RP 9 - Titration curves

Question 1

constructing a pH curve method

Answer 1

• transfer 25cm3 of acid to a conical flask with a volumetric pipette
• measure initial pH of the acid with a pH meter
• add alkali in small amounts noting the volume added
• stir mixture to equalise the pH
• measure and record the pH to 1dp
• repeat steps 3-5 but when approaching end point add in smaller volumes of alkali
• add until alkali in excess

Question 2

why should u calibrate meter first by measuring known pH of a buffer solution

Answer 2

• necessary because pH meters can lose accuracy on storage
• most pH probes are calibrated by putting probe in a set buffer and pressing a calibration button / setting for that pH.
• sometimes this is repeated with a second buffer at a diff pH
• can also improve accuracy by maintaining constant temp

Question 3

EQ : suggest a suitable piece of apparatus that could be used to measure out sodium hydroxide solution and explain why it’s more suitable than a pipette (2)

Answer 3

burette
can deliver variable volumes

Question 4

EQ : describe briefly how you would ensure that a reading from a pH meter is accurate (2)

Answer 4

• calibrate meter with solutions of known pH / buffers
• Adjust meter / plot calibration curve

Question 5

EQ : suggest a reason for removing the funnel after it has been used for filling the burette (1)

Answer 5

as a droplet from the funnel could enter the burette / affect volume / readings / titre

Question 6

EQ : suggest one other source of error in using the burette to carry out a titration (1)

Answer 6

Air bubble in jet

Question 7

EQ : during the titration the inside of the conical flask is rinsed with distilled water . Suggest why rinsing improves the accuracy of the titre (1)

Answer 7

Ensures all reagents are able to react / mix / come into contact

Question 8

EQ : suggest how the experimental procedure could be slightly modified in order to give a more reliable value for the end point (1)

Answer 8

take more pH readings around the end point / add smaller volumes of NaOH near the end point

Question 9

EQ : in order to obtain a pH curve , you’re provided with a conical flask containing 25cm3 of a 0.1moldm3 carboxylic acid solution and a burette filled with 0.1 moldm-3 sodium hydroxide solution . You are also provided with a calibrated pH meter . Describe how you would obtain the pH curve for the titration (5)

Answer 9

• measure pH of the acid
• add alkali in known small portions
• stir mixture
• measure pH after every addition
• repeat until alkali in excess
• add in smaller increments near end point