Question

25. Nicotine is a weak base (C10H14N2, Kb = 1.0x10). A researcher prepares a solution containing 0.300 M nicotine and 0.050 M NaOH. Determine the concentration of monoprotonated nicotine, (C10H24N2H, the conjugate acid) in the solution. 26. A buffer is diluted to 25% of its initial strength with water. What happens to the buffer capacity? Example The buffer capacity decreases to (one-quarter, one- choices: half, three-quarters) compared to original buffer capacity. The buffer capacity increases by adding (one-quarter, one-half, three-quarters) to the original buffer capacity. The buffer capacity remains the same as before. 27. A chemist wants to make a pH 7.29 buffer. Two chemical combinations are available: hypochlororous acid (HClO, Kg = 3.5x108) with sodium hypochlorite, or sodium dihydrogen phosphate (NaH2PO4, Ka = 6.2x10-8) with sodium hydrogen phosphate. Which chemical combination is best for the buffer? What [base]/(acid) ratio should be used?

Nicotine is a weak base (C10H14N2, Kb= 1.0×10^-6. A researcher prepares a solution containing 0.300 M nicotine and 0.050 M NaOH. Determine the concentration of monoprotinated nicotine, (C10H14N2H+, the conjugate acid) in the solution.

Answer 1

Q25. initial concentration nicotine = 0.300 M

initial concentration NaOH = 0.050 M

initial [OH^-] = [NaOH] = 0.050 M

K_b = 1.0 x 10^-6

ICE table	C10H14N2 (aq)	H2O (l)	$\rightleftharpoons$	C10H14N2H+ (aq)	OH- (aq)
Initial conc.	0.300 M	-		0	0.050 M
Change	-x	-		+x	+x
Equilibrium conc.	0.300 M - x	-		+x	0.050 M + x

K_b = [C₁₀H₁₄N₂H⁺]_eq[OH^-]_eq / [C₁₀H₁₄N₂]_eq

1.0 x 10^-6 = [(x) * (0.050 M + x)] / (0.300 M - x)

1.0 x 10^-6 = (0.050x + x²) / (0.300 - x)

Solving this quadratic equation for x, x = 6.0 x 10^-6 M

equilibrium concentration of monoprotonated nicotine C₁₀H₁₄N₂H⁺ = x = 6.0 x 10^-6 M