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In: Chemistry

What is the pH of a solution made by dissolving 2.81 grams of calcium fluoride in...

What is the pH of a solution made by dissolving 2.81 grams of calcium fluoride in enough water to make 450 mL of solution? The Ka for HF is 6.8x10–4.

Write a chemical equation for the hydrolysis reaction that explains why an aqueous solution of CH3NH3Cl is acidic.

Calculate the pH of a 1.22×10-2 M solution of the decongestant ephedrine hydrochloride if the pKb of ephedrine (its conjugate base) is 3.86.

Solutions

Expert Solution

1)

Given that,

Mass of calcium fluoride = 2.81 g

Molar mass of caF2 = 78 g/mol

We know, moles = mass / molar mass

= 2.81 / 78

= 0.036 mol

Now, calculate the molarity of the calcium fluoride solution,

Molarity = moles /volume * 1000

= 0.036 / 450 * 1000

= 0.08 M

Let us consider a reaction,

CaF2 ---> Ca2+ +2F-

As 1 mole of CaF2 gives 2 moles of F- ions.

So, 0.08 M solution of CaF2 will have = 0.08*2

= 0.16 M F- ions

The fluoride ions hydrolyze in water to form hydrofluoric acid as follows:

F- +H2O --->HF+OH-

The hydrolysis constant for fluoride ion is

Kh = Kw / Ka

= 10-14 / 6.8x10–4

= 1.4 x 10-11

Now, calculate the concentration of OH- ions released at equilibrium as follows:

Kh = [HF] [OH-] / [F-]

1.4 x 10-11 = [OH-]2 / 0.16

[OH-] =1.49 x 10-6

Now, calculate the pOH of the solution ,

pOH = - log[OH-]

= - log[1.49 x 10-6]

= 5.82

Let's calculate the pH of the solution is,

pH = 14- pH

= 14 - 5.82

= 8.18

2)

In water CH3NH3Cl dissociates as follows:

CH3NH3Cl --- H2O--- > CH3NH3 + (aq) + Cl-(aq)

CH3NH3+ (aq) reacts with water to form H3O+, which make solution acidic.

CH3NH3 + (aq) + H2O (l) ---- > CH3NH2 (aq) + H3O (aq)


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