Question

Determine the pH of a solution that is 1.85% NaOH by mass. Assume that the solution has density of 1.01 g/mL.

Answer 1

Solution

1.85 % NaOH by mass

density = 1.01 g/ml

pH= ?

lets assume we have 100 ml solution (0.100 L)

so lets calculate the mass of solution

mass of solutin = volume * density

mass of solution = 100 ml * 1.01 g per ml = 101 g solution

now lets calculate the mass of the NaOH present in the solution using the mass percent of the NaOH

101 g solution * 1.85 % NaOH / 100 % = 1.8685 g NaOH

now lets calculate moles of NaOH

moles of NaOH = mass in gram / molar mass

                       =1.8685 g / 40.0 g er mol

                      = 0.04671 mol NaOH

now lets calculate molarity of the NaOH

molarity = moles/ Liter

molarity of NaOH= 0.04671 mol / 0.100 L = 0.4671 M

since aOH is strong base therefore it dissociate completely

hence concnetration of the [OH-] is same as concnetration of the NaOH

tehrefore [OH-] = 0.4671 M

now lets calculate the pOH using this concnetration of the [OH-]

pOH= -log[OH-]

pOH= -log[ 0.4671]

pOH = 0.33

now lets find the pH

pH+ pOH= 14

therefore pH= 14- OH

               pH= 14 -0.33

               pH= 13.67