Presentation on theme: "Acids & Bases How We Measure Acids and Bases pH Blue Base."— Presentation transcript:
1Acids & BasesHow We Measure Acids and BasespHBlue Base
2What 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.
3Where 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.
4What 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.
5Where 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
6How 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/LThe pH is the negative Log of the Hydrogen Ion Concentration, [H+], so for water:pH = - Log [1.0 × 10-7mol/L]pH = 7
7Let’s Look at some of the Concentrations How do the concentrations of [H+] compare to pH?1×10-14 mol/L1×10-121×10-101×10-81×10-61×10-41×10-21×100
8Let’s PracticepH 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
9Let’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 H2SO > 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
10Let’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
11pH 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 = 7Interestingly, the pOH and the pH are the same for water. That is because the concentration of H and OH are the same.[H+] = [OH-]
12Can 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
13Let’s Practice pOHThe 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?
14Let’s Practice pOHA reference solution of sodium hydroxide is 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-] = 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?
15Is 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 = pOH = 9.6