Question

Calculate the pH of the solution made by adding 0.50 mol of HOBr and 0.30 mol of KOBr to 1.00 L of water. The value of Ka for HOBr is 2.0×10−9.

Answer 1

Solution :-

HOBr is the weak acid and KOBr is the conjugate base of the acid

Therefore the solution will form a buffer solution

Lets find the molarities of the HOBr and KOBr

Molarity = moles / volume in liter

Molarity of HOBr= 0.50 mol / 1 L = 0.50 M

Molarity of KOBr = 0.30 mol / 1 L = 0.30 M

Now using the Henderson equation we can calculate the pH of the buffer solution

Equation is as follows

pH= pka + log ([base]/[acid])

we have the ka value using that lets calculate the pka

pka = -log [ka]

pka = -log [2.0*E-9]

pka = 8.7

now lets use this pka value in the Henderson equation

pH= 8.7 + log ([0.30]/[0.50])

pH= 8.7 +(-0.222)

pH= 8.48

Therefore pH of the solution = 8.48