Question

Observe the following species: AlCl$_3$, NH$_3$, H$^+$, Co$^{3+}$, $^-$OH, Mg$^{2+}$, BF$_3$, Cl$^-$ How many Lewis acids are present in the above list?

• A. 5
• B. 4
• C. 2
• D. 3

Hint:

Remember the general trends: positively charged ions (cations) and molecules with central atoms having incomplete octets are often Lewis acids. Species with lone pairs of electrons (anions or molecules with lone pairs) are often Lewis bases.

Answer 1

The Correct Option is A

Step 1: Define a Lewis acid.
A Lewis acid is a chemical species that can accept an electron pair.
Step 2: Analyze each species in the list. \begin{itemize} \item AlCl$_3$: Aluminum has an incomplete octet and can accept an electron pair. (Lewis acid) \item NH$_3$: Nitrogen has a lone pair of electrons and can donate it. (Lewis base) \item H$^+$: A proton has an empty 1s orbital and can accept an electron pair. (Lewis acid) \item Co$^{3+$}: The cobalt(III) ion is electron deficient and can accept electron pairs, especially in forming coordination complexes. (Lewis acid) \item $^-$OH: The hydroxide ion has lone pairs of electrons and can donate them. (Lewis base) \item Mg$^{2+$}: The magnesium(II) ion is electron deficient and can accept electron pairs, particularly in aqueous solutions forming hydrated ions. (Lewis acid) \item BF$_3$: Boron has an incomplete octet and can accept an electron pair. (Lewis acid) \item Cl$^-$: The chloride ion has lone pairs of electrons and can donate them. (Lewis base) \end{itemize}
Step 3: Count the number of Lewis acids.
The Lewis acids in the list are AlCl$_3$, H$^+$, Co$^{3+}$, Mg$^{2+}$, and BF$_3$. There are 5 Lewis acids. Final Answer: \[ \boxed{5} \]