IB Chemistry: The pH Scale

IB Chemistry: The pH Scale The pH Scale 1. Distinguish between aqueous solutions that are acidic, neutral or alkaline using thepH scale. A pH scale gives us a measure of how acidic or basic a substance is. A pH of 7 means that the substance is neutral. A pH of less than 7 means that the substance is acidic A pH greater than 7 means that the substance is alkaline. 2. Identify which of two or more aqueous solutions is more acidic or alkaline using pHvalues. Using either a pH meter or a universal indicator will tell you the pH value of a substance. It’s pretty easy to work out which substance more acid or alkaline. The substance with a lower pH value is more acidic whilst the one with the higher pH value is more alkaline. 3. State that each change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H+(aq)]. The pH of a substance is actually the measure of the concentration of the hydrogen ions present. The pH scale itself is logarithmic, meaning that a change of pH of 1 equals a change in [H + ] of 10. The equation for pH itself is: -log 10 [H + ] So if the concentration of H + ions in a solution is 0.1 mol dm -3 , it has a pH of 1. Likewise, if the [H + ] of a solution is 0.01 mol dm -3 , it has a pH of 2. 4. Deduce changes in [H+(aq)] when the pH of a solution changes by more than one pH unit. The equation for changing pH back into [H + ] is : 10 -(pH) So if the pH of a solution is 7, then its concentration of H + ions is 10 -7 . Hopefully, you’ve been paying attention in maths class and know how to use indices. So if the pH of a solution changes from 3 to 9: @ pH 3, [H + ] = 10 -3 mol dm -3 @ pH 9, [H + ] = 10 -9 mol dm -3 The order of difference in concentration from pH 3 to pH 9 is 10 6 times less.

A pH change from 6-7 may not seem like alot. However, that’s 10 times! If your blood levels increased by 1 whole pH, that is going to be fatal. Question hub: Below are a list of questions you might want to try out. Don’t worry, solutions are below as well! 1. Object X has pH of 4, Object Y pH of 9. Determine which is: 1. More acidic, or less, and: 2. How many times the [H+] concentration is in the more acidic object. Solution: The answer for part (1 ) is object X of pH 4. As we previously identified in this topic, the lower the pH, the more acidic a substance is. Therefore, Object X is definitely more acidic. As we also previously examined, pH works on a logarithmic scale. Therefore, a change in pH of about 1 means x10 in the [H+] concentration. Using the formula learned in 8.4.4, [H + ] is : 10 -(pH) [H+] concentration at pH 4 = 10 -(4) = 0.0001 moldm-3 [H+] concentration at pH 9 = 10 -(9) = 0.000000001 moldm-3 Using mathematics, we can see that the magnitude of difference between the two values is 10 (5)