Titrations

What percentage of HF (pKa = 3.17) dissociates in one liter of solution containing two moles of the acid?

10^-3.17 = x^2/(2.0M) x = 1.4x10^-1.58 = [F-] [F-]/[HF] = [1.4x10^-1.58]/[2.0M] = 2%

Acetylsalicylic acid has a pKa of 3.5. What is the pH of a 1 M solution of acetylsalicylic acid?

10^-3.5 = x^2/[1M] x = 10^-1.75 pH = -log[1x10^-1.75] = 1.75

Twenty milliliters of a 8.4 x 10-3 M solution of HBr is added to sixty milliliters of water. What is the final pH of the solution?

2.7 This problem requires us to perform a dilution before calculating the pH. Using C1V1 = C2V2, we find our final concentration to be 2.1 x 10-3 M. Since we are dealing with a strong acid, the concentration of hydronium ion is the same as the molarity. To find the pH, we need only find -log [H+] and find our answer to be between a pH of 2 and 3 after appropriate rounding to 1 x 10-2 M and 1 x 10-3 M

Which of the following would exhibit a color change only at the end of the titration of 20 milliliters of 0.75 M NaOH into 60 milliliters of 0.25 M HClO4?

Bromothymol blue (pKa = 7.1) Thus 0.75 M NaOH x 0.020 L gives 0.150 mol NaOH 0.25 M HClO4 x 0.060 L gives 0.150 mol HClO4. The neutralization reaction here involves equal quantities of a strong acid and strong base resulting in a neutral solution (pH = 7). When the solution pH approaches that of the pKa of the indicator, the protonation state of the compound changes and the color of the indicator changes. Therefore, we will select an indicator with a pKa nearest the pH we wish to see a color change.

What is the pOH of a mixture made by adding 150 mL of 0.20 M sodium hydroxide to 50 mL of 0.20 M hydrochloric acid?

From the values in the question, there is an excess of the strong base, sodium hydroxide, so the solution must be basic. This should give a pH value higher than 7.0 and a pOH value LOWER than 7.0 (eliminate choices A and B). pOH is defined as -log[OH-], so find the molar concentration of NaOH. The moles of hydroxide are: M x V = (0.20 M)(150 x 10-3 L) = 30 x 10-3 mol NaOH The moles of acid added are: M x V = (0.20 M)(50 x 10-3 L) = 10 x 10-3 mol HCl This leaves 20 x 10-3 mol of base in excess. The final concentration of base will be: Start Fraction: Numerator: Start Parenthesis: 20 times 10 raised to the power of negative 3: moles: End Parenthesis Denominator: Start Bracket: Start Parenthesis: 150 times 10 raised to the power of negative 3: liters: End Parenthesis: plus: Start Parenthesis: 50 multiplication sign 10 raised to the power of negative 3: liters: End Parenthesis: End Bracket: End Fraction: equality sign: Start Fraction: Numerator: 20 Denominator: 200: End Fraction: equality sign: 0 decimal point 1 Molar Sodium Hydroxide or NaOH so pOH = -log(0.1) = 1.0

Half equivalence point

HF = F- pH = pKa

Which of the following best accounts for the difference in acid strength between H2S and H2Se?

Increased stability is observed in HSe- compared to HS- At equilibrium, the side of the reaction with the greatest stability predominates. In this instance, if HSe- is more stable than HS-, the dissociation of H2Se will occur to a greater extent and helps explain why H2Se is a stronger acid Se is larger than S, therefore it is able to spread out its negative charge better making it more stable

Mixing which of the following with an equal volume of 1 M HCl would generate a final solution with pH 7? A. 1 M H2SO4 B. 1 M Mg(OH)2 C. 1 M KOH D. 1 M CH3COOH

KOH

Which of the following salts would yield acidic aqueous solutions when dissolved in water? NH4ClO4 AgNO3 NaC2H3O2

Silver nitrate, AgNO3, also contains the conjugate base (NO3-) of a strong acid, and its cation is reactive with water to form an insoluble hydroxide. Since Ag+ pulls hydroxide out of the solution, it lowers the pH of the solution by producing excess hydrogen ions (Ag+ + H2O → AgOH + H+).

What is pH at equivalence point?

Slighty above 7: WA titrated with SB At 7: SA titrated with SA Slightly below 7: SA titrated with WB

Titrant

Solution of known composition and concentration

Equivalence Point

moles OH added = moles H+ present

HF titrated with NaOH, what is pH at half equivalence point?

pKa of HF

Analyte

solution being analyzed