Questions
Which of the following is the net ionic equation for the overall reaction that occurs as the cell operates?
(A) \(Ag(s) + Zn^{2+}(aq) \rightarrow Ag^{2+}(aq) + Zn(s)\)
(B) \(Ag^{+}(aq) + Zn^{2+}(aq) + 3 e \rightarrow AgZn(s)\)
(C) \( Ag^{+}(aq) + Zn(s) \rightarrow Ag(s) + Zn^{2+}\)(aq) + e^{-}\)
(D) \(2 Ag^{+}(aq) + Zn(s) \rightarrow 2 Ag(s) + Zn^{2+}\)(aq)
▶️Answer/Explanation
Ans: D
To determine the net ionic equation for the overall reaction, we need to consider the two half-reactions and their standard reduction potentials given in the table:
Agt(aq) + e -+ Ag(s) E° = +0.80 V Zn2+(aq) + 2 e- – Zn(s) E° = -0.76 V
The half-reaction with the more positive reduction potential (Ag/Ag) will be the reduction reaction, while the half-reaction with the less positive (or more negative) reduction potential (Zn2+/Zn) will be the
oxidation reaction.
The overall reaction is obtained by multiplying the two half-reactions to cancel out the electrons transferred and then adding them:
Reduction: Ag*(aq) + e — > Ag(s) Oxidation: Zn(s) -> Zn2+(ag) + 2 e-
Overall reaction: Agt(aq) + Zn(s) -> Ag(s) + Zn2+(aq)
Therefore, the correct answer is (C): Ag+(aq) + Zn(s) -> Ag(s) + Zn2+(aq) + e-
Questions
Another sample of eggshell reacts completely with 4.0 mL of an HCl(aq) solution of unknown concentration. If the reaction produced 0.095 atm of gas, the concentration of the HCl(aq) solution was at least
(A) 0.0020 M
(B) 0.050 M
(C) 0.50 M
(D) 1.0 M
▶️Answer/Explanation
Ans: D
\(P=0.095\) atm \(\rightarrow\) corresponds to \(0.20 \mathrm{~g} \mathrm{CaCO}_3\)
So, moles of \(\mathrm{CaCO}_3=\frac{0.20 \mathrm{~g}}{100 \mathrm{gm}{mol}^{-1}}=0.002 \mathrm{moles}\)
moles of \(\mathrm{HCl}=\left(\right.\) moles of \(\left.\mathrm{CaCO}_3 \times 2\right) \Rightarrow 0.002 \times 2\)
So, molarity of \(\mathrm{HCl}=\frac{\text { moles }}{\text { Volume in } \mathrm{L}} \Rightarrow \frac{0.004}{4} \times 1000\)
Concentration of \(\mathrm{HCl}=1 \mathrm{~m} \rightarrow\) option \((\mathrm{D})\)
Questions
The mass percent of \(CaCO_{3}\)(s) in the eggshell is closest to
(A) 30%
(B) 45%
(C) 60%
(D) 75%
▶️Answer/Explanation
Ans: D
(36) Pressure of gas \(=0.870-0.800=0.07 \mathrm{~atm}\) Mass of \(\mathrm{CaCO}_3\) corresponding to it \(\Rightarrow 0.15 \mathrm{~g}\) (approx)
So, \(\% \mathrm{CaCO}_3=\frac{0.15}{0.20} \times 100 \Rightarrow 75 \%\)