Question

A) If a 10.0 mL solution of "acid rain" is measured to have a pH of 2.75, what is the molar concentration of strong acid HNO3 formed? Hint: pH=-log[H+] and 10^-pH=[H+] is in molarity units.

B) The total acid concentration (nitric + nitrous acid) of the 10.0 mL solution of "acid rain" is determined by an acid-base titration method. You add 5.00 mL of the titrant sodium hydroxide with a concentration of 0.0114 M to reach the endpoint.

i. What is the number of moles of hydroxide ions added?

ii. What is the total number of moles of hydrogen ions in the "acid rain" sample?

iii. What is the total concentration (in moles/L) of hydrogen ions in the "acid rain" sample?

iv. What is the concentration (in moles/L) of the weak acid, nitrous acid, in the solution of "acid rain" considered above in A?

v. Refer to the equilibrium reaction (HNO2 --> <-- H+ + NO2-) for the weak acid, HNO2. How will the equilibrium shift if the concentration of hydrogen ions decreases?

Please show your work/explain steps...

Answer 1

A Ans.

Volume of the acid rain = 10 mL

pH = 2.75

The hydrogen ions is due to HNO₃

Therefore, the molar concenration of HNO₃ is 1.78 x 10^-3M

B Ans.

Volume of NaOH = 5mL

Concentration of NaOH = 0.114M

No. of mol of hydroxide ions added is

Therefore, the no. of mol of hydroxide ion added is 0.0570 x 10^-3mol

ii)

The no. of mol of the hydroxide ion required to reach the end point is the total no. of mol of hydrogen ions.

Therefore, the no. of mol of hydrogen ions is also equal to 0.0570 x 10^-3 mol

iii)

Volume of the acid rain = 10mL = 10 x 10^-3L

Concentration of hydrogen ions is

Therefore, the total concentration of hydrogen ions i the acid rain sample is 5.70 x 10^-3mol/L.

iv)

Concentration of hydrogen ions from nitric acid = 1.78 x 10^-3mol/L

Total concentration of hydrogen ions in the sample = 5.78 x 10^-3 mol/L

Thereore, hydrogen ions concentration due to nitrous acid is

Therefore, the concentration of nitrous acid is 3.92 x 10^-3mol/L

v)

If the concentration of hydrogen ion decreases, the equilibrium will shift towards the left.