Please show work/explain your answers. Thanks :) 1. To the nearest hundredths ,

Question

Please show work/explain your answers. Thanks :)

1. To the nearest hundredths, what is the pH of a 0.15 M solution of a weak acid (HA) that has a pKa of 5.37? Hint: assume that x, which corresponds to [H+], is small compared to the initial concentration of the solution. If this assumption is reasonable (which it should be), you can avoid solving a quadratic equation.

2. A 0.24 M solution of a weak acid HA dissociates such that 99.6% of the weak acid remains intact (i.e., remains as HA). To the nearest hundredths, what is the pH of the solution?

3. A 0.25 M solution of a weak acid HA dissociates such that 99.5% of the weak acid remains intact (i.e., remains as HA). What is the pKa of the weak acid? Enter your answer to the nearest hundredths.

Explanation / Answer

Ka= [H+] [A-]/[HA]= Equilibrium constant. Given Pka= 5.37 and Ka= 10(-5.37)= 4.3*10-6

Given [HA] = 0.15 M initially

Let x =drop in concentration of HA to reach equilibrium

At equilibrium [HA] =0.15-x, [H+] =[A-] =x

Hence x2/(0.15-x)= 4.3*10-6

Looking at the magnitude of Ka, it is reasonable to assume 0.15-x =0.15

Hence x2 =4.3*0.15*10-6 ,x =0.008M. pH= -log [H+] =2.08

Hence [H+] =0.24**(1-0.996)= 0.00096

pH= 3.01

Ka= [H+] [A-]/[HA] = 0.00125*0.00125/ 0.24875 =6.3*10-6

pKa= 5.2

Related Questions

Navigate

• Browse (All)
• Browse P
• Subjects