Q: A solution is prepared by dissolving 58 grams of NaCl (molecular mass = 58 g/mol) in enough water to make 1 liter of solution. What is the molarity of NaCl in this solution?

$$\frac{58\,g}{58\,g/mol} = 1\,mol$$; $$\text{Molarity} = \frac{1\,mol}{1\,L} = 1\,M$$

Q: A solution is made by dissolving 42.23 g of $$NH_4Cl ($$molar mass = 53.50 g/mol) in enough water to make 500.0 mL of solution. What is the molarity of $$NH_4Cl in $$this solution?

$$\frac{42.23\,g}{53.50\,g/mol} = 0.7893\,mol$$; $$\text{Molarity} = \frac{0.7893\,mol}{0.5000\,L} = 1.579\,M$$

Question 1

Which property of water allows insects like water striders to walk on its surface, and what is the molecular basis for this property?

Accepted Answer

Cohesion resulting from hydrogen bonding between water molecules

Question 2

A solution is prepared by dissolving 58 grams of NaCl (molecular mass = 58 g/mol) in enough water to make 1 liter of solution. What is the molarity of NaCl in this solution?

Accepted Answer

$$\frac{58\,g}{58\,g/mol} = 1\,mol$$; $$	ext{Molarity} = \frac{1\,mol}{1\,L} = 1\,M$$

Question 3

Why does ice float on liquid water, and what is the biological significance of this phenomenon?

Accepted Answer

Ice is less dense than liquid water because water molecules form a crystalline lattice, insulating aquatic environments

Question 4

Which of the following best explains why water has a high specific heat?

Accepted Answer

Water's high specific heat is due to hydrogen bonds that absorb heat when broken and release heat when formed

Question 5

A student adds 0.0235 M HCl to water. What is the pH of the resulting solution? (Use $$pH = -\log[H^+])$$

Accepted Answer

$$pH = -\log(0.0235) = 1.629$$

Question 6

Which of the following substances would water NOT dissolve effectively?

Accepted Answer

Oil, because it is nonpolar and hydrophobic

Question 7

What is the difference between hydrophobic and hydrophilic substances?

Accepted Answer

Hydrophobic substances have relatively nonpolar bonds and do not interact with water; hydrophilic substances have polar or ionic bonds and interact well with water

Question 8

A buffer solution contains a weak acid and its corresponding base. What is the main function of a buffer in biological systems?

Accepted Answer

To minimize changes in concentrations of $$H^+$$ and $$OH^-$$ ions, maintaining pH stability

Question 9

Which equation correctly relates the concentrations of $$H^+$$ and $$OH^- in $$any aqueous solution at 25°C?

Accepted Answer

$$[H^+][OH^-] = 10$$^{-14}$$

Question 10

Ocean acidification is primarily caused by which process, and what is its impact on marine organisms?

Accepted Answer

Excessive absorption of $$CO_2 by $$oceans, leading to formation of carbonic acid and reduction of carbonate ions needed for shell formation

Question 11

A solution is made by dissolving 42.23 g of $$NH_4Cl ($$molar mass = 53.50 g/mol) in enough water to make 500.0 mL of solution. What is the molarity of $$NH_4Cl in $$this solution?

Accepted Answer

$$\frac{42.23\,g}{53.50\,g/mol} = 0.7893\,mol$$; $$	ext{Molarity} = \frac{0.7893\,mol}{0.5000\,L} = 1.579\,M$$

Question 12

Which of the following best describes the process of evaporative cooling and its importance for living organisms?

Accepted Answer

Evaporative cooling occurs when water molecules with the highest kinetic energy evaporate, lowering the temperature of the remaining liquid and helping organisms regulate body temperature

Question 13

If the pH of a solution is 1.629, what is the pOH of the solution? (Use $$pOH = 14.000 - pH)$$

Accepted Answer

$$pOH = 14.000 - 1.629 = 2.371$$

Question 14

Which of the following is NOT one of the four emergent properties of water that contribute to Earth's suitability for life?

Accepted Answer

High reactivity with nonpolar molecules

Question 15

A scientist creates a solution by mixing 3.60 x $$10^{-3}$$ moles of NaOH and 5.95 x $$10^{-4}$$ moles of HCl in 1.00 L of water. What is the resulting pH? (Assume complete dissociation and that NaOH and HCl neutralize each other)

Accepted Answer

First, subtract moles of HCl from NaOH: $$3.60 	imes 10$$^{-3}$$ - 5.95 	imes 10$$^{-4}$$ = 3.00 	imes 10$$^{-3}$$ moles NaOH remain. $$[OH^-] = 3.00 	imes $$10^{-3} M$$. $$pOH = -\log(3.00 	imes 10$$^{-3}$$) = 2.52$$. $$pH = 14.00 - 2.52 = 11.48$$

Question 16

Which of the following best describes the role of hydrogen bonds in water's ability to moderate temperature?

Accepted Answer

Hydrogen bonds allow water to absorb and release large amounts of heat with little change in temperature

Question 17

Which statement best explains why water is considered a 'universal solvent'?

Accepted Answer

Water's polarity allows it to dissolve many ionic and polar substances by forming hydration shells around solute ions and molecules

Question 18

Which of the following scenarios would most likely result in a decrease in ocean pH?

Accepted Answer

Increased absorption of atmospheric $$CO_2 by $$seawater

Ch. 3Water and Life: Properties, Chemistry, and Biological Importance

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