Question

Calculate the PH of the following solutions

a) 25g HCIO4 in 500ml aqueous solution

b) 0.2M hydrofluoric acid

c) 0.02M Ca(OH)2

d) 25g HCIO and 30g Ca(CIO)2 in 1000ml aqueous solution

Answer 1

a)

The concentration of the acid is:

MW=100.45g/mol

Perchloric acid is a strong acid thus the pH is given by the concentration of the acid:

b)

Hydrofloric acid is a weak acid with pKa=3.17

the pH is given by the dissociation of the acid in the equilibrium

Using an ICE table:

	HF	--->	H+	F-
Initial	0.2M		0	0
Change	-X		+X	+X
Equilibrium	0.2-X		X	X

Using the mass action equation:

------> using the concentration in the equilibrium

, rearranging the equation

----->solving with the quadratic formula

<-----concentration of H+

c)

Ca(OH)2 is a strong base

From a 0.02M solution of Ca(OH)2 you will get a OH concentration of 0.04M, thus.

d)

You have a mix weak acid and its conjugate base, the pKa for this system is pKa=7.54

You need the final concentration of both:

HClO MW=52.45g/mol

for Ca(ClO)2 MW=142.9g/mol

For the Ca(ClO)2:

the concentration of ClO- is =2*0.21=0.42M

And uisng the henderson hasselbach equation, you can calculate the buffer concentration