obtained; ignore outliers when calculating the mean.
• Uncertainty Each burette reading has an uncertainty of plus or minus 0.05 cm3, so a titre has plus or minus 0.10 cm3. Percentage uncertainty = uncertainty / measured value x 100.
• Calculations Moles = concentration x volume in dm3. Use the mole ratio from the equation, for example Na2CO3 + 2HCl gives 2NaCl + H2O + CO2. Concentration in g dm-3 = mol dm-3 x molar mass.

Common mistakes in 1.9 Acid-Base Titrations

• Forgetting to convert cm3 to dm3. Divide by 1000 before using moles = concentration x volume.
• Rinsing the burette with water only. Rinse it with the solution it will contain, otherwise that solution is diluted.
• Using phenolphthalein for a strong acid with a weak base. Use methyl orange, which changes colour in the steep acidic region.
• Averaging all titres including the rough one. Use only concordant titres.
• Saying a weak acid is a dilute acid. Strength is about dissociation; concentration is about how much acid is in a given volume.

Try three 1.9 Acid-Base Titrations exam-style questions