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

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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.

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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.

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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.

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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 (H 3 O + ). Hydronium ion (H 3 O + ) and Hydrogen ion (H + ) can be used interchangeably in discussions or math with no problem. Water is Amphoteric – Both an Acid and a Base

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HOW DO WE GET THE NUMBERS? When water self-ionizes, the concentrations of the hydrogen ions and the hydroxide ions are 1 × 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 × mols/L The pH is the negative Log of the Hydrogen Ion Concentration, [H + ], so for water: pH = - Log [1.0 × mol/L] pH = 7

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LET’S LOOK AT SOME OF THE CONCENTRATIONS How do the concentrations of [H + ] compare to pH? 1× mol/L 1× × × × × × ×10 0

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Let’s Practice pH is a measure of the hydrogen ion concentration pH is calculated using the following formula: pH = -log 10 [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 = -log 10 [H + ] pH = -log 10 [0.2] = 0.7

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Let’s Practice (2) Find the pH of a 0.2 mol/L (0.2M) solution of H 2 SO 4 Write the balanced equation for the dissociation of the acid H 2 SO > 2H + (aq) + SO 4 2- (aq) Use the equation to find the [H+]: 0.2 mol/L H 2 SO 4 produces 2 x 0.2 = 0.4 mol/L H + since H 2 SO 4 is a strong acid that fully dissociates Calculate pH: pH = -log10[H + ] pH = -log 10 [0.4] = 0.4

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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

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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 × 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 - ]

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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

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Let’s Practice pOH The hydroxide ion concentration of a solution is found to be 3.4 × mol/L What is the pOH? pOH = - log [OH-] pOH = - log (3.4 × 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?

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Let’s Practice pOH A 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?

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Is it Acid Rain? A solution of rainwater has a hydronium ion concentration of 3.98 × mol/L. What is its pH? Is it acid rain? pH = - log [H 3 O + ] (Remember that H 3 O + = H + ) pH = - log (of 3.98 × ) 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

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EXTRA DIAGRAM

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