Question

18. Novocaine, used as a local anesthetic in dentistry, is a weak base (at 25 °C.K. = 8.91 x 10“). What is the ratio of the concentration of the base to that of the acid in the blood plasma (pH = 7.40) of the patient? (Base): (acid) = ? (Note: For example, if (Base)/(acid) -0.25 then it's a 1:4 ratio but (Base)/(acid) = 4 then it's a 4:1 ratio) a. 223:1 b 1:35 C. 1:5 d. 1:9 e. 30:1

can someone please explain the answers by showing steps please? i have an exam tommorow

Answer 1

Question 18.

According to the Henderson-Hasselbulch equation:

pH = pKa + Log([base]/[acid])

Here, pKa = -Log(Ka) = -Log(10^-14/K_b) = -Log(10^-14/8.91*10^-6) = 8.95

Now, 7.4 = 8.95 + Log([base]/[acid])

i.e. Log([base]/[acid]) = 7.4-8.95 = -1.55

i.e. [base]/[acid] = 10^-1.55 = 0.028 ~ 1:35

Question 10.

Ka = [H⁺][NO₂^-]/[HNO₂]

i.e. 4.6*10^-4 = 0.1*[NO₂^-]/0.1

Therefore, [NO₂^-] = 4.6*10^-4 M

Note: Due to common ion effect, [H⁺] = [HCl] = 0.1 M

Question 11.

Millimoles of HCl = 25 mL * 0.23 mmol/mL = 5.75 mmol

Millimoles of Ba(OH)₂ = 20 mL * 0.1 mmol/mL = 2 mmol

Now, the moles of HCl consumed in the reaction with Ba(OH)₂ = 2*2 = 4 mmol

Note: Ba(OH)₂ + 2HCl ----> BaCl₂ + 2H₂O

Now, the remaining [HCl] = (5.75-4) mmol/(25+20) mL = 0.0389 M

Now, pH = -Log[H⁺] = -Log[HCl] = -Log(0.0389) = 1.41