Q79 · Practice questionComplex familiar4 marks
QUESTION 79 (4 marks)
A purified amino acid with isoelectric point 5.8 is placed at the origin of the electrophoresis apparatus. The buffer has pH 8.0.
a) Predict its net charge and direction of migration. [2 marks]
b) Predict its behaviour if the buffer pH is changed to 5.8, and explain. [2 marks]
WORKED SOLUTION
4 marksPractice marking scheme
At pH 8.0 the amino acid is net negative and moves towards the anode. At pH 5.8 its average net charge is zero, so there is no net migration in the ideal model.
Above its pI, deprotonated forms predominate. At the pI the positive and negative charges balance on average.
- Identifies net negative charge at pH 8.0. [1 marks]
- Predicts migration towards positive anode. [1 marks]
- Identifies zero average net charge at pH = pI. [1 marks]
- Predicts no net migration and links it to charge balance. [1 marks]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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