Which pH value represents a solution with the lowest OH- ion concentration?

Which pH value represents a solution with the lowest OH- ion concentration?

• A. 7
• B. 14
• C. 1
• D. 10

Answer: (C)

The key idea is that the amount of hydroxide ions in water is tied to how acidic or basic the solution is. In water at a given temperature, pH and pOH add up to about 14, and the product [H+][OH−] equals Kw (about 1×10−14). So when the pH is very low (more acidic), [H+] is large and [OH−] must be very small to keep the product Kw constant. Among the given values, the smallest pH is 1, which means a high [H+] of 0.1 M and, using Kw, an [OH−] of about 1×10−13 M. That is much lower than the hydroxide levels at higher pH values (neutral pH 7 gives ~1×10−7 M OH−, pH 10 gives ~1×10−4 M OH−, and pH 14 gives ~1 M OH−). Therefore, the solution with the lowest OH− concentration corresponds to the pH value of 1.

The key idea is that the amount of hydroxide ions in water is tied to how acidic or basic the solution is. In water at a given temperature, pH and pOH add up to about 14, and the product [H+][OH−] equals Kw (about 1×10−14). So when the pH is very low (more acidic), [H+] is large and [OH−] must be very small to keep the product Kw constant. Among the given values, the smallest pH is 1, which means a high [H+] of 0.1 M and, using Kw, an [OH−] of about 1×10−13 M. That is much lower than the hydroxide levels at higher pH values (neutral pH 7 gives ~1×10−7 M OH−, pH 10 gives ~1×10−4 M OH−, and pH 14 gives ~1 M OH−). Therefore, the solution with the lowest OH− concentration corresponds to the pH value of 1.