molar solubility, help!

[wait4me]

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Why do we solve these two problems differently? Is there something basic that I am not getting?

"What is the concentration of I ions in .2Msoln magnesium Iodide (MgI2)
You can just do .2*2=.4M since there are 2 molec I

"What is the concentration of F ions in 2M soln HF?? (ka=6.8*10^-4)

Here Ka= [F][H]/[HF]

So you would do X^2/2=Ka and solve for X. Why couldnt this one be done the same way as the first problem and ignore the Ka? what is the difference? I never know which way to go about these problems!

 

[wait4me]

is it because we assume Ka=1 in the first question? Help!

 

[inaccensa]

Why do we solve these two problems differently? Is there something basic that I am not getting?

"What is the concentration of I ions in .2Msoln magnesium Iodide (MgI2)
You can just do .2*2=.4M since there are 2 molec I

"What is the concentration of F ions in 2M soln HF?? (ka=6.8*10^-4)

Here Ka= [F][H]/[HF]

So you would do X^2/2=Ka and solve for X. Why couldnt this one be done the same way as the first problem and ignore the Ka? what is the difference? I never know which way to go about these problems!

You are doing 2 different things here. What is MgI2? Its a salt which is present in solid state. You are dealing with solubility as in

Ksp = (Mg2+) (2I-)^2

Now HF is a weak acid (not solid) and thus you use Ka = (H+)(F-)/(HF)

Since you ask why we do this, the answer boils down to the basics here. Solids are never used in such equations.