Question

Nitrous acid was added to 100 gallons of pure water until the pH of the solution dropped to 3.50. How much acid (in grams) was added to the water? Relevant information can be found near the end of the Acid-Base PowerPoint presentation.

Answer 1

Sol:-

Given pH = 3.50

i.e - log [H⁺] = 3.50

[because pH = - log [H⁺] ]

so

[H⁺] = 10^-pH

[H⁺] = 10^-3.50

[H⁺] =0.000316 M

we know

HNO₂ is monobasic acid , therefore Molarity of HNO₂ i.e

[HNO₂] = [H⁺] = 0.000316 M

also

Molarity = Number of moles of solute i.e HNO₂ / volume of solution in L

Number of moles of solute i.e HNO₂ = mass of HNO₂ / gram molar mass of HNO₂

so

Molarity = mass of HNO₂ / gram molar mass of HNO₂ x volume of solution in L .............(1)

gram molar mass of HNO₂ = 47.013 g/mol

Volume of solution = 100 gallons = 378.541 L

because 1 gallon = 3.78541 L

Now from equation (1), we have

0.000316 M = Mass of HNO₂ / (47.013 g/mol) (378.541 L )

Mass of HNO₂ = 0.000316 mol/L x 47.013 g/mol x 378.541 L

Mass of HNO₂ = 5.62 g

Hence Mass of HNO₂ added is equal to 5.62 g