The conjugate base of an acid will be the same compound, short one H atom. ClO_4– is the only one that meets this criterion.

A Lewis base is an electron-pair donor. Only PH3 has a pair of nonbonding electrons and can act as a donor. 

The conjugate base has one less H atoms and one unit greater negative charge because of this. Thus, the correct answer is HC₂O_4–

HCN, HCl, CH₃COOH, and NH₄Cl are all acids (NH₄₊ is the ammonium ion). That only leaves KCN as the correct answer.

A Lewis acid is a species that can accept an electron pair. In the BCl₃ molecule, B does not have a complete octet (3 covalent bonds, thus 6 electrons around it rather than 8). Thus, it can accept another electron pair, making it a Lewis acid.

A Lewis acid is a 2-electron acceptor, and a Lewis base is a 2-electron donor.

, while it is acidic, cannot accept two electrons, so it is not a Lewis acid.

can accept two electrons, so it is a Lewis acid.

is a Lewis base.

, like hydrochloric acid, is acidic, but cannot accept two electrons, so it is not a Lewis acid.

All of the listed salts will dissolve into ions when in water. When the ions are in solution, they can act as acids or bases by donating or accepting protons. Chloride, bromide, and iodide ions are all conjugate bases of strong acids, so they will not accept protons. Sodium and potassium ions are the conjugate acids of strong bases, which dissociate completely, so they will not accept hydroxide ions.

Ammonium is the conjugate acid of ammonia, a weak base. The ammonium ion can donate a proton to the solution. This will make the solution slightly acidic. As a result, ammonium bromide is a salt that will make an acidic solution.

A salt will dissociate completely in an aqueous solution, forming its respective ions. Knowing this, we can make predictions on how the ions will affect the pH of the solution, given their strengths as conjugate acids and bases. Fluoride ions are the conjugate base of hydrofluoric acid, which is a weak acid. As a result, fluoride ions will attach to protons in solution and decrease the proton concentration of the solution. This will make the solution more basic.

Chloride and bromide ions are conjugate bases of strong acids. Since these strong acids would dissociate completely in water, these ions will not associate with hydrogen ions in solution and will not affect the solution pH.

The above reaction describes the reaction that occurs when carbon dioxide dissolves in water and reacts to form carbonic acid, which is responsible for acid rain.

The phenomenon seen by dry ice is simply the sublimation of carbon dioxide. Acids and bases do not usually react in a violent fashion. The reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid) is given below. The production of carbon dioxide gas is responsible for the bubbles seen in this reaction.

The conjugate base of an acid is the same formula, minus one proton. The only option that fits this description is the one that includes the hydronium ion (H₃O+) and water (H₂O).