Helpful hints To make it simple:  For this unit, all acids/bases are going to be aqueous (except for water = pure liquid) General equations:  Strong Bases: MOH (aq)  M + (aq) + OH - (aq)  Weak Bases: M (aq) + H 2 O (l) MH + (aq) + OH - (aq)  Strong Acids: HX (aq) + H 2 O (l)  H 3 O + (aq) + X - (aq)  Weak Acids: HX (aq) + H 2 O (l) H 3 O + (aq) + X - (aq)

pH Scale

pH pH is just another way to express [H + ], the hydrogen ion concentration of an acidic or basic solution. Hydrogen acid concentrations are often small numbers, such as 1.3 × pH is a method of transforming this number into something that is a little easier to work with. Why do we measure it? pH matters for everything from swimming pools, soil conditions for plants, medical diagnosis, soaps and shampoos, etc.

Practice using Logs on your Calculator Example: Find the log of 1.0 ×10 5 You should get an answer of "5". Try the examples shown on the right. Again, be sure you are getting the correct answers - if not, try changing the order than you enter items into your calculator. NumberLog 1 × × ×

pH pH = -log[H + ] Because hydrogen ion concentration is generally less than one (for example, 1.3 x ), the log of the number will be a negative number. To make pH even easier to work with, pH is defined as the negative log of [H + ], which will give a positive value for pH. **If you ever see a lowercase “p” in front of some value in chemistry, it always means –log**

pH = -log[H + ] Try the examples shown on the right. Find the pH, given [H + ]. 1.0 ×10 -7 is the [H + ] in pure water. Pure water therefore has a pH of 7. [H + ]pH 1 × × × × ×

pH Scale Acids  pH < 7  The lower the pH, the stronger the acid Basic Bases  pH > 7  The higher the pH, the stronger the base Neutral solutions pH = 7

Finding [H + ] when you know pH Sometimes you need to work "backwards" - you know the pH of a solution and need to find [H + ], or even the concentration of the acid solution. How do you do that? pH = -log[H + ] How would we find [H + ] if we were given pH?  To convert pH into [H + ] involves taking the antilog of the negative value of pH. [H + ] = 10 -pH

Finding [H + ] when you know pH Example: We have a solution with a pH = 8.3. What is [H + ]?  With some calculators you will do things in the following order:  Enter 8.3 as a negative number (use the key with both the +/- signs, not the subtraction key)  Use your calculator's 2nd or Shift or INV (inverse) key to type in the symbol found above the LOG key. The shifted function should be 10 x.  You should get the answer 5.0 × 10 -9

pOH In addition to pH, we can also define pOH: pOH = - log [OH - ] For bases, once we find [OH - ] for a base, we can quickly determine pOH: For example:  [OH - ] =4.2 ×  pOH = -log [OH - ]  = -log (4.2×10 -4 )  pOH= 3.4 answer

pH & pOH pH + pOH = 14 Does the number 14 ring a bell? Remember K w = 1.0 × The negative log of 1.0 × = 14 Once we find pOH, it is a simple matter to find pH:  pH + pOH = 14  pH = 14 - pOH  =  pH = 10.6

Calculating pH, pOH pH = -log 10 [H 3 O + ] pOH = -log 10 [OH - ] Relationship between pH and pOH pH + pOH = 14 Finding [H 3 O + ], [OH - ] from pH, pOH [H 3 O + ] = 10 -pH [OH - ] = 10 -pOH

Examples of Calculating pH 1. Calculate the pH of a 0.01M HNO 3 solution. 2. Find the pH of a M solution of calcium hydroxide.

Find the hydronium ion concentration in a solution with a pH of Is this solution an acid or a base? How do you know?

A 0.24M solution of the weak acid, H 2 CO 3, has a pH of Determine K a for H 2 CO 3 (carbonic acid). *Assume only the first (or one) H + ionizes.

Find My pH Assignment In pairs, visit the stations around the room to determine the pH of common household substances Once you have the pH of seven substances – you may clean up and sit down You must then use the pH to fill in the rest of hte chart Complete this assignment for HW  It will be handed in for marks