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Acids & Bases

Acids & Bases. How W e M easure 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 +

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Acids & Bases

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

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. 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 × 10-7mols/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

  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

  8. 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-1H+ since HCl is a strong acid that fully dissociates • Calculate pH: pH = -log10[H+] pH = -log10[0.2] = 0.7

  9. 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

  10. 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

  11. pH and pOH– What is pOH? • 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-]

  12. 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

  13. 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?

  14. 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 (0.005mol/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?

  15. Is it Acid Rain? • A solution of rainwater has a hydroniumion 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

  16. Extra Diagram

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