Acids & Bases How We Measure Acids and Bases pH Blue Base

What is pH? A pH scale is how we measure how acidic or basic something is. When we dissolve an acid in water, it ionizes and produces excess Hydrogen Ions, H+ pH is formally how we measure the activity of those Hydrogen Ions in solutions.

Where Do Common Substances Fall on the Scale? pH is a range that usually is expressed as going from 0 to 14, though it can go beyond that range.

What Exactly does the pH Number Mean? When we studied solutions, we learned about Molarity, or moles-per-liter. pH is a special way of measuring the moles of Hydrogen Ions in a solution. The problem is that the number of moles can vary by many orders of magnitude. To solve that problem, we use a logarithmic scale, proposed by the Danish Scientist Soren Sorensen in 1909.

Where do The Numbers Come From? In a liter of water, a very small potion of the molecules will self-ionize, and split into a hydrogen ion (H+) and a hydroxide (OH-) ion. In this process the Hydrogen attaches to another water and creates something called a hydronium ion (H3O+). Hydronium ion (H3O+) and Hydrogen ion (H+) can be used interchangeably in discussions or math with no problem . Water is Amphoteric – Both an Acid and a Base

How do We Get the Numbers? When water self-ionizes, the concentrations of the hydrogen ions and the hydroxide ions are 1 × 10-7 moles/liter (they are the same, because each split water molecule produces one of each) We show concentrations by placing them in square brackets, like this: [H+] = 1.0 × 10-7 mols/L The pH is the negative Log of the Hydrogen Ion Concentration, [H+], so for water: pH = - Log [1.0 × 10-7mol/L] pH = 7

Let’s Look at some of the Concentrations How do the concentrations of [H+] compare to pH? 1×10-14 mol/L 1×10-12 1×10-10 1×10-8 1×10-6 1×10-4 1×10-2 1×100

Let’s Practice pH is a measure of the hydrogen ion concentration pH is calculated using the following formula: pH = -log10[H+] Find the pH of a 0.2mol/L-1 (0.2M) solution of HCl Write the balanced equation for the dissociation of the acid HCl -----> H+(aq) + Cl-(aq) Use the equation to find the [H+]: 0.2 mol/L- HCl produces 0.2 mol/L-1 H+ since HCl is a strong acid that fully dissociates Calculate pH: pH = -log10[H+] pH = -log10[0.2] = 0.7

Let’s Practice (2) Find the pH of a 0.2 mol/L (0.2M) solution of H2SO4 Write the balanced equation for the dissociation of the acid H2SO4 -----> 2H+(aq) + SO42-(aq) Use the equation to find the [H+]: 0.2 mol/L H2SO4 produces 2 x 0.2 = 0.4 mol/L H+ since H2SO4 is a strong acid that fully dissociates Calculate pH: pH = -log10[H+] pH = -log10[0.4] = 0.4

Let’s Practice (3) A hydrogen ion concentration is found to be [H+] = 2.3×10-5 mol/L what is the pH between (without a calculator). 1×10-5 would be a pH of 5, but this is more than that. 1×10-4 would be a pH of 4, but is less, so it is between the two. What is the actual pH? (us your calculator) pH = - log[H+] pH = - log(2.3×10-5) pH = 4.6

pH and pOH – What is pOH? [H+] = [OH-] so for water: Remember that water splits into both an OH- and an H+ when it ionizes. Because of this, we define the pOH just like pH, but it is the negative log of the hydroxide ion concentration instead of the hydrogen ion concentration: pOH = - log[OH-] so for water: pOH = - Log [1.0 × 10-7 mol/L] pOH = 7 Interestingly, the pOH and the pH are the same for water. That is because the concentration of H and OH are the same. [H+] = [OH-]

Can we convert between pH and pOH? Yes – Start with what you have and use the formulas at the bottom to convert between pH and pOH

Let’s Practice pOH The hydroxide ion concentration of a solution is found to be 3.4 × 10-4 mol/L What is the pOH? pOH = - log [OH-] pOH = - log (3.4 × 10-4 mol/L ) pOH = 3.5 What is the pH of that same solution? 14 = pH + pOH pH = 14 – pOH pH = 14 – 3.5 pH = 10.5 Is the solution an acid or a base?

Let’s Practice pOH A reference solution of sodium hydroxide is 0.005 M (mol/L). What should its pH be? Write the balanced equation for the dissociation of the base NaOH-----> Na+(aq) + OH-(aq) there is one mole of OH- for each mole of base, so [OH-] = 0.005 mol/L Find the pOH first: pOH = - log [OH-] pOH = - log (0.005 mol/L ) pOH = 2.3 What is the pH of that same solution? 14 = pH + pOH pH = 14 – pOH pH = 14 – 2.3 pH = 11.7 Is the solution an acid or a base?

Is it Acid Rain? A solution of rainwater has a hydronium ion concentration of 3.98 × 10-5 mol/L . What is its pH? Is it acid rain? pH = - log [H3O+] (Remember that H3O+ = H+) pH = - log (of 3.98 × 10-5 ) pH = 4.4 Acid rain has a pH of less than 5, so yes – it is acid rain pOH = 14 – pH, so pOH = 14 - 4.4 pOH = 9.6

Extra Diagram