Final answer:
When you raise the pH between pKa units, the charge on the molecule typically becomes more negative. This is due to the loss of protons as the pH increases, leading to a reduction in positive charge. Therefore, correct option is c.
Step-by-step explanation:
As you raise the pH between pKa units, the charge becomes more negative. This occurs because increasing the pH means that you are adding hydroxide ions (OH-) or removing hydrogen ions (H+), which decreases the number of protons available to bind to the molecule in question.
As a result, if the molecule was initially in a protonated (positively charged) state, it will lose protons and thus become more negatively charged.
For instance, amino acids have specific pKa values for their carboxylic acid and amine groups. Raising the pH above the pKa of the carboxylic acid group will deprotonate it, leading to a more negative charge.
Similarly, raising the pH above the pKa of the amine group will remove a hydrogen ion, although this typically leads to a neutral rather than a negative charge, unless the amino acid was initially positively charged.