Tonicity and Osmoticity of saline with weak acid

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uzi kiko
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Hi all

I got an exercise:
Cells were floated in a mass in a solution with 150mM of NaCl (isotonic conditions). A weak acid was added to the solution with a pK of 7.0 and concentration of 50mM in the solution.
What is the tonicity and osmoticity of the new solution if PH=4

I know that I should use the following equation:
log([A-]/[HA]) = PH - pKa

But It seem that I am getting the wrong result.
Can somebody help?
 
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The answer in case somebody else will interest:

Need to understand that only the fraction of the drug that is uncharged can diffuse across membranes.
So in case PH=4 and pKa=7 the solution will be acidic so the weak acid remain almost in the same concentration (50mM)

so the solution is isotonic and hyperosmotic