Question 1

A slice of pizza has 500 kcal. If we could burn the pizza and use all the heat to warm a 50-L container of cold water, what would be the approximate increase in the temperature of the water? (Note: A liter of cold water weighs about 1 kg.)

Accepted Answer

To solve this problem, we need to use the formula Q = mcΔT, where Q is the amount of heat transferred, m is the mass of the water, c is the specific heat capacity of water, and ΔT is the change in temperature.

Since we have 50 L of water, the mass is approximately 50 kg. The specific heat capacity of water is 1 calorie/gram·°C, or 4.184 joule/gram·°C. Thus, the total heat required to increase the temperature of the water by 1°C is:

Q = (50 kg) x (4.184 J/g·°C) x (1°C) = 2092 J

Since 1 calorie is approximately equal to 4.184 joules, this is equivalent to:

Q = (50 kg) x (1 kcal/kg·°C) x (1°C) = 50 kcal

Therefore, burning a slice of pizza with 500 kcal would provide enough heat to increase the temperature of the water by:

ΔT = (500 kcal) / (50 kg x 1 kcal/kg·°C) = 10°C

So the approximate increase in temperature of the water would be 10°C. Therefore, the best answer choice is E.

Question 2

You have two beakers. One contains pure water, the other contains pure methanol (wood alcohol). The covalent bonds of methanol molecules are nonpolar, so there are no hydrogen bonds among methanol molecules. You pour crystals of table salt (NaCl)into each beaker. Predict what will happen.

Accepted Answer

NaCl crystals are composed of charged ions (Na+ and Cl-). In water, the polar water molecules attract and surround the ions, allowing them to separate and dissolve. However, in methanol, the nonpolar methanol molecules are unable to attract and surround the ions, so the NaCl crystals will not dissolve. Therefore, NaCl crystals will dissolve readily in water but will not dissolve in methanol.

Question 3

How many grams of acetic acid (C₂H₄O₂)would you use to make 10 L of a 0.1 M aqueous solution of acetic acid? (Note: The atomic masses, in daltons, are approximately 12 for carbon, 1 for hydrogen, and 16 for oxygen.)

Accepted Answer

The molar mass of acetic acid (C₂H₄O₂) is calculated as follows:

2(12.01 g/mol) + 4(1.01 g/mol) + 2(16.00 g/mol) = 60.05 g/mol

To make 10 L of a 0.1 M solution of acetic acid, we need:

0.1 mol/L × 10 L = 1 mol of acetic acid

Therefore, we need 1 mol × 60.05 g/mol = 60.05 g of acetic acid to make 10 L of a 0.1 M solution of acetic acid. The closest answer choice is D) 60 g.

Question 4

Measurements show that the pH of a particular lake is 4.0. What is the hydroxide ion concentration of the lake?

Accepted Answer

The hydroxide ion concentration ([OH⁻]) in a solution can be found using the relationship between the hydrogen ion concentration ([H⁺]), the hydroxide ion concentration ([OH⁻]), and the ion product of water (Kw), where Kw = [H⁺][OH⁻] = 1.0 x 10⁻¹⁴ at 25°C. Given the pH is 4.0, [H⁺] = 10⁻⁴ M. Therefore, [OH⁻] = Kw / [H⁺] = (1.0 x 10⁻¹⁴) / (10⁻⁴) = 10⁻¹⁰ M.

Question 5

You have two beakers. One contains a solution of HCl at pH = 1.0. The other contains a solution of NaOH at pH = 13. Into a third beaker, you slowly and cautiously pour 20 mL of the HCl and 20 mL of the NaOH. After complete stirring, the pH of the mixture will be

Accepted Answer

When an acidic solution (HCl) and a basic solution (NaOH) are mixed in equal amounts, they neutralize each other and form water and a salt (NaCl). Since NaCl is a neutral compound, the resulting solution will have a neutral pH of 7.0. Therefore, the correct choice is C) 7.0.

Question 6

Measurements show that the pH of a particular lake is 4.0. What is the hydrogen ion concentration of the lake?

Accepted Answer

The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration (pH = -log[H+]). Therefore, to find the hydrogen ion concentration, we take the antilogarithm of the negative pH value: [H+] = 10^(-pH). Thus, [H+] = 10^(-4) M, which is answer choice C. The other answer choices are incorrect, as 4.0 M is a very high concentration (and cannot be achieved for H+ in water), 10^-10 M is very low (and would indicate a very basic solution), 10^4 M is extremely high (and impossible for H+ in water), and 4% does not make sense in the context of hydrogen ion concentration.

Question 7

We can be sure that a mole of table sugar and a mole of vitamin C are equal in their

Accepted Answer

A mole of any substance contains the same number of entities, which is Avogadro's number (6.022 x 10^23). Therefore, a mole of table sugar and a mole of vitamin C both contain the same number of molecules, making E the correct choice. The mass of each substance in grams will be different since they have different molar masses, making B incorrect. The volume of a substance depends on its density and is not necessarily the same for different substances, making C incorrect. The number of atoms in a substance will depend on its molecular formula and is not necessarily the same for different substances, making D incorrect. The term "mass in daltons" is synonymous with molar mass, making A redundant.

Question 8

The bonds that are broken when water vaporizes are

Accepted Answer

When water vaporizes, the bonds that are broken are the hydrogen bonds between water molecules. This is because the heat energy provided during vaporization is enough to overcome the attractive forces between the water molecules which are due to hydrogen bonding. There are no ionic bonds in water, and the covalent bonds within water molecules are not broken during vaporization. Nonpolar covalent bonds are not present in water, and while water does have polar covalent bonds, these bonds are not broken during vaporization.

Question 9

Which of the following is a hydrophobic material?

Accepted Answer

Hydrophobic materials are non-polar in nature and repel water. Wax is a common hydrophobic material used to coat surfaces to make them water-resistant.

Question 10

Many mammals control their body temperature by sweating. Which property of water is most directly responsible for the ability of sweat to lower body temperature?

Accepted Answer

Sweating involves the breaking of hydrogen bonds between water molecules on the surface of the skin, which absorbs heat from the body, and allows the sweat to evaporate and carry heat away from the body. This property of water is directly responsible for the ability of sweat to lower body temperature.

Quiz 3: Water and Life