Required Practical 9 - Investigating pH changes

Question 1

Suggest why it is necessary to calibrate a pH meter

Answer 1

So that it gives accurate pH values

Question 2

Describe how a pH meter is calibrated

Answer 2

• rinsed with deionised water
• place in buffer solutions of different known pH values
• rinse in deionised water before taking each reading
• plot calibration curve of pH readings against actual pH values

Question 3

Suggest how to improve accuracy to pH values

Answer 3

• calibrate pH meter

- plot a calibration curve and convert pH readings to more accurate pH values

Question 4

Suggest why a beaker is used rather than a conical flask for this titration

Answer 4

Cannot use a conical flask as pH meter will not fit and difficult to stir using glass rod

Question 5

Suggest why it is important not to wipe pH meter after rinsing with deionised water

Answer 5

Could produce a static charge which could interfere with pH reading of electrode

Question 6

Suggest a suitable piece of apparatus to measure small/precise volumes of solutions

Answer 6

• VOLUMETRIC pipette

- burette

Question 7

Explain why a burette is more suitable than a pipette to add alkali to standard solution of acid in conical flask

Answer 7

delivers VARIABLE volumes (not small/precise volumes since pipette does same)

Question 8

State why calibrating a pH meter just before use improves accuracy of pH measurements

Answer 8

over TIME meter does not give accurate readings

Question 9

Describe how you would obtain a pH curve for a titration

Answer 9

• record initial pH of solution in conical flask
• add SMALL volumes with burette
• swirl or stir contents on conical flask
• repeat until reagent added is in excess (specify)
• add even smaller increments near end point (1cm^3)

Question 10

Suggest a reason for anomalous reading on pH curve

Answer 10

ineffective stirring/swirling of mixture

Question 11

Suggest why an indicator cannot be used for a reaction with a gradual change in pH

Answer 11

Indicator would not change colour rapidly / changes colour over a range of volumes