<u>Answer:</u>
<em>A. 10.25</em>
<em></em>
<u>Explanation:</u>
Pkb =4.77
So pka = 14 - pka = 9.23


Initial 0.50M 0 0
Change -x +x +x
Equilibrium 0.50M-x +x +x


(-x is neglected) so we get

![pH=-log[H^3 O^+]\\\\pH=-log[1.72\times10^{-5}]\\\\pH=4.76](https://tex.z-dn.net/?f=pH%3D-log%5BH%5E3%20O%5E%2B%5D%5C%5C%5C%5CpH%3D-log%5B1.72%5Ctimes10%5E%7B-5%7D%5D%5C%5C%5C%5CpH%3D4.76)
pOH = 14 - pH
= 14 - 4.76
pOH = 9.24 is the answer
Option A - 10.25 is the answer which is close to 9.24
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A solution is a homogenous mixture meaning it has a uniform appearance throughout. (example: salt water)
heterogeneous mixture is visibly composed of different things. (example: chicken noodle soup)
<u>Answer:</u> The mass of sample A after given time is 99.05 g.
<u>Explanation:</u>
All the radioactive reactions follows first order kinetics.
The equation used to calculate half life for first order kinetics:

We are given:

Putting values in above equation, we get:

Rate law expression for first order kinetics is given by the equation:
![k=\frac{2.303}{t}\log\frac{[A_o]}{[A]}](https://tex.z-dn.net/?f=k%3D%5Cfrac%7B2.303%7D%7Bt%7D%5Clog%5Cfrac%7B%5BA_o%5D%7D%7B%5BA%5D%7D)
where,
k = rate constant = 
t = time taken for decay process = 84.2 s
= initial amount of the reactant = 250 g
[A] = amount left after decay process = ?
Putting values in above equation, we get:
![0.011s^{-1}=\frac{2.303}{84.2s}\log\frac{250}{[A]}](https://tex.z-dn.net/?f=0.011s%5E%7B-1%7D%3D%5Cfrac%7B2.303%7D%7B84.2s%7D%5Clog%5Cfrac%7B250%7D%7B%5BA%5D%7D)
![[A]=99.05g](https://tex.z-dn.net/?f=%5BA%5D%3D99.05g)
Hence, the mass of sample A after given time is 99.05 g.