Question

Use the Henderson-Hasselbalch equation to calculate the Ph pf each solution.

A) A solution that contains 1.00% C2H5NH2 by mass and 1.19% C2H5NH3 Br by mass. Express your answer using two decimal places. (pH=?)

B) A solution that is 13.0g HC2H3O2 and 11.5g of NaC2H3O2 in 150.0 mL of solution. Express your answet using two decimal places. (pH=?)

Answer 1

A)

let the mass of the solution be 100 g

so

mass of C2H5NH2 = 1 g

mass of C2H5NH3 = 1.19 g

we know that

moles = mass / molar mass

so

moles of C2H5NH2 = 1 / 45 = 0.022

moles of C2H5NH3 = 1.19 / 46 = 0.025869

according to hasselbach hendersen equation

p0H = pKb + log [conjugate acid / base ]

so

pOH = pKb + log [ C2H5NH3 / C2H5NH2]

pOH = 3.25 + log [ 0.025869 / 0.022]

pOH = 3.316

pH = 14 - pOH

pH = 14 - 3.316

pH = 10.68

so the pH is 10.68

2)

mass of HC2H3O2 = 13 g

moles of acetic acid ( HC2H3O2 ) = 13 / 60 = 0.2166

moles of NaC2H3O2 = 11.5 / 82 = 0.14

according to hasselbach equation

for a acidic buffer

pH = pKa + log [ conjugate base / acid ]

pH = pKa + log [ NaC2H3O2 / HC2H3O2 ]

we know that

molarity = moles / volume

as the final volume is same for both

ratio of molarity = ratio of moles

pH = 4.76 + log [ 0.14 / 0.2166]

pH = 4.57

so the pH is 4.57