Chem 2 exam

The pKa of a weak acid was determined to be 3.39 by measuring the pH of a solution containing the weak acid at 0.20 M and its conjugate base. The measured pH was 3.27. What was the concentration of the conjugate base?

0.15 M

How many moles of sodium acetate must be added to 500 mL of 0.25 M acetic acid solution to produce a buffer with a pH of 4.94? The pKa of acetic acid is 4.74. You can neglect the change in volume when sodium acetate dissolves in the acetic acid solution.

0.198 moles

To simulate the pH of blood, which is 7.4, an undergraduate researcher in a biology lab produced a buffer solution by dissolving sodium dihydrogen phosphate (Ka = 6.2 ´ 10-8) and disodium hydrogen phosphate (Ka = 3.6 ´ 10-13) together in an aqueous solution. What mole ratio of Na2HPO4/NaH2PO4 did she need to use?

1.6

What is the pH of a solution containing 0.30 M lactic acid and 0.50 M of lactate ion (Ka for lactic acid = 1.38 ´ 10-4)?

4.08

Which one of the following would make the best buffer? (Ac = acetate)

A solution of ammonia and ammonium chloride, NH3 and NH4Cl

What reaction occurs as a hydrochloric acid solution is added to a solution containing equal concentrations of acetic acid and sodium acetate?

CH3COO- + H= ---> CH3OOH

Which of the following can be mixed together in water to produce a buffer solution?

HCl and NaCl

What are the characteristics of a pH indicator? A pH indicator ________

I, II, and III

You have a summer job as an intern in an analytical chemistry laboratory. You are about to titrate a solution of nicotine (pKb = 6.0), which is a base similar to ammonia, with a standard hydrochloric acid solution. Two pH indicators are available for you to use: thymol blue (pKa = 3.47) and phenolphthalein (pKb = 9.4). Which indicator do you select as being the more appropriate one to use?

Thymol blue is the better choice because the sample pH at the equivalence point will be close to the pKa value of 3.47.

Acid-base indicators need to have very intense colors so that a very low concentration is visible. Could there be a problem in using too much indicator?

Yes, the indicator could affect the acid-base chemistry being measured.

Which of the following would be the best choice for preparing a buffer with a pH = 3.5?

a solution of formic acid and sodium formate, Ka = 1.8 x 10^-4

Which of the following would be the best choice for preparing a buffer with a pH = 8.0?

a solution of hypochlorous acid and sodium hypochlorite, Ka = 3.5 x 10^8

A buffer system is set up with [HA] = 2[A-]. If pKa = 5.5, what is the pH of the buffer?

a. 5.2

Which of the following is not a buffer system? A solution containing roughly equal concentrations of ________

bromide ion and hydrobromic acid.

The pKa of a weak acid was determined by measuring the pH of a solution containing the weak acid at 0.40 M and its conjugate base at 0.60 M. The measured pH was 7.8. What is the pKa of the weak acid?

c. 7.6

Identify all the correct statements about an acid-base buffer solution. I. It can be prepared by combining a strong acid with a salt of its conjugate base. II. It can be prepared by combining a weak acid with a salt of its conjugate base. III. It can be prepared by combining a weak base with its conjugate acid. IV. The pH of a buffer solution does not change when the solution is diluted. V. A buffer solution resists changes in its pH when an acid or base is added to it.

c. II, III, IV, and V

A solution that contains a weak acid and its conjugate base in roughly equal concentrations is ________

c. a buffer.

What is the pH of a buffer solution where [HA] = [A-]?

c. pH = pKa

Which of the following cannot be mixed together in water to produce a buffer solution?

e. HNO3 and NaNO3

Which combination of solutions is the best choice for making a buffer solution?

equal volumes of 2 M ammonia (NH3) and 1 M hydrochloric acid (HCl)

Acid-base indicators change color ________

generally over a range of 1 or 2 pH units.

The most suitable acid-base indicator for a titration of acetic acid with NaOH has ________

pKin = pH of a sodium acetate solution.

Research with biochemical systems commonly requires buffers because ________

proteins have a critical pH dependence in their structure and function.

Bromocresol green is yellow in its acidic form and blue in its basic form. When is it green?

when the solution pH equals its pKa