Question

The conjugate base for bronsted acids ${\mathrm{H}}_2\mathrm{O}$ and $\mathrm{HF}$ are?

• A. ${\mathrm{OH}}^{-}$and ${\mathrm{H}}_2{\mathrm{F}}^{+}$
• B. ${\mathrm{H}}_3{\mathrm{O}}^{+}$and ${\mathrm{F}}^{-}$
• C. ${\mathrm{OH}}^{-}$and ${\mathrm{F}}^{-}$
• D. ${\mathrm{H}}_3{\mathrm{O}}^{+}$and ${\mathrm{H}}_2{\mathrm{F}}^{+}$

Answer 1

The correct option is C

${\mathrm{OH}}^{-}$and ${\mathrm{F}}^{-}$

The correct option is option C

The explanation for the correct option:

1. According to the Bronsted-Lowry theory, a conjugate acid is the species that gives a hydrogen cation or proton to a reaction, whereas a conjugate base is the remaining species or the one that accepts a proton or hydrogen.
2. An anion may be recognized as the conjugate base.
3. They have the ability to take in a proton and then change back into an acid.
4. For finding conjugate acid or base of any bronsted acid or base we need to make it react with water.
5. Water(as bronsted acid)will lose one of its hydrogens to form hydroxide ions. $\underset{\mathrm{Water}(\mathrm{Bronsted}\mathrm{acid})}{{\mathrm{H}}_2\mathrm{O}}+{\mathrm{H}}_2\mathrm{O}\to \underset{\mathrm{Conjugate}\mathrm{base}}{{\mathrm{OH}}^{-}}+{\mathrm{H}}_3{\mathrm{O}}^{+}$
6. From the above reaction Hydroxide is the anion form and is capable of accepting a proton so it is the conjugate base of water.
7. Also Hydrogen fluoride losses its hydrogen atom to form fluoride ion. $\underset{\mathrm{Hydrogen}\mathrm{fluoride}}{\mathrm{HF}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+\underset{\mathrm{Conjugate}\mathrm{base}}{{\mathrm{F}}^{-}}$
8. From the above reaction the fluoride is the anion formed and is capable of accepting a proton so it is the conjugate base for hydrogen fluoride.

The explanation for the incorrect options:

Option (A): ${\mathrm{OH}}^{-}$and ${\mathrm{H}}_2{\mathrm{F}}^{+}$

1. Hydrogen fluoride losses its hydrogen atom to form a fluoride ion. $\underset{\mathrm{Hydrogen}\mathrm{fluoride}}{\mathrm{HF}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+\underset{\mathrm{Conjugate}\mathrm{base}}{{\mathrm{F}}^{-}}$
2. From the above reaction the fluoride is the anion formed and is capable of accepting a proton so it is the conjugate base for hydrogen fluoride.
3. Hence, it is an incorrect option.

Option (B): ${\mathrm{H}}_3{\mathrm{O}}^{+}$and ${\mathrm{F}}^{-}$

1. Water(as bronsted acid)will lose one of its hydrogens to form hydroxide ions. $\underset{\mathrm{Water}(\mathrm{Bronsted}\mathrm{acid})}{{\mathrm{H}}_2\mathrm{O}}+{\mathrm{H}}_2\mathrm{O}\to \underset{\mathrm{Conjugate}\mathrm{base}}{{\mathrm{OH}}^{-}}+{\mathrm{H}}_3{\mathrm{O}}^{+}$
2. From the above reaction Hydroxide is the anion form and is capable of accepting a proton so it is the conjugate base of water.
3. Hence, it is an incorrect option.

Option (D): ${\mathrm{H}}_3{\mathrm{O}}^{+}$and ${\mathrm{H}}_2{\mathrm{F}}^{+}$

1. Hydrogen fluoride losses its hydrogen atom to form a fluoride ion. $\underset{\mathrm{Hydrogen}\mathrm{fluoride}}{\mathrm{HF}}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+\underset{\mathrm{Conjugate}\mathrm{base}}{{\mathrm{F}}^{-}}$
2. From the above reaction the fluoride is the anion formed and is capable of accepting a proton so it is the conjugate base for hydrogen fluoride.
3. Hence, it is an incorrect option.

Final answer: The conjugate base for bronsted acids ${\mathrm{H}}_2\mathrm{O}$ and $\mathrm{HF}$ are ${\mathrm{OH}}^{-}$and ${\mathrm{F}}^{-}$