Question

State whether each of the following statements is true or false. Justify your answer in each case. (a) Sulfuric acid is a monoprotic acid. (b) \(\mathrm{HCl}\) is a weak acid. (c) Methanol is a base.

Short answer

(a) False. Sulfuric acid is a diprotic acid, not a monoprotic acid, as it can donate two protons per molecule in a solution. (b) False. \(\mathrm{HCl}\) is a strong acid, not a weak acid, due to its complete dissociation in water. (c) False. Methanol is not a base; it is an alcohol that does not exhibit the proton-accepting behavior of bases.

Step-by-step solution

(Statement (a) - Sulfuric acid is a monoprotic acid)

We will first determine if this statement is true or false by understanding the definition of a monoprotic acid and verifying if sulfuric acid fits that definition. A monoprotic acid is an acid that donates one proton (\(\mathrm{H^+}\)) per molecule upon being dissolved in water. Sulfuric acid (\(\mathrm{H_2SO_4}\)), on the other hand, has two hydrogen atoms that can be donated as protons when dissolved in water. Thus, the statement is False. Sulfuric acid is not a monoprotic acid; it is a diprotic acid because it can donate two protons per molecule in a solution.

(Statement (b) - \(\mathrm{HCl}\) is a weak acid)

Now, we will examine the second statement by understanding the distinction between weak and strong acids and determining if hydrochloric acid (\(\mathrm{HCl}\)) belongs to the weak acid group. A weak acid is an acid that does not dissociate completely in water, meaning not all acid molecules lose their hydrogen ions (\(\mathrm{H^+}\)) when dissolved in water, leaving a significant amount of un-dissociated acid molecules in the solution. Strong acids, on the other hand, dissociate completely in water, with essentially all the acid molecules losing their hydrogen ions. Hydrochloric acid is a strong acid, which is characterized by its complete dissociation in water: \[\mathrm{HCl \rightarrow H^+ + Cl^{-}}\] Thus, the statement is False. \(\mathrm{HCl}\) is not a weak acid; it is a strong acid due to its complete dissociation in water.

(Statement (c) - Methanol is a base)

Lastly, let's analyze the third statement and determine if methanol is a base. A base is a substance that can accept a proton (\(\mathrm{H^+}\)), typically releasing a hydroxide ion (\(\mathrm{OH^{-}}\)) in water. Methanol (\(\mathrm{CH_3OH}\)) is an alcohol, specifically the simplest one. In an aqueous solution, methanol does not readily act as a proton acceptor, neither does it release a hydroxide ion. Instead, methanol can act as a solvent or a reactant in various chemical reactions, but it does not possess the characteristics of a base. Thus, the statement is False. Methanol is not a base; it is an alcohol that does not exhibit the proton-accepting behavior of bases.

Key concepts

Monoprotic Acid

When you hear the term "monoprotic acid," it is referring to an acid that can donate exactly one proton (\(\mathrm{H^+}\)) per molecule when dissolved in water. This means each molecule is capable of releasing a single hydrogen ion into the solution.

• Common examples include hydrochloric acid (\(\mathrm{HCl}\)) and nitric acid (\(\mathrm{HNO_3}\)).
• Monoprotic acids play a crucial role in titration experiments due to their straightforward stoichiometry.

Sulfuric acid (\(\mathrm{H_2SO_4}\)) is often confused but it's actually diprotic, not monoprotic. This is because it can release two protons per molecule. Remember, monoprotic acids change the pH of their solutions by releasing just one proton per molecule.

Diprotic Acid

Diprotic acids are capable of donating two protons (\(\mathrm{H^+}\)) per molecule in solution. This characteristic gives them versatility in acid-base reactions as they can undergo two stages of ionization.

• An example of a diprotic acid is sulfuric acid (\(\mathrm{H_2SO_4}\)).
• The release of each proton in diprotic acids is a stepwise process, often with the first dissociation being stronger than the second.

Understanding diprotic acids is essential as they can generate more complex chemical reactions and titration curves due to their dual proton donation. This property is what makes them different from monoprotic acids.

Strong Acid

The strength of an acid is mainly determined by its ability to fully dissociate in water. Strong acids dissociate completely, releasing nearly all their hydrogen ions (\(\mathrm{H^+}\)) into the solution.

• Examples of strong acids include hydrochloric acid (\(\mathrm{HCl}\)) and sulfuric acid (\(\mathrm{H_2SO_4}\)), for its first dissociation step.
• In practical terms, solutions of strong acids have a higher concentration of hydrogen ions, leading to a lower pH.

Because of their complete dissociation, strong acids are often preferred in situations requiring a consistent pH for reactions. It's important to distinguish them from weak acids, which do not dissociate fully.

Weak Acid

Weak acids are quite the opposite of strong acids. They do not dissociate completely in water, leaving a notable amount of undissociated molecules (\(\mathrm{HA}\)) within the solution.

• Acetic acid (\(\mathrm{CH_3COOH}\)) is a classic example of a weak acid.
• This incomplete dissociation results in weak acids having a higher pH than strong acids, with a mixture of \(\mathrm{H^+}\), \(\mathrm{A^-}\), and undissociated acid in solution.

Due to these properties, weak acids are often used in buffer solutions, as they can resist pH changes upon the addition of small amounts of acid or base. This ability to partially dissociate makes them less aggressive compared to strong acids.

Alcohol and Bases

Alcohols and bases are easily confused, but they have very different properties. Alcohols, like methanol (\(\mathrm{CH_3OH}\)), consist of an –OH (hydroxyl) group but do not display basic behavior.

• Alcohols can act as solvents or reactants, playing roles in various chemical reactions, but they do not generally change the pH of solutions as bases do.
• Bases, in contrast, are substances that can accept protons or release hydroxide ions (\(\mathrm{OH^-}\)) in solutions, leading to an increase in pH.

It's crucial to distinguish between these to avoid misunderstandings in acid-base reactions. For example, methanol does not provide the hydroxide ions needed to be classified as a base.