Question

Nitrous acid was added to 100 gallons of pure water until the pH of the solution...

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.

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