# Chemistry and Logarithms

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#1
Hi, I have two questions that I am stuck on:

1. The pK a of HEPES buffer at room temperature is 7.5. What is the acid dissociation constant K a?
a)18.7 nM-1
b)18.7 nM
c)31.6 nM-1
d)31.6 nM

I think it is d) 31.6, but not sure.

2. The radioactivity of an isotope R decays exponentially with time according to R = R0exp(-0.35t) where t is the time in days and R0 is the radioactivity at zero time.
What fraction of the initial radioactivity will remain after 6 days?

a)12.2%
b)8.3%
c)6.1%
d)10.5%

Don't even know where to start on this one

Thanks
Last edited by izaakha; 9 months ago
0
9 months ago
#2
1. d) is right
2. R/R0 = exp(-0.35t) is the fraction of initial radioactivity at time t. You are given t, so just sub in.
0
#3
(Original post by lordaxil)
1. d) is right
2. R/R0 = exp(-0.35t) is the fraction of initial radioactivity at time t. You are given t, so just sub in.
Sorry, I don't understand. I've never done exponentials before. I know t=6. (-0.35x6)= 2.1 x e = -5.708 ??? Am I even on the right lines.
0
#4
(Original post by lordaxil)
1. d) is right
2. R/R0 = exp(-0.35t) is the fraction of initial radioactivity at time t. You are given t, so just sub in.
I think I have done it, 12.2?
0
9 months ago
#5
yes, well done.
0
9 months ago
#6
How did you work it out?
0
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