Asked by kate-------------------------------------

reposted again

a 275mL sample of vapor in equilibrium with 1-propylamine at 25C is removed and dissolved in 0.500L of H20. For 1-propalamine, Kb=3.72*10^-4 and the vapor pressure at 25C is 215 torr.
(a)What should be the pH of the aqueous solution?
(b) How many mg of NaOH dissolved in 0.500L of water give the same pH


how do i now which values to use for part b? for a i got pH of 11.2
Answered by kate-------------------------------------
its 316 not 215 torr
Answered by DrBob222
pH = 11.2. Convert to pOH.
pH + pOH = pKw = 14, then convert pOH to (OH^-) and that will be in moles/L.
For 0.5 L you will have half that number of moles. grams = moles x molar mass
Then convert grams to mg.
Answered by kate-----------------------------------
will it be 3.17*10^-3mg?
Answered by DrBob222
I find those digits but I have a different position for the decimal. If I didn't goof it is 31.7 mg.
Answered by kate-----------------------------------
so the correct answer is 31.7mg? I don't see what i did wrong. Could You show me the steps please?
Answered by DrBob222
If you will post you work I will look for the error.
Answered by Avogadro
pH=11.23
[H+]= 10^(-11.23)=5.888*10^-12
(1*10^-14)/(5.888*10^-12)=[OH]=.001698M/L
.001698/2= .0008491 moles/.5L

.0008491*40(molar mass of NaOH)=.033964*1000= 33.96mg
Answered by DrBob222
I agree with 33.96 when using 11.23 for pH but I would round it to 34.0 mg to three s.f.. My answer of 31.7 mg is correct using 11.2 for part a.
There are no AI answers yet. The ability to request AI answers is coming soon!