Unit 2 Review

Buffers

100

What is [OH−] in a solution of 0.125 M CH₃NH₂ and 0.130 M CH₃NH₃Cl?

CH₃NH₂⁢(𝑎⁢𝑞) + H₂O(𝑙) ⇌ CH₃NH₃⁢+ ⁢(𝑎⁢𝑞) + OH−⁢(𝑎⁢𝑞)

𝐾b=4.4×10−4

[OH−] = 4.2 × 10−4 M

200

What will be the pH of a buffer solution prepared from 0.20 mol NH₃, 0.40 mol NH₄NO₃, and just enough water to give 1.00 L of solution?

Kb for NH3: 1.8×10−5

pH = 8.96

300

What mass of NH₄Cl must be added to 0.750 L of a 0.100-M solution of NH₃ to give a buffer solution with a pH of 9.26? (Hint: Assume a negligible change in volume as the solid is added.)

pKa for NH4+ = 9.25

3.92 g  NH₄Cl

400

Consider a solution initially containing 0.300 mol of hydrogen fluoride (HF). How many grams of NaF (42.0 g/mol) would be needed to set the pH = 3.00? 

(HF, Ka = 7.2 × 10^-4 )?

9.07 g

Titration problems

100

Calculate the pH at the following points in a titration of 40 mL (0.040 L) of 0.100 M barbituric acid (Ka = 9.8 × 10−5) with 0.100 M KOH:

a) 20 mL of KOH solution added

pH = 4.01

200

8.65 g of an unknown monoprotic weak acid was titrated to the equivalence point with 10.35 mL of 2.35 M NaOH. What is the molar mass of the unknown weak acid?

3.56 × 10² 𝑔/𝑚𝑜𝑙

300

If you start with 80.0 mL of 0.40 M HNO₃, calculate the [H+ ] concentration following addition of 40.0 mL of 0.60 M KOH.

0.0667 M

400

A 25.0 mL solution of quinine was titrated with 1.00 M hydrochloric acid, HCl. It was found that the solution originally contained 0.125 moles of quinine. What was the pH of the solution after 50.00 mL of the HCl solution were added? 

Quinine is monobasic with pKb = 5.10

pH = 9.08

Solubility problems (Ksp, molar solubility, etc.)

100

Write the ionic equation for the dissolution and the Ksp expression for each of the following slightly soluble ionic compounds:

(a) CaCO₃

(a) CaCO₃⁢(𝑠) ⇌ Ca²⁺⁢(𝑎⁢𝑞) + CO₃⁢^2-⁢(𝑎⁢𝑞)

𝐾sp=[Ca²⁺]⁢[CO₃⁢^2-]

200

Stalactites—the long, icicle-like formations that hang from the ceilings of caves—are formed from calcium carbonate. The Ksp of calcium carbonate is 4.5 × 10^-9 What is the concentration of calcium ions in a saturated calcium carbonate solution?

6.71 × 10^-5 M

300

The solubility of CaF₂ is 3.9 x 10^-11 mol/L. What is the Ksp of CaF₂?

Ksp = 2.4 x 10^-31

400

What is the solubility of barium sulfate in a solution also containing 0.050 M sodium sulfate (which is fully soluble)? 

The Ksp value for barium sulfate is 1.1 × 10^-10

2.2 × 10^-9 M