Amphetamine (c9h13n is a weak base with a pkb of 4.2. calculate the ph of a solution containing an amphetamine concentration of 205 mg/l.

The answer to the question is;

The pH of a solution containing an amphetamine (with  pKb of 4.2) concentration of 205 mg/l is  10.4.

Explanation:

To solve the question we note that the

The mass of amphetamine = 205 mg/l

The molar mass of amphetamine  = 135.2062 g/mol

Number of moles of amphetamine  = (205 mg)/(135.2062 g/mol) = 1.5×10⁻³ moles    

That is the initial concentration of amphetamine = 1.5×10⁻³ M

We have therefore = = 6.3×10⁻⁵

Then   = 6.3×10⁻⁵

Which gives 6.3×10⁻⁵ =  

Solving, we get  

From which by factorizing gives (x+3.42×10⁻⁴)×(x-2.79×10⁻⁴) = 0

That is x = -3.42×10⁻⁴ or 2.79×10⁻⁴ which gives x= 2.79×10⁻⁴  since we are dealing with concentration

However x = [OH⁻] = 2.79×10⁻⁴

pOH = -log[OH⁻] = -log(2.79×10⁻⁴) =   3.5538

pH = 14 - pOH = 14 - 3.5538 = 10.446 ≈ 10.4

the pH of a solution containing an amphetamine concentration of 205 mg/l = 10.4.

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