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5A: Unique Nature of Water and Its Solutions

Acids, bases, buffers, solubility and titration — the chemistry of everything dissolved in the water that living systems actually run on.

22 topics · 92 questions

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5A. Brønsted-Lowry and Lewis Acid-Base Definitions

  1. How it worksHow does a Brønsted-Lowry acid hand off a proton, and what does the base do to catch it?
  2. How it worksHow does a Lewis acid accept a whole electron pair when it has no proton to donate?
  3. Common mistakeWhy isn't BF3 a Brønsted-Lowry acid even though it clearly behaves like an acid toward ammonia?
  4. Worked exampleWhich reactant is the Lewis acid when NH3 meets BF3, and where exactly does the new bond form?
  5. Going furtherWhat changes about water's role when it meets HCl instead of when it meets ammonia?
Your turneasy1 of 3
In a flask, 15 millimoles of BF3 are combined with 15 millimoles of NH3 and a single product forms. Which reactant is the Lewis acid, and where does the new bond appear?

5A. Conjugate Acids and Bases

  1. How it worksHow do you build the conjugate base of an acid by stripping off exactly one proton?
  2. Worked exampleFind the conjugate acid and the conjugate base of HCO3- and show why it can be both.
  3. Common mistakeWhy doesn't a strong acid leave behind a strong conjugate base, when strength sounds like it should carry over?
  4. Going furtherWhat happens to the conjugate pair picture when a species like H2PO4- sits in the middle of a chain?
Your turneasy1 of 3
A 7 tenths molar solution of H2PO4- is prepared. Which species is its conjugate base, and which its conjugate acid?

5A. Ionization of Water, Kw and the pH of Pure Water

  1. How it worksHow do two water molecules trade a proton to make one hydronium ion and one hydroxide ion?
  2. Common mistakeWhy isn't water at 60 degrees C still exactly pH 7 even though it is still perfectly neutral?
  3. Worked exampleFind the hydroxide concentration in a solution whose hydrogen ion concentration is 2.0 x 10−3 M.
  4. Going furtherWhat happens to Kw and to the neutral pH value when you heat water from 25 up to 100 degrees C?
Your turneasy1 of 3
A solution at 25 degrees C has a hydrogen ion concentration of 1.0 x 10−9 M. Find its pOH.

5A. Estimating pH and pOH Without a Calculator

  1. How it worksHow does taking a negative log turn a concentration like 10−5 M into the single number 5?
  2. Worked exampleFind the pH of a 4.0 x 10−5 M strong acid solution using only log estimation and no calculator.
  3. Worked exampleFind the pH of 0.0025 M HNO3 by splitting the log of 2.5 away from the log of 10−3.
  4. Common mistakeWhy doesn't doubling the acid concentration double the pH the way intuition keeps insisting it should?
  5. Ties togetherHow does the same log trick used for pH carry straight over to pKa, pKb and pKw?
Your turneasy1 of 3
Find the pH of a strong monoprotic acid solution whose concentration is 1.0 x 10−4 M.

5A. Strong Versus Weak Acids and Bases

  1. How it worksHow does HCl come apart completely in water while acetic acid stops partway and sits there?
  2. Worked exampleCompare the pH of 0.10 M HCl with the pH of 0.10 M acetic acid, whose Ka is 1.8 x 10−5.
  3. Common mistakeWhy isn't acid strength the same thing as acid concentration, and how do students keep swapping them?
  4. Going furtherWhat changes about a weak acid's percent dissociation when you dilute the solution tenfold?
Your turnmedium1 of 3
A 1.0 M solution of a weak monoprotic acid has Ka = 1.0 x 10−8. Find its pH.

5A. Ka, Kb, pKa and pKb

  1. How it worksHow does writing the equilibrium expression for HA collapse an acid's behavior into one number?
  2. How it worksHow do Ka and Kb for a single conjugate pair multiply together to give exactly Kw?
  3. Worked exampleFind the pKa of an acid whose Ka is 6.3 x 10−5, and say what that value predicts about it.
  4. Common mistakeWhy doesn't a larger pKa mean a stronger acid, when a larger Ka definitely does?
Your turneasy1 of 3
An acid has Ka = 1.0 x 10−9. Find its pKa.

5A. Calculating the pH of a Weak Acid Solution

  1. How it worksWalk me through setting up an ICE table for a weak acid dissociating in pure water.
  2. Worked exampleFind the pH of 0.20 M acetic acid, Ka 1.8 x 10−5, using the x-is-small shortcut.
  3. Worked exampleFind the pH of 0.010 M HF, Ka 7.2 x 10−4, and check whether that shortcut still survives.
  4. Common mistakeWhy doesn't the x-is-small approximation hold once Ka gets close to the starting concentration?
Your turnmedium1 of 3
A weak monoprotic acid with Ka = 1.0 x 10−8 is dissolved at 1.0 x 10−2 M. Using the x-is-small shortcut, find the pH.

5A. Hydrolysis of Salts of Weak Acids and Bases

  1. How it worksHow does an acetate ion pull a proton off a water molecule and leave hydroxide behind?
  2. Worked exampleFind the pH of 0.10 M sodium acetate given that acetic acid has a Ka of 1.8 x 10−5.
  3. Worked exampleFind whether 0.10 M ammonium chloride comes out acidic, basic or neutral, and by roughly how much.
  4. Common mistakeWhy isn't every salt neutral, given that each one came from neutralizing an acid with a base?
  5. Going furtherWhat happens to the pH when both ions of a salt hydrolyze at once, as in ammonium acetate?
Your turneasy1 of 3
Ammonium chloride is dissolved in pure water at 7 tenths molar. Is the solution acidic, basic or neutral?

5A. Polyprotic Acids and Stepwise Dissociation

  1. How it worksHow does pulling one proton off a molecule make the next proton harder to remove?
  2. Common mistakeWhy doesn't a diprotic acid make twice the hydrogen ion concentration of a monoprotic acid?
  3. Worked exampleFind the pH of 0.10 M H3PO4 from Ka1 = 7.5 x 10−3, and show why Ka2 barely matters.
  4. Going furtherWhat happens to the dominant phosphate species as the pH climbs from 2 through 7 up to 12?
Your turnmedium1 of 3
A triprotic acid with Ka1 = 1.0 x 10−6 is dissolved at 1.0 M. Find the pH, using only the first ionization.

5A. Buffers and the Henderson-Hasselbalch Equation

  1. How it worksHow does a weak acid paired with its conjugate base soak up added acid and added base?
  2. How it worksHow does the Henderson-Hasselbalch equation fall straight out of the Ka expression with one log?
  3. Worked exampleFind the pH of a buffer holding 0.30 M acetic acid and 0.15 M acetate, with pKa 4.76.
  4. Worked exampleFind the base-to-acid ratio needed to hold a phosphate buffer at pH 7.4 when pKa is 7.2.
  5. Common mistakeWhy doesn't adding water to a buffer shift its pH the way adding water to an acid does?
Your turneasy1 of 3
A buffer is prepared from a weak acid of pKa 8 and its conjugate base, with the base-to-acid concentration ratio equal to 101. Find the pH.

5A. The Buffer Region and Buffer Capacity

  1. How it worksHow does the flat buffer region line up with pKa plus or minus one full pH unit?
  2. Common mistakeWhy isn't a buffer with a huge acid-to-base ratio a good buffer, when it has plenty of acid in it?
  3. Worked exampleFind how far the pH of a 0.10 M acetate buffer moves when 0.010 mol of HCl goes into a liter of it.
  4. Going furtherWhat happens to the pH the instant a buffer's conjugate base is completely used up?
Your turneasy1 of 3
A weak acid has pKa 8. Taking the buffer region as pKa plus or minus one unit, find the lowest pH at which this acid still buffers usefully.

5A. Ions in Solution: Names, Charges, Hydration and Concentration

  1. How it worksHow do water molecules turn themselves around to build a hydration shell on a sodium ion?
  2. How it worksHow do you get from grams of solute to molarity fast enough for a test with no calculator?
  3. Worked exampleFind the molarity when 5.85 g of NaCl is dissolved to make 250 mL of solution.
  4. Worked exampleFind the charges on ammonium, sulfate and phosphate, and show how those charges fix the formulas.
  5. Going furtherWhat changes about the hydration shell when the ion is small and highly charged like Al3+?
Your turneasy1 of 3
Find the molarity when 29 g of NaOH, molar mass 40 g/mol, is dissolved to make 684 mL of solution.

5A. The Solubility Product Ksp and Molar Solubility

  1. How it worksHow does a saturated solution settle into a dynamic equilibrium between solid and loose ions?
  2. Worked exampleFind the molar solubility of AgCl from a Ksp of 1.8 x 10−10.
  3. Worked exampleFind the molar solubility of CaF2 from Ksp 3.9 x 10−11 and watch a factor of 4s3 appear.
  4. Common mistakeWhy doesn't a larger Ksp always mean a more soluble salt when the two salts have different formulas?
Your turnmedium1 of 3
A salt MX dissolves to give M+ and X-. Its Ksp is 1.0 x 10−13. Find its molar solubility, in mol/L.

5A. The Common-Ion Effect and Laboratory Separations

  1. How it worksHow does adding an ion the solid already contains push its equilibrium backward toward solid?
  2. Worked exampleFind the solubility of AgCl in 0.10 M NaCl from Ksp 1.8 x 10−10 and compare it with pure water.
  3. Going furtherWhat happens when you use a common ion deliberately to pull one salt out of a two-salt mixture?
Your turnmedium1 of 3
A 1:1 salt MX has Ksp = 1.0 x 10−11. Find its molar solubility in a solution already 1.0 x 10−2 M in X-.

5A. Complex Ion Formation and Solubility

  1. How it worksHow do ligands wrap around a metal center to build a complex ion with its own formation constant?
  2. Worked exampleFind why AgCl dissolves in excess ammonia by multiplying Ksp 1.8 x 10−10 by Kf 1.7 x 107.
  3. Common mistakeWhy doesn't complex formation break Ksp, when it plainly dissolves more solid than Ksp allows?
  4. Ties togetherHow does complex-ion chemistry explain the way EDTA strips metal ions out of a blood sample?
Your turnmedium1 of 3
A sparingly soluble salt has Ksp = 1.0 x 10−11, and its metal ion forms a complex with a ligand whose Kf is 1.0 x 108. The overall dissolution constant is 1.0 x 10y. Find y.

5A. Solubility and pH

  1. How it worksHow does protonating the anion of a barely soluble salt drain its solubility equilibrium dry?
  2. Worked exampleCompare the solubility of Mg(OH)2 at pH 7 with its solubility at pH 10, using Ksp 5.6 x 10−12.
  3. Going furtherWhat changes about a weakly basic drug's absorption as it travels from the stomach into the intestine?
  4. Common mistakeWhy doesn't lowering the pH make AgCl more soluble the way it does for calcium carbonate?
Your turnhard1 of 3
A metal hydroxide M(OH)2 has Ksp = 1.0 x 10−11. Find its molar solubility in a solution buffered at pH 12.

5A. Precipitation and the Ion Product Q

  1. How it worksHow does holding the ion product Q up against Ksp tell you whether a solid is about to appear?
  2. Common mistakeWhy isn't a precipitate guaranteed just because you mixed two solutions containing an insoluble pair?
  3. Worked exampleFind whether mixing 50 mL of 0.0020 M Pb(NO3)2 with 50 mL of 0.0020 M KI drops PbI2, Ksp 7.1 x 10−9.
  4. Going furtherWhat happens to Q the moment you dilute a mixture that was sitting exactly at saturation?
Your turnhard1 of 3
Equal volumes of 1.0 x 10−5 M Pb(NO3)2 and 1.0 x 10−5 M Na2SO4 are mixed. Find the ion product Q for PbSO4 immediately after mixing.

5A. Strong Acid-Strong Base Titration Curves

  1. How it worksHow does simple mole bookkeeping at each stage of a strong-strong titration set the pH?
  2. Worked exampleFind the pH after 10.0 mL of 0.10 M NaOH goes into 25.0 mL of 0.10 M HCl.
  3. Worked exampleFind the pH at the equivalence point when 25 mL of 0.10 M HCl is titrated with 0.10 M NaOH.
  4. Common mistakeWhy doesn't the height of the jump depend on which strong acid you started with?
Your turnmedium1 of 3
A 50.0 mL sample of 0.10 M HCl is titrated with 0.10 M NaOH. Find the hydrogen ion concentration, in mol/L, after 33 mL of base has been added.

5A. Weak Acid Titration and the Half-Equivalence Point

  1. How it worksHow does the half-equivalence point make pH equal pKa exactly, with no calculation at all?
  2. Worked exampleFind the pKa of an unknown weak acid whose titration curve reads pH 4.8 at half-equivalence.
  3. Worked exampleFind the pH at the equivalence point when 0.10 M acetic acid is titrated with 0.10 M NaOH.
  4. Common mistakeWhy isn't the equivalence point of a weak acid titration sitting at pH 7?
  5. Ties togetherHow does reading a titration curve backwards let you name an unknown acid from a table of pKa values?
Your turneasy1 of 3
An unknown weak acid is titrated with NaOH and the curve reads pH 5.6 at the half-equivalence point. Find its pKa.

5A. Indicators and Choosing an Endpoint

  1. How it worksHow does an indicator's own conjugate pair swap color across roughly two pH units?
  2. Common mistakeWhy isn't the endpoint the same thing as the equivalence point, and when does that gap bite?
  3. Worked exampleWhich indicator fits titrating acetic acid with NaOH, phenolphthalein at 8.2 or methyl orange at 3.7?
  4. Going furtherWhat happens to the concentration you report when your indicator changes color far too early?
Your turneasy1 of 3
Acetic acid is titrated with NaOH and the equivalence point falls near pH 9. Which indicator should be chosen?

5A. Polyprotic Titration Curves

  1. How it worksHow does each equivalence point on a polyprotic curve mark the loss of one more proton?
  2. Worked exampleFind pKa1 and pKa2 of a diprotic acid whose two buffer plateaus sit at pH 2.1 and pH 7.2.
  3. Going furtherWhat changes about the curve when two Ka values fall within a factor of ten of each other?
  4. Common mistakeWhy doesn't the third proton of phosphoric acid give a visible equivalence point in water?
Your turneasy1 of 3
A diprotic acid is titrated with NaOH and its FIRST equivalence point arrives after 31 mL. Find the total volume of titrant, in mL, needed to reach the second equivalence point.

5A. Redox Titration

  1. How it worksHow does a permanganate titration announce its own endpoint with no added indicator?
  2. Worked exampleFind the moles of Fe2+ in a sample that consumes 20.0 mL of 0.0200 M KMnO4 under acid.
  3. Going furtherWhat changes about the stoichiometry when permanganate is reduced in base instead of in acid?
Your turnmedium1 of 3
A sample of Fe2+ in acid consumes 31 mL of 4 hundredths molar KMnO4 to reach the endpoint. Find the millimoles of Fe2+ present.

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