FPGEE for National Association of Boards of Pharmacy (NABP) Practice Exam

In the context of oxy-acids, which acid will typically be more acidic?

1. The acid with fewer oxygen atoms

2. The acid with more hydrogen ions

3. The acid with more oxygen atoms

4. The acid with a higher negative charge

The correct answer is: The acid with more oxygen atoms

The correct answer is that the acid with more oxygen atoms typically exhibits greater acidity. This concept is rooted in the stability of the conjugate base formed when the acid donates a proton (H+). In oxy-acids, increasing the number of oxygen atoms usually correlates with an increase in the acidity due to the effect of the electronegative oxygen atoms, which can help stabilize the negative charge of the conjugate base. More oxygen atoms can distribute the negative charge more effectively after deprotonation, leading to a more stable conjugate base. This process is known as resonance stabilization since the negative charge can be delocalized over multiple oxygen atoms. Additionally, the increased electronegativity of additional oxygen atoms pulls electron density away from the hydrogen-oxygen bond, weakening it. This results in a stronger acid because it favors the release of the proton. This principle can apply in comparing acids where the only difference is the number of oxygen atoms. The more oxygen atoms present, the stronger the acid generally is, making it the most favorable option for increased acidity in the case of oxy-acids.