Question 1

When a hydronium ion concentration equals 1 × 10^-10 moles per liter, what is the pH of the solution? Is the solution acidic or basic?

Accepted Answer

The pH is calculated as the negative logarithm (base 10) of the hydronium ion concentration. For a concentration of 1 × 10^-10, the pH is 10, which is basic.

Question 2

If the pH of a solution was 7 and you were to increase the hydronium ion concentration 1000x, what would the pH be?

Accepted Answer

pH = -log[H3O+], so if the H3O+ concentration is increased 1000 times, the new H3O+ concentration will be 1000 M.

pH = -log(1000) = -3

pH = 7 -3 = 4.

Question 3

When the hydronium ion concentration equals 1 mole per liter, what is the pH of the solution? Is the solution acidic or basic?

Accepted Answer

The pH is calculated as the negative logarithm (base 10) of the hydronium ion concentration. When the concentration is 1 mole per liter, the pH = -log(1) = 0. Since pH less than 7 is considered acidic, the solution is acidic.

Question 4

If you had a 1 M solution of a weak acid what would be its pH?

Accepted Answer

Weak acids partially dissociate in solution, resulting in a pH less than 7 but greater than the pH of a strong acid solution.

Question 5

When a hydronium ion concentration equals 1 × 10^-4 moles per liter, what is the pH of the solution? Is the solution acidic or basic?

Accepted Answer

The pH is calculated as the negative logarithm (base 10) of the hydronium ion concentration. For a concentration of 1 × 10^-4, the pH is 4, which is acidic.

Question 6

What does the value of K_w say about the extent to which water molecule react with themselves?

Accepted Answer

K_w is the ion product constant of water, which represents the extent to which water molecules react with themselves to form ions. The value of K_w is very small, indicating that only a small fraction of water molecules undergo such a reaction. Therefore, Choice B is the best answer. Choice A is incorrect because an increase in K_w would indicate a larger extent of ionization, not a smaller one. Choice C is incorrect because the rate of self-ionization of water is constant only under specific conditions, such as constant temperature and pressure. Choice D is incorrect because the addition of an acid to water would not directly affect the value of K_w, although it would affect the concentration of H+ and OH- ions in the solution via the acid-base equilibrium.

Question 7

If the pH of a solution is 10, what is the hydroxide ion concentration?

Accepted Answer

The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration, so: pH = -log[H+]. To determine the hydroxide ion concentration in a solution, we can use the equation Kw = [H+][OH-], where Kw is the ionization constant of water (1.0 × 10^-14 M^2). Rearranging this equation gives: [OH-] = Kw/[H+]. At pH 10, [H+] = 1.0 × 10^-10 M. Substituting this value into the equation for [OH-], we get: [OH-] = 1.0 × 10^-14 M^2 / 1.0 × 10^-10 M = 1.0 × 10^-4 M. Therefore, the correct answer is A) 1 × 10^-4 M.

Question 8

If the pH of a solution was 7 and you were to increase the hydroxide ion concentration 100x, what would the pH be?

Accepted Answer

Increasing the hydroxide ion concentration in a solution shifts the pH towards the basic side. A pH of 7 is neutral. Increasing the hydroxide ion concentration 100 times would decrease the hydrogen ion concentration by the same factor, moving the pH up by 2 units (since pH is logarithmic), resulting in a pH of 9.

Question 9

Why do we use the pH scale to indicate the acidity of a solution rather than simply stating the concentration of hydronium ions?

Accepted Answer

The pH scale is used because it is more convenient due to the very small concentration of hydronium ions in solutions. The scale provides a simpler way to express these concentrations, which can vary widely and be extremely small, making direct numerical representation cumbersome.

Question 10

Along with the pH scale, there is the pOH scale, which indicates the level of "basicity" in a solution. Accordingly, pOH = -log[OH^-]. What is the sum of the pH and the pOH of a solution always equal to?

Accepted Answer

The pH and pOH scales are inversely related, meaning that an increase in pH results in a decrease in pOH and vice versa. The product of the hydronium and hydroxide ions is always equal to Kw, which is 1.0 x 10^-14 at 25°C. Therefore, the sum of the pH and pOH must always equal the negative log of Kw, which is 14.

Question 11

Which of the following solutions is the most acidic?

Accepted Answer

A solution with a pH of 7 is neutral, and all other options are basic (pH > 7). Among the given options, the neutral solution (pH = 7) is the most acidic.

Question 12

If you had a 1 M solution of a weak base what would be its pH?

Accepted Answer

Since the solution is a weak base it will have a pH greater than 7. A 1 M solution of a weak base is not necessarily dilute, but it is not concentrated either. Based on this, we can assume that the pH will be around 8.

Question 13

If you had a 1 M solution of a strong acid what would be its pH?

Accepted Answer

A 1 M solution of a strong acid would be highly acidic with a pH of 0. Strong acids completely dissociate in water, meaning that all of the acid molecules ionize into their constituent H+ ions and anions. At a concentration of 1 M, this would result in a very high concentration of H+ ions and a very low pH.

Question 14

What happens to the pH of an acidic solution as water is added?

Accepted Answer

When water is added to an acidic solution, the concentration of hydronium ions decreases as the solution becomes more dilute. This results in an increase in pH, making the solution less acidic.

Question 15

What is the hydroxide ion concentration in an aqueous solution when the hydronium ion concentration equals 1 × 10^-10moles per liter?

Accepted Answer

In water at 25°C, the product of the concentrations of hydronium and hydroxide ions is always 1 × 10^-14. Given [H₃O⁺] = 1 × 10^-10, [OH⁻] = 1 × 10^-14 / 1 × 10^-10 = 1 × 10^-4.

Question 16

What is the hydroxide ion concentration in an aqueous solution where the pH = 5?

Accepted Answer

The pH of 5 indicates that the hydronium ion concentration is 1 × 10^-5 M. In a neutral solution, the concentration of hydroxide ions would also be 1 × 10^-5 M, but in this case, we know that the solution is acidic, so the hydroxide ion concentration must be lower than 1 × 10^-5 M. Using the Kw expression, we can see that [H3O+][OH-] = 1 × 10^-14, so [OH-] = 1 × 10^-14 / [H3O+] = 1 × 10^-14 / 1 × 10^-5 = 1 × 10^-9 M. Therefore, the hydroxide ion concentration is 1 × 10^-9 M.

Question 17

When the hydronium ion concentration equals 10 moles per liter, what is the pH of the solution? Is the solution acidic or basic?

Accepted Answer

The answer of When the hydronium ion concentration equals 10...

Question 18

The amphoteric reaction between two water molecules is endothermic, which means that this reaction requires the input of energy in order to proceed: Energy + H₂O + H₂O → H₃O⁺ + O H^- The warmer the temperature of the water, the more thermal energy is available for this reaction, and the more hydronium and hydroxide ions are formed. Based on this information, should the value of K_w be expected to increase, decrease, or stay the same with increasing temperature?

Accepted Answer

The value of KW increases with increasing temperature because there will be a higher concentration of both hydronium and hydroxide ions due to the endothermic reaction between water molecules. This is consistent with Le Chatelier's principle, as the increase in temperature shifts the equilibrium towards the side with more products to absorb the added heat. Therefore, the correct answer is C - the value of KW will increase with increasing temperature.

Question 19

What would be the concentration of hydronium ions in a solution that had a pH = -3? Why would such a solution be impossible to prepare?

Accepted Answer

The concentration of hydronium ions would be 10^3 M, which is 1000 M. Such a high concentration is impossible because it exceeds the solubility limits of acids in water.

Question 20

As the pH increases, the hydroxide ion concentration ________.

Accepted Answer

As the pH increases, it means that the concentration of hydronium ions (H3O+) decreases. As the concentration of H3O+ decreases, the concentration of hydroxide ions (OH-) increases, as they are being produced from the dissociation of water (H2O). Therefore, the hydroxide ion concentration gets larger as the pH increases.

Quiz 18: Two Classes of Chemical Reactions

When a hydronium ion concentration equals 1 × 10-10 moles per liter, what is the pH of the solution? Is the solution acidic or basic?