Titrations · Grade 12
In Grade 12 you get fluent with: titration apparatus and the end point; concordant titres and the mean titre; titration calculations.
In a titration, a pipette measures an exact volume of one solution into a conical flask and a burette adds the other until they just react, shown by a sudden color change of an indicator: the end point. Repeat until the titres are concordant and take their mean. From the known solution’s moles and the ratio in the equation, you can find the unknown concentration.
Key facts
- In a titration a pipette measures one solution (often 25.0 cm³) into a flask, and a burette adds the other until the indicator changes color: the end point.
- Repeat until the titres are concordant (within 0.10 cm³ of each other) and use their mean.
- 1 dm³ = 1 liter = 1000 cm³: divide a volume in cm³ by 1000 to get dm³.
- concentration (mol/dm³) = moles ÷ volume (dm³), so moles = concentration × volume.
Worked examples
Example 1
Which piece of apparatus measures exactly 25.0 cm³ of alkali into the flask for a titration?
Answer: a pipette (with a pipette filler)
Example 2
Which indicator would you use for a titration of a strong acid with a strong alkali?
Answer: phenolphthalein or methyl orange (a sharp single color change)
Example 3
In a titration, why is the conical flask swirled while the acid is added?
Answer: to mix the solutions so the end point is seen accurately
Example 4
Why are titrations repeated until concordant results are obtained?
Answer: to check the results are repeatable and take a reliable mean
Example 5
Titres (cm³): rough 27.10, then 25.95, 26.00, 26.40, 26.05. Concordant results are within 0.10 cm³ of each other. What is the mean titre, to 2 decimal places?
Answer: 26.00 cm³
Example 6
25.0 cm³ of 0.100 mol/dm³ sodium hydroxide is neutralized by 20.0 cm³ of hydrochloric acid. NaOH + HCl → NaCl + H2O. Find the concentration of the acid.
Answer: 0.125 mol/dm³
Example 7
25.0 cm³ of 0.200 mol/dm³ potassium hydroxide is neutralized by 12.5 cm³ of sulfuric acid. 2KOH + H2SO4 → K2SO4 + 2H2O. Find the concentration of the sulfuric acid.
Answer: 0.200 mol/dm³
Example 8
25 cm³ of 0.1 mol/dm³ chem KOH is exactly neutralized by 23.1 cm³ of chem H2SO4. 2KOH + H2SO4 → K2SO4 + 2H2O. What is the concentration of the chem H2SO4, in mol/dm³, to 3 significant figures?
Answer: 0.0541 mol/dm³