Water and Life

Which statement is true of water's tensile strength?

(a) It results from hydrogen bonding. (b) It helps to pull water through plants. (c) It involves both cohesion and adhesion

Which factor is important in making it possible to cool yourself by sweating?

(a) Molecules collide with varied angles and speeds. (b) Hydrogen bonds are relatively weak. (c) Water has more energy at the body surface.

calculate the pH of a solution with [H3O+] = 6 x 10-12 M.

11.2

What is the pH of an aqueous solution with the hydronium ion concentration [H3O+] = 2 x 10-14 M ?

13.7

Which of the following can be attributed to water's high specific heat?

A lake heats up more slowly than the air around it

Which of the following effects can occur because of the high surface tension of water?

A raft spider can walk across the surface of a small pond.

Which answer helps to explain why all living cells need pH buffers?

Amino acid side chains have many carboxyl and amino groups.

Ammonia reacts with water as shown below. Which statement best explains why ammonia is considered to be a base? NH3 + H2O ⇌ NH+4 + OH− Ammonia Water Ammonium Ion Hydroxide Ion

Bases are compounds that remove H+ from solutions

How does the way a buffer stabilizes pH during addition of acid differ from the way the same buffer stabilizes pH during addition of base?

It's the same reaction running backward or forward

Your tank of swamp fish needs a pH of 5, and the pH is 7 at present. What should you do to the H+ concentration?

Raise it to 100 times its present value

Your tank of alkali fish needs a pH of 8, and the pH is 6 at present. What should you do to the H+ concentration?

Reduce it to 1/100 of its present value.

Increased atmospheric CO2 concentrations might have what effect on seawater?

Seawater will become more acidic, and carbonate concentrations will decrease.

Which statement is true of pH buffers?

They consist of weak acids and weak bases

Which of the following statements is true about buffer solutions?

They maintain a relatively constant pH when either acids or bases are added to them.

Dissolving is best described as ...

a mingling of molecules and/or ions.

following can be considered bases?

a) Na2CO3 (b) KOH

Buffers work best when ..

about half of the buffer molecules are dissociated

The open spaces in water's crystal structure make it possible for ...

aquatic life to exist at the North Pole.

Water molecules can form hydrogen bonds with _____.

compounds that have polar covalent bonds

A strong acid like HCl _____.

dissociates completely in an aqueous solution

A compound is an acid if it ...

donates H+ to water.

The tendency of an atom to pull electrons toward itself is referred to as its _____

electronegativity

Each water molecule is joined to _____ other water molecules by ____ bonds.

four ... hydrogen

A solution contains only glycerol and fructose. There is ten times as much glycerol as fructose. Therefore

glycerol is the solvent.

Which of the following is a property of liquid water? Liquid water _____.

has a heat of vaporization that is higher than that for most other substances

To make a buffer, you need to ..

have a weak acid or a weak base half ionized in water.

Water is a source of ______________ for chemical reactions in cells.

hydrogen atoms and oxygen atoms

Which type of bond must be broken for water to vaporize?

hydrogen bonds

Water has surface tension because ...

hydrogen bonds between surface water molecules resist being stretched

Despite its cohesion, water can flow because ...

hydrogen bonds break and re-form rapidly.

Though you add heat, the temperature of boiling water remains constant because ...

it takes energy to break hydrogen bonds.

To determine whether a base is weak or strong, ...

look for undissociated molecules of base.

Hydrophobic substances such as vegetable oil are _____.

nonpolar substances that repel water molecules

Sulfur is in the same column of the periodic table as oxygen, but has electronegativity similar to carbon. Compared to water molecules, molecules of H2S will _____.

not form hydrogen bonds with each other

A 1 M solution contains ...

one mole of solute per liter of solution.

The water molecule has a bent shape because ...

oxygen has two unbonded pairs of valence electrons

The partial negative charge in a molecule of water occurs because _____.

the electrons shared between the oxygen and hydrogen atoms spend more time around the oxygen atom nucleus than around the hydrogen atom nucleus

A solution is aqueous if ...

water is the solvent.

Which of the following is a hydrophobic material?

wax

Factory smoke has led to acid rain that lowered the pH of a lake to 5.0. What is the H+ concentration in the lake?

10- 5 M

In a washing machine, the detergent raised the pH to 9.0. The concentration of OH- in the solution is .

10-5 M

What is the molar concentration of [H3O+] in a cola that has a pH of 3.120?

7.59 × 10−4 The molar concentration of [H3O+] in a cola with a pH of 3.120 can be calculated this way: [H3O+]=10−pH [H3O+]=10−3.120 [H3O+]=7.59×10−4M Ingestion of large amounts of phosphoric acid found in cola can upset the body's regulation of bone metabolism and reduce the absorption of calcium from the diet. For this reason, people who are at risk of developing osteoporosis are often advised not to drink much cola.

Which of the following helps most to explain why water has a high specific heat?

A water molecule can make 4 hydrogen bonds When you heat water, much of the heat is used to break hydrogen bonds. Only the remaining heat can increase molecular motion, raising the temperature.

Which statement helps to explain why ice is less dense than liquid water?

A) Water molecules make hydrogen bonds at definite angles B) Cold molecules move less than warm molecules.

A buffer consists of undissociated acid (HA) and the ion made by dissociating the acid (A-). How does this system buffer a solution against decreases in pH?

A- reacts with H+ to become HA.

In water, every sulfuric acid molecule transfers H+ to water and becomes an HSO4 - ion. Some HSO4 - ions give off another H+ to form SO4 -2 ions. Which statement is true? H2SO4→H++HSO−4⇌H++SO2−4

H2SO4 is a strong acid and HSO4 - is a weak acid

In water, phosphoric acid dissociates according to the chemical reactions written below. Which of the following statements about the dissociation of phosphoric acid is true? H3PO4⇌H2PO−4+H+ H2PO−4⇌HPO2−4+H+ HPO2−4⇌PO3−4+H+

H3PO4, H2PO4 - and HPO4 2- are all weak acids.

Which of these bonds is weakest?

HYDROGEN Hydrogen Bonds are weaker than covalent bonds because they do not involve sharing of electrons, and they are weaker than ionic bonds because they involve the attraction of partial (not full) opposite charges.

Which of the following takes place as an ice cube cools a drink?

Kinetic energy in the liquid water decreases

Which of the following can be considered strong bases

NaOH

Which answer is a true statement about pH values?

None of the above.

Compounds that release OH- are bases because ...

OH- combines with H+ and removes it from solution.

Which statement is part of the explanation for water's high cohesion?

Oxygen has four valence orbitals. Oxygen is much more electronegative than hydrogen. The electronegativity difference creates charges that make hydrogen bonds possible, and the four outer orbitals permit water to make four hydrogen bonds. The result is a three-dimensional network of bonds between water molecules.

Which answer helps to explain how many hydrogen bonds a water molecule can form?

Oxygen's valence shell has four orbitals.

Which statement must be mentioned in explaining why amphipathic molecules line up at a water surface?

Polar groups attract one another. Polar water molecules pull together, holding the polar parts of the amphipathic molecules and squeezing out the nonpolar parts. That leaves the nonpolar parts waving in the air.

Why doesn't oil mix with water?

Polar molecules attract one another.

Why does ice float in liquid water?

Stable hydrogen bonds keep water molecules of ice farther apart than water molecules of liquid water.

Which statement is true of water?

The H atoms in water have partial positive charges.

What conditions must be met in order for an aqueous solution to be called "neutral"?

The concentrations of hydronium and hydroxide ions are equal.

Dr. Haxton told his class that a water molecule can make 4 hydrogen bonds, all of them in the same plane as the three atoms. What would a good student reply?

Wrong! Two hydrogen bonds are not in the plane of the atoms.

The unequal sharing of electrons within a water molecule makes the water molecule _

polar The electrons spend more time with the oxygen of the water molecule than with the hydrogens of water. Thus, the oxygen has a net negative charge and the hydrogens have a net positive charge.

In this molecule, what type of bond is found between the oxygen and hydrogens?

polar covalent While oxygen shares electrons with hydrogen, the electrons spend more time with the oxygen than with the hydrogens.

In a single molecule of water, two hydrogen atoms are bonded to a single oxygen atom by

polar covalent bonds