Intermediate 2 Chemistry - Unit 3(A)

1. In water and aqueous solutions there is an _______________ between hydrogen and hydroxide ions and water molecules.

In water and aqueous solutions, there is a dynamic balance or equilibrium between the hydrogen and hydroxide ions and water molecules. This means that the concentration of these ions and molecules remains relatively constant over time, as they continuously react and form each other. The equilibrium is maintained by the dissociation and recombination of water molecules, resulting in the presence of equal amounts of hydrogen and hydroxide ions.

Explanation

In water and aqueous solutions, there is a dynamic balance or equilibrium between the hydrogen and hydroxide ions and water molecules. This means that the concentration of these ions and molecules remains relatively constant over time, as they continuously react and form each other. The equilibrium is maintained by the dissociation and recombination of water molecules, resulting in the presence of equal amounts of hydrogen and hydroxide ions.

Submit

2. An alkaline solution has a pH of above 7 and contains a higher concentration of hydroxide ions than hydrogen ions.

True

False

An alkaline solution is characterized by having a pH value above 7, indicating that it is basic rather than acidic. This means that the concentration of hydroxide ions (OH-) in the solution is higher than the concentration of hydrogen ions (H+). In other words, there are more hydroxide ions available to accept protons than there are protons available to donate. Therefore, the statement that an alkaline solution has a higher concentration of hydroxide ions than hydrogen ions is true.

Explanation

An alkaline solution is characterized by having a pH value above 7, indicating that it is basic rather than acidic. This means that the concentration of hydroxide ions (OH-) in the solution is higher than the concentration of hydrogen ions (H+). In other words, there are more hydroxide ions available to accept protons than there are protons available to donate. Therefore, the statement that an alkaline solution has a higher concentration of hydroxide ions than hydrogen ions is true.

3. The concentration (in mol l^-1) of a solution made by dissolving 0.25mol of sodium chloride in 500cm³ of water is...

The concentration of a solution is calculated by dividing the amount of solute (in moles) by the volume of the solution (in liters). In this case, 0.25 mol of sodium chloride is dissolved in 500 cm3 of water. To convert the volume from cm3 to liters, we divide by 1000. So, the volume of the solution is 500/1000 = 0.5 L. Therefore, the concentration is 0.25 mol / 0.5 L = 0.5 mol L-1.

Explanation

The concentration of a solution is calculated by dividing the amount of solute (in moles) by the volume of the solution (in liters). In this case, 0.25 mol of sodium chloride is dissolved in 500 cm3 of water. To convert the volume from cm3 to liters, we divide by 1000. So, the volume of the solution is 500/1000 = 0.5 L. Therefore, the concentration is 0.25 mol / 0.5 L = 0.5 mol L-1.

Submit

4. In pure water, there are no hydrogen or hydroxide ions, only water molecules.

True

False

In pure water, there are actually very small amounts of hydrogen and hydroxide ions present due to the process of self-ionization. This is when water molecules can break apart into hydrogen ions (H+) and hydroxide ions (OH-). Therefore, the statement that there are no hydrogen or hydroxide ions in pure water is false.

Explanation

In pure water, there are actually very small amounts of hydrogen and hydroxide ions present due to the process of self-ionization. This is when water molecules can break apart into hydrogen ions (H+) and hydroxide ions (OH-). Therefore, the statement that there are no hydrogen or hydroxide ions in pure water is false.

5. When a reversible reaction is in equilibrium...

...the concentrations of reactants and products are equal

...the concentrations of reactants and products are constant

In a reversible reaction at equilibrium, the concentrations of reactants and products remain constant. This means that the rate of the forward reaction is equal to the rate of the reverse reaction, resulting in a stable concentration of both reactants and products. This state of equilibrium is achieved when there is no net change in the concentrations of reactants and products over time.

Explanation

In a reversible reaction at equilibrium, the concentrations of reactants and products remain constant. This means that the rate of the forward reaction is equal to the rate of the reverse reaction, resulting in a stable concentration of both reactants and products. This state of equilibrium is achieved when there is no net change in the concentrations of reactants and products over time.

6. Diluting an acid or an alkali brings the pH closer towards 7 because this lowers the concentration of hydrogen and hydroxide ions.

True

False

Diluting an acid or an alkali reduces the concentration of hydrogen and hydroxide ions in the solution. Since the pH scale measures the concentration of these ions, dilution will result in a decrease in the pH value. As a result, the pH of the solution will move closer towards 7, which is considered neutral on the pH scale. Therefore, the given statement is true.

Explanation

Diluting an acid or an alkali reduces the concentration of hydrogen and hydroxide ions in the solution. Since the pH scale measures the concentration of these ions, dilution will result in a decrease in the pH value. As a result, the pH of the solution will move closer towards 7, which is considered neutral on the pH scale. Therefore, the given statement is true.

7. How many grams of sodium hydrxide are required to make up 500cm³ of a 0.1 mol l^-1 solution?

To calculate the grams of sodium hydroxide required, we need to use the formula:

grams = moles x molar mass

Given that the solution is 0.1 mol/l and the volume is 500 cm3, we can calculate the number of moles:

moles = concentration x volume
moles = 0.1 mol/l x 0.5 l
moles = 0.05 mol

The molar mass of sodium hydroxide (NaOH) is 40 g/mol. Plugging in the values, we can calculate the grams:

grams = 0.05 mol x 40 g/mol
grams = 2 g

Therefore, 2 grams of sodium hydroxide are required to make up 500 cm3 of a 0.1 mol/l solution.

Explanation

To calculate the grams of sodium hydroxide required, we need to use the formula:

grams = moles x molar mass

Given that the solution is 0.1 mol/l and the volume is 500 cm3, we can calculate the number of moles:

moles = concentration x volume
moles = 0.1 mol/l x 0.5 l
moles = 0.05 mol

The molar mass of sodium hydroxide (NaOH) is 40 g/mol. Plugging in the values, we can calculate the grams:

grams = 0.05 mol x 40 g/mol
grams = 2 g

Therefore, 2 grams of sodium hydroxide are required to make up 500 cm3 of a 0.1 mol/l solution.

Submit

8. Which of the following substances produce an acid(s) when added to water?

Sodium chloride

Hydrogen chloride

Copper(II) oxide

Sodium oxide

Carbon dioxide

Silicon oxide

Sodium carbonate

Sodium sulphate

Sodium hydroxide

Ammonia

Hydrogen chloride (HCl) is a substance that produces an acid when added to water. When HCl dissolves in water, it dissociates into H+ ions and Cl- ions, resulting in the formation of hydrochloric acid (HCl). This acid increases the concentration of H+ ions in the solution, making it acidic. Similarly, carbon dioxide (CO2) also produces an acid when added to water. When CO2 dissolves in water, it reacts with water to form carbonic acid (H2CO3), which increases the concentration of H+ ions, making the solution acidic.

Explanation

Hydrogen chloride (HCl) is a substance that produces an acid when added to water. When HCl dissolves in water, it dissociates into H+ ions and Cl- ions, resulting in the formation of hydrochloric acid (HCl). This acid increases the concentration of H+ ions in the solution, making it acidic. Similarly, carbon dioxide (CO2) also produces an acid when added to water. When CO2 dissolves in water, it reacts with water to form carbonic acid (H2CO3), which increases the concentration of H+ ions, making the solution acidic.

Submit

9. Which of the following substances produce an alkali(s) when added to water?

Sodium chloride

Hydrogen chloride

Copper(II) oxide

Sodium oxide

Carbon dioxide

Silicon oxide

Sodium carbonate

Sodium sulphate

Sodium hydroxide

Ammonia

Sodium oxide, sodium carbonate, sodium hydroxide, and ammonia produce an alkali when added to water. These substances have the ability to react with water to form hydroxide ions (OH-) which give the solution an alkaline pH. Sodium oxide (Na2O) reacts with water to form sodium hydroxide (NaOH). Sodium carbonate (Na2CO3) also reacts with water to form sodium hydroxide. Sodium hydroxide (NaOH) itself is a strong alkali. Ammonia (NH3) dissolves in water to form ammonium hydroxide (NH4OH), which is also an alkali.

Explanation

Sodium oxide, sodium carbonate, sodium hydroxide, and ammonia produce an alkali when added to water. These substances have the ability to react with water to form hydroxide ions (OH-) which give the solution an alkaline pH. Sodium oxide (Na2O) reacts with water to form sodium hydroxide (NaOH). Sodium carbonate (Na2CO3) also reacts with water to form sodium hydroxide. Sodium hydroxide (NaOH) itself is a strong alkali. Ammonia (NH3) dissolves in water to form ammonium hydroxide (NH4OH), which is also an alkali.

Submit

10. Which of the following statements relating to strong and weak acids and bases is/are true?

Hydrochloric acid and nitric acid are both strong acids

Ammonia is a strong base

A strong acid is fully dissociated into ions in aqueous solution

A strong acid will always have a lower pH than a weak acid

0.1 mol/l HCl and 0.1 mol/l HNO3 have the same pH

0.1 mol/l HCl and 0.1 mol/l H2SO4 have the same pH

Equimolar solutions (equal concentrations) of ethanoic acid and hydrochloric acid will conduct electricity equally well

Equimolar solutions of ethanoic acid and hydrochloric acid will react at the same rate with magnesium ribbon

Equimolar solutions of ammonia and sodium hydroxide will have the same pH

A solution of ammonia has more ions present in it than a solution of sodium hydroxide

Hydrochloric acid and nitric acid are both strong acids because they completely dissociate into ions in aqueous solution. This means that they break apart into their component ions, H+ and Cl- for hydrochloric acid, and H+ and NO3- for nitric acid. This complete dissociation results in a high concentration of H+ ions in the solution, leading to a low pH. Therefore, a strong acid will always have a lower pH than a weak acid, which only partially dissociates in solution.

Explanation

Hydrochloric acid and nitric acid are both strong acids because they completely dissociate into ions in aqueous solution. This means that they break apart into their component ions, H+ and Cl- for hydrochloric acid, and H+ and NO3- for nitric acid. This complete dissociation results in a high concentration of H+ ions in the solution, leading to a low pH. Therefore, a strong acid will always have a lower pH than a weak acid, which only partially dissociates in solution.

Submit