AP Chemistry Acids Part II – The Calculations

Molarity This is actually “old business” as we covered Molarity in our lessons involving solutions. The “Molarity Pyramid” still works for these systems. Molarity Moles Liters

Continuing This calculation will give you the concentration of an acid solution, the same way that it would give you the concentration of a salt- water solution (or any other dissolved solute for that matter). But – and this is important – we will need to use this calculation to get to other more commonly requested measurements.

The Concentration of Hydronium This is an easy one. If you look back at the first presentation, you see that acid molecule react with water to form hydronium ( H 3 O + ). In every single example presented, one acid molecule reacted with one water molecule to produce one hydronium ion. example: HCl + H 2 O  H 3 O + + Cl -

Meaning ???? Meaning that the concentration of hydronium will be exactly what the concentration of the acid solution was. So, if the HCl solution is prepared to be 0.50 molar, the concentration of H 3 O + will be molar

No Big Deal There… What’s Next ? What’s next is something called pH. pH is a measurement that communicates the relative acidity of an acid (or base – but that discussion occurs later) solution. It is simply a number value – and it does not have units The best place to begin a discussion of pH is to understand the pH scale. As you will see on the next slide, the common scale runs from 0 to 14 with 7 in the middle. Acid solutions have pH values less than 7 and base solutions have pH values greater than 7 An actual value of 7 is considered “neutral” – neither acidic nor basic.

The pH Scale Note that the word “alkaline” is a synonym for “basic”. While this visual does not show it, the scale does extend to 0 on the left and 14 on the right.

More on the pH Scale Need to realize that the “ends” of the scale are “more”. – As you move to the left, the solutions become more acidic. – As you move to the right, the solutions become more basic. That means that a pH = 2 solution is more acidic than a pH = 3 solution. (and a pH 1 solution would be more acidic than a pH 2 solution) Or, a pH 10 solution is more basic than a pH 9 solution.

What about the Calculations? The pH scale is actually a logarithmic scale, so we will be taking the logs of numbers. The equation to calculate pH is: pH = - log [H 3 O + ] pH is the opposite of the log of the concentration of the hydronium in a solution.

Keystrokes So, once you have the molarity of the acid solution… – You know that the concentration of the hydronium is the same thing – Take its log – Change the sign On the TI – 80 series calculator – Push the sign change button (-) – Push the “log” button – Enter the numerical value of the molarity – Hit “enter” Now you have the pH

Going “Backwards” It is possible to take a pH value and determine what the concentration of hydronium is in the solution. Basically, you will be taking an “antilog” which will be “INV” then “log” on the calculator. On the TI – 80 series: – Push “INV” or “2 nd ” – Push “LOG” – Push the sign change (-) – Enter the pH – Hit “enter” You have the original [H 3 O + ] in the solution.

Your Task There are two different worksheets that will provide some quick practice in calculations involving molarity and pH.