The second one is the way to go.
Na₂S(aq) + Cd(NO₃)₂(aq) = CdS(s) + 2NaNO₃(aq)
v=25.00 mL
c=0.0100 mmol/mL
M(Na₂S)=78.046 mg/mmol
n(Na₂S)=n{Cd(NO₃)₂}=cv
m(Na₂S)=M(Na₂S)n(Na₂S)=M(Na₂S)cv
m(Na₂S)=78.046*0.0100*25.00≈19.5 mg
Answer:

Explanation:
Hello there!
In this case, according to the given information, it turns out firstly necessary to write out the described chemical reaction as shown below:

Now, we set up the expression for the calculation of the standard free energy change, considering the free energy of formation of each species, specially those of H2 and F2 which are both 0 because they are pure elements:

Regards!
Answer:
The reason is because Flagstaff is at a higher elevation than Phoenix.
Explanation:
The air is thinner at higher elevations. You can google Flagstaff's elevation compared to Phoenix but the simple answer is that air is thinner at higher elevations and some people used to 'thicker' air find it harder to breath, especially after some strenuous exercise.