Common ion in solution

[sergey_le]

Summary:: finding concetrations

[Thread moved from the technical forums]

i did a lab experiment and I'm a little confused about what i should do.

i got a solution of CaCl(2) 1.5 gr and Ca(OH)2 5gr in 100ml that i prepared 2 weeks ahead
. after filttering the solution i got a saturated solution of CaCl(2) +Ca(OH)2.i tested the pH of it.(it is 12.43)

from the Ph value i got that OH concetration is 0.027M. the next quetion i need to answer is whet is the concentration of Ca. so i was seeing a lab report from a year ago, and to calculate the Ca consentration they first used the OH saturation (1:2) ratio. so 0.027/2=0.0135M. after that they calculated the amount of Ca from the CaCl(2) salt (1.5gr/110.98(gr/mol)=0.0137m/0.1l) which is 0.137M. they concluded that the ca consentration is 0.137+0.0135M.

i don't understand why. we calculate the consentration for a saturated solution, so there are no Ca ions from the CaCl(2) just floating and reacting...they already reacted and the axcess already sunk and turned to a solid, no?

 

[Borek]

Nope, doesn't look like a correct approach to me.

Hint: what is the Ksp of Ca(OH)₂?

 

[sergey_le]

Nope, doesn't look like a correct approach to me.

Hint: what is the Ksp of Ca(OH)₂?

my approach is wrong or the one in the lasts year lab report?

 

[Borek]

Sorry, report approach looks wrong.

Technically you just expressed your concerns but never stated what would be your approach to calculations