hw 3 exam 1

The pH of blood is affected by

- the reaction of CO2 with H2O to form carbonic acid. - the ionization of aqueous carbonic acid to hydronium ions and the bicarbonate anions. - the decrease of the blood pH due to the production of hydronium ions. - the excretion of bicarbonate and ammonium ions from the kidneys.

Kw, the ionization constant of water, is _____ at _____.

10-14; 25°C

Phosphoric acid is a polyprotic acid, with pK values of 2.14, 6.86, and 12.38. Which ionic form predominates at pH 9.3?

HPO42-

Sheets composed of two layers of amphipathic molecules arranged with the hydrophilic groups on the surface and the hydrophobic groups buried in the center that form in water are called

bilayer membranes

To make an acetate buffer at pH 4.76 starting with 500 mL of 0.1 M sodium acetate (pK acetic acid = 4.76), you could add

0.025 mol HCl

The pK1 of citric acid is 3.09. What is the citric acid : monosodium citrate ratio in a 1.0 M citric acid solution with a pH of 2.09?

10:1

You mix 999 mL pure water and 1 mL of 2.0 M NaOH. Calculate the pH of the resulting solution. (assume the total volume is 1.0 L).

11.3

To make a phosphate buffer at pH 6.82 starting with one liter of 10 mM phosphoric acid (pKs are 2.15, 6.82, and 12.38), you could add

15 millimoles of KOH

Urea is a water-soluble product of nitrogen metabolism. How many hydrogen bonds can one urea molecule donate to surrounding water molecules?

4

A solution is made by mixing 0.05 mL of 1.0 M HCl with 999.95 mL of pure water. Calculate the pH of the resulting solution (assume the total volume is 1.0 L).

4.3 M1V1=M2V2 pH = -log[H+]

The pKs of succinic acid are 4.21 and 5.64. How many grams of monosodium succinate (FW = 140 g/mol) and disodium succinate (FW = 162 g/mol) must be added to 1 L of water to produce a solution with a pH 5.28 and a total solute concentration of 100 mM? (Assume the total volume remains 1 liter, answer in grams monosodium succinate, grams disodium succinate, respectively.)

9.7 g, 4.9 g

Which of the following statements about water is incorrect?

Cations are solvated by shells of water molecules oriented with their hydrogen atoms pointed toward the ions.

A graduate student at SDSU wants to measure the activity of a particular enzyme at pH 4.0. To buffer her reaction, she will use a buffer system based on one of the acids listed below, which acid is most appropriate for the experiment?

Formic acid (Ka 1.78 × 10-4)

Phosphate (pK1 = 2.15, pK2 = 6.82, and pK3 = 12.38) will be mostly in the HPO42- form at pH 7.2. At pH 5.82 it is mostly in the _______ form.

H2PO4-

A phosphate buffer solution at pH = pK1 = 2.15 would have equal amounts of phosphate in the _______ form and the H2PO4- form.

H3PO4

Which of the following statements about water is not true? It can form two hydrogen bonds per water molecule. It has a high dielectric constant. It dissolves salts and polar substances. In the liquid state it is only 15% less hydrogen bonded than in the solid state at 0°C. It packs in a hexagonal (honeycomb) shaped lattice when the temperature falls below 0°C

It can form two hydrogen bonds per water molecule.

Which of the following statements about water is incorrect? It is a small, polar molecule with a low dielectric constant. It has a marked dipole moment. Acid-base reactions are very fast due to the mobility of hydronium ions in water which is a consequence of the ability of individual protons to "jump" from one water molecule to another. It is largely hydrogen bonded, although any single H-bond exists only for a very short period of time (~10-12 s ). It has a bent geometry with each O-H bond approximately 0.958 Å long and with an O-H bond energy of approximately 460 kJ/mol.

It is a small, polar molecule with a low dielectric constant.

Which of the following is the best explanation for the hydrophobic effect?

It is an entropic effect, caused by the desire of water molecules to increase their entropy by excluding hydrophobic groups, which they must otherwise surround with highly ordered structures.

What is the approximate pKa of a weak acid HA if a solution 0.1 M HA and 0.3 M A- has a pH of 6.5?

Ph=Pk+Log[A-]/[HA] Pk=6.5-log 0.3/0.1 Pk = 6.0

Which of the following statements about hydrophobic interactions is not true? They are entropy driven. They are the driving force for micelle formation. When nonpolar molecules come in contact with water, a highly-ordered shell of water molecules forms at the interface between the nonpolar molecules and water. A hydrophobic interaction is caused by the desire of water molecules to regain the entropy lost during this organization around the nonpolar substance by excluding the substance from interaction with water molecules. They are the main driving force for protein folding into three dimensional structures. They are caused by hydrophobic molecules interacting strongly with each other.

They are caused by hydrophobic molecules interacting strongly with each other.

Which of the following statements about water is not true? Water is a key player in the energetics of hydrophobic interactions. The electron-rich oxygen atom of one water molecule can interact with the electron-poor proton on another water molecule to form a hydrogen bond. Liquid water is only 15% less hydrogen bonded than ice. Water is a nonpolar molecule that with a bent molecular geometry. Water can form highly ordered, cage-like, structures around nonpolar molecules.

Water is a nonpolar molecule that with a bent molecular geometry.

The pH of coffee is 5.6. The pH of grapefruit juice is 2.6. This means that the proton concentration in coffee is

a thousand times lower than in grapefruit juice

Physical properties that depend on the amounts of various species, rather than the identities of those species, are called

colligative properties

The pH at the midpoint in the titration of an acid with a base is

equal to the pK of the corresponding acid.

_____ is exceptionally soluble in water due to the formation of hydrogen bonds.

ethanol

Amphiphilic molecules

have both hydrophilic and hydrophobic groups.

Globules of up to several thousand amphiphilic molecules arranged with the hydrophilic groups on the surface and the hydrophobic groups buried in the center that form in water are called

micelles

You mix equal volumes of 0.05 M NaH2PO4 and 0.05 M Na2HPO4 (pK's for phosphoric acid are 2.15, 6.82 and 12.38). Which of the following best describes the resulting solution?

pH 6.82 and well buffered

In the energetics of transferring hydrocarbons from water to nonpolar solvents, the factor TΔS is commonly

positive

Hydrophobic interactions between nonpolar molecules

require the presence of surrounding water molecules.

osmotic pressure measures

solute concentration

In a hydrogen bond between a water molecule and another biomolecule

the hydrogen bond will typically form between a hydrogen atom of the water molecule and either a nitrogen, sulfur, or oxygen atom of the other molecule.

The capacity of a buffer to resist changes in pH upon addition of protons or hydroxide ions depends on

the pKa of the weak acid in the buffer. the pH of the buffer. the total concentration of the weak acid and its conjugate base in the buffer.

Molecules such as methanol and ethanol are very soluble in water because

they contain O─H groups that can form multiple H-bonds with water.

The boiling point of water is 264°C higher than the boiling point of methane because

water molecules are connected to each other by H-bonds.

Fatty acids such as palmitate and oleate are usually characterized as

amphiphilic

Hydrogen bonds within liquid water

are dipole-dipole attractions.

weak acids

are only partially ionized in an aqueous solution.

The pH of a 0.1 M solution of sodium acetate would be

basic, because of the acetate ion reacts with water to form acetic acid and OH-.