Question
An example of a strong acid solution is perchloric acid, \({\text{HCl}}{{\text{O}}_{\text{4}}}\), in water. Which statement is correct for this solution?
A. \({\text{HCl}}{{\text{O}}_{\text{4}}}\) is completely dissociated in the solution.
B. \({\text{HCl}}{{\text{O}}_{\text{4}}}\) exists mainly as molecules in the solution.
C. The solution reacts only with strong bases.
D. The solution has a pH value greater than 7.
▶️Answer/Explanation
A
Strong acid releases H+ in water and is completely dissociated in the solution.
\({\text{HCl}}{{\text{O}}_{\text{4}}}\) will form H+ and ClO4– ions.
Strong acids can react with weak bases also.
The solution would be acidic and have a pH value less than 7.
Question
The pH of a solution changes from \({\text{pH}} = 2\) to \({\text{pH}} = 5\). What happens to the concentration of the hydrogen ions during this pH change?
A. It decreases by a factor of 1000
B. It increases by a factor of 1000
C. It decreases by a factor of 100
D. It increases by a factor of 100
▶️Answer/Explanation
A
[H+] = 10-pH .
Initially, [H+] = 10-2 .
Finally, [H+] = 10-5 .
Thus concentration decreases by a factor of 1000.
Question
What is the pH of the solution formed when \({\text{10 c}}{{\text{m}}^{\text{3}}}\) of HCl(aq) with pH 1.0 is added to \({\text{990 c}}{{\text{m}}^{\text{3}}}\) of water?
A. 1.5
B. 2.0
C. 2.5
D. 3.0
▶️Answer/Explanation
D
\({\text{10 c}}{{\text{m}}^{\text{3}}}\) of HCl(aq) with pH 1.0 has 10-1 mol/l H+ ions. i.e. (10-1)(0.01ltr) = 10-3 moles H+ ions.
Total volume after adding water = \({\text{1000 c}}{{\text{m}}^{\text{3}}}\) = 1 ltr
Final concentration of H+ ions = 10-3 moles per ltr
pH = -Log[H+]
pH = 3
Question
Three aqueous solutions of nitric acid are listed below.
W. \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}{\text{ HN}}{{\text{O}}_{\text{3}}}{\text{(aq)}}\)
X. \({\text{0.001 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}{\text{ HN}}{{\text{O}}_{\text{3}}}{\text{(aq)}}\)
Y. \({\text{0.010 mol}}\,{\text{d}}{{\text{m}}^{ – 3}}{\text{ HN}}{{\text{O}}_{\text{3}}}{\text{(aq)}}\)
What is the correct order of increasing pH of these solutions?
A. \({\text{W}} < {\text{X}} < {\text{Y}}\)
B. \({\text{W}} < {\text{Y}} < {\text{X}}\)
C. \({\text{X}} < {\text{W}} < {\text{Y}}\)
D. \({\text{X}} < {\text{Y}} < {\text{W}}\)
▶️Answer/Explanation
B
pH = -Log[H+]
As [H+] decreases, pH increases.
Order of [H+] : \({\text{W}} > {\text{Y}} > {\text{X}}\)
Hence, correct order of increasing pH of these solutions: \({\text{W}} < {\text{Y}} < {\text{X}}\)
Question
A solution of acid HX has a pH = 1 and a solution of acid HY has a pH = 3. Which statement must be correct?
A. HX is a stronger base than HY.
B. HY is a stronger acid than HX.
C. The \({\text{[}}{{\text{H}}^ + }{\text{]}}\) in the solution of HX is 100 times greater than the \({\text{[}}{{\text{H}}^ + }{\text{]}}\) in the solution of HY.
D. The \({\text{[}}{{\text{H}}^ + }{\text{]}}\) in the solution of HY is 100 times greater than the \({\text{[}}{{\text{H}}^ + }{\text{]}}\) in the solution of HX.
▶️Answer/Explanation
C
pH = -Log[H+]
As [H+] decreases, pH increases.
[H+] in HX = 10-1 mol/l
[H+] in HY = 10-3 mol/l
[H+] in HX is 100 times greater than HY.
Lower the pH, more is the [H+], stronger the acid.
Hence, HX is stronger acid than HY.