Chemistry Please confirm my Activity Coefficient Calculations

[Yokoko]

Homework Statement

I've had a go at these but am unsure about the result. I thought the values would have to be closer to each other.

Relevant Equations

See picture

 

[mjc123]

You have not calculated I correctly. I (not √I) = 1/2∑cz² for ALL ions present.
Then you haven't used your calculated value for I in the equation for logγ. You used c. Why?
(Do it correctly, and it does come out close to the experimental value.)

 

[Yokoko]

Thanks for your answer. Again, I'm not sure if I understood this correctly.

I used c as I thought s = c for simple ions. I now took the root of √I to get I.

I assumed the C for KNO3 = 0.0050 and C for CaSO4 = s = 0.0154

This might be where I am wrong?

 

[mjc123]

No! 1/2Σcz² is I, not √I, as I said before. (What are the units?) I = 0.0666M.

 

[Yokoko]

Oops thanks, I misread your earlier message.
With I = 0.0666 I get an overall value of 0.41 which fits.

Thanks again!