EXPERIMENT 7: DETERMINATION OF THE Ks OF A SLIGHTLY SOLUBLE COMPOUND Date: Name:

Question

EXPERIMENT 7: DETERMINATION OF THE Ks OF A SLIGHTLY SOLUBLE COMPOUND Date: Name: Instructor Post-Lab Questions (Turn in to your Instructor with the post laboratory report for this experiment.) 1. The literature value for the K, of Ca(OH), at 25 'C is 4.68 x 10*. How does the value you determined in the lab compare with this result? Explain what may have caused your result to differ from this value. (Clearly explain what may have happened. Should you have obtained the literature value? Clearly explain how this could have happened.) How would your results have changed if there had been a trace of basic (hydroxide containing) impurity present in the Erlenmeyer flasks you used? 2. eerimest 7 | Determination of theofa Slightly Seluble Compounte

Explanation / Answer

I need your data and the experimental procedure to answer question 1. I will answer question 2, since it relates to the general concept of equilibrium constant.

2) Ca(OH)2 undergoes ionizations as below.

Ca(OH)2(s) <======> Ca2+(aq) + 2 OH-(aq)

The solubility product constant, Ksp is expressed as

Ksp = [Ca2+][OH-]2

Due to the 1:1 nature of dissociation as expressed by the equation, the solubility of Ca(OH)2 is equal to the solubility of Ca2+.

When the Erlenmeyer flask contains traces of OH-, the concentration of OH-, i.e, [OH-] is increased. Ksp is a thermodynamic equilibrium constant and must stay constant at a constant temperature. Therefore, to keep Ksp constant, [Ca2+] must decrease. Therefore, the solubility of Ca(OH)2 will decrease when the flask has traces of OH-.

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