Chemistry 5070/11 — October/November 2011
Cambridge O-Level · Multiple Choice · answer key with instant marking and worked solutions
Topics Atoms, Elements and Compounds · Organic Chemistry · Metals · Experimental Techniques and Chemical Analysis · Acids, Bases and Salts · Chemistry of the Environment · +5 more
Tap an option under each question to check it — your score builds as you go.
In a titration between an acid (in the burette) and an alkali, you may need to re-use the same titration flask.
Which is the best procedure for rinsing the flask?
Options
A Rinse with distilled water and then with the alkali.
B Rinse with tap water and then with distilled water.
C Rinse with tap water and then with the acid.
D Rinse with the alkali.
Working
In a titration the conical flask contains the measured volume of alkali transferred by the pipette. The amount of alkali that reacts must be fixed by the pipette, not by extra solution left behind in the flask.
- Rinsing with the alkali (A or D) leaves extra alkali behind, so the titre would be too high.
- Rinsing with the acid (C) would neutralise some alkali before the titration starts, so the titre would be too low.
- Tap water may contain dissolved impurities, so a final rinse with distilled water is needed to remove them.
Therefore the best procedure is rinse with tap water then distilled water: B.
Answer
B
B
Walkthrough
In a titration, the conical flask is the vessel in which the reaction happens. It does not need to contain a precise volume of solution.
The known amount of alkali is delivered by a pipette. Once that alkali is in the flask, the only amount that must be controlled is the acid added from the burette. If the flask is rinsed with the alkali before the pipetted sample is added, an unknown extra volume of alkali remains. More acid is then needed to neutralise it, so the measured titre is larger than the true value.
Rinsing the flask with the acid would start neutralising the alkali before the burette reading is taken. The apparent titre would then be too small.
Tap water may contain dissolved ions and other impurities. A rinse with tap water cleans the flask, but a final rinse with distilled water removes those impurities. A little distilled water left behind only dilutes the solution; it does not change the number of moles of alkali, so the result is unaffected.
Hence the correct choice is B: rinse with tap water, then with distilled water.
Key Takeaways
- The conical flask is rinsed with distilled water only, not with the solutions being measured.
- The burette is rinsed with the acid and the pipette is rinsed with the alkali before use.
- Tap water is not used as the final rinse because it contains dissolved impurities.
- A few drops of distilled water left in the flask are harmless because they only dilute, not change the amount of reactant.
Common Mistakes
- Choosing A or D: rinsing with the alkali adds extra alkali and makes the titre too high.
- Choosing C: rinsing with the acid would neutralise some alkali and make the titre too low.
- Thinking the flask must be rinsed with a reactant to maintain concentration, when only the burette and pipette need that rinsing.
Things to Be Careful About
- The flask does not need to be dry; distilled water in it is acceptable.
- Never use tap water as the final rinse because of dissolved ions.
- Remember the order in option B: tap water first to clean, then distilled water to remove the tap water impurities.
The rest of this paper
39 more questions- Q2Acids, Bases and Salts1M
- Q3Experimental Techniques and Chemical Analysis · Chemistry of the Environment1M
- Q4The Periodic Table1M
- Q5Atoms, Elements and Compounds1M
- Q6Atoms, Elements and Compounds1M
- Q7Atoms, Elements and Compounds1M
- Q8Atoms, Elements and Compounds1M
- Q9Atoms, Elements and Compounds1M
- Q10Atoms, Elements and Compounds1M
- Q11Stoichiometry1M
- Q12Stoichiometry1M
- Q13Electrochemistry1M
- Q14Electrochemistry1M
- Q15Chemistry of the Environment · Chemical Energetics1M
- Q16Chemical Reactions1M
- Q17Chemical Reactions1M
- Q18Acids, Bases and Salts1M
- Q19Acids, Bases and Salts1M
- Q20Experimental Techniques and Chemical Analysis1M
- Q21The Periodic Table1M
- Q22The Periodic Table1M
- Q23The Periodic Table1M
- Q24Metals1M
- Q25Metals1M
- Q26Metals1M
- Q27Metals1M
- Q28Chemistry of the Environment1M
- Q29Metals1M
- Q30Atoms, Elements and Compounds1M
- Q31Chemical Reactions1M
- Q32Experimental Techniques and Chemical Analysis1M
- Q33Chemistry of the Environment1M
- Q34Organic Chemistry1M
- Q35Organic Chemistry1M
- Q36Organic Chemistry1M
- Q37Stoichiometry1M
- Q38Organic Chemistry1M
- Q39Organic Chemistry1M
- Q40Organic Chemistry · Acids, Bases and Salts1M