Name number. Section TA. Laboratory Date Experiment 8 Post-Laboratory Questions

Question

Name number. Section TA. Laboratory Date Experiment 8 Post-Laboratory Questions 1. Justify why bromocresol indicator to use in experiment and why it is this superior to Purple is a good for acid-base titrations. phenolphthalein which is used 2. How would the measured solubility of filtrate? effected if undissolved Ca(oH was allowed to pass into the some The laboratory ran out magnesium nitrate (MgoNO,2), and your partner suggests of MgoCH,Co.)) in steps IV.1-2 to investigate the acetate 3. substituting or why not? uncommon ion effect. Do you think this is a good suggestion? Why 4. Would steps IIL1-2 with a filtered saturated solution of magnesium hydroxide (MgoH)2) substituted for Ca(oH02 be a good approach for determining the K p of Mg(OHr? Why or why not? 5. Use the measured solubility increase of Ca(oh)2 in the presence of Mg(NO3)2 (salt effect) to correct the measured solubility decrease of Ca(oH)2 in the presence ofca(NO3)2 (common ion effect) for the general salt effect which is also present.

Explanation / Answer

Q1.

Bromocresol purple range --> 5.2 - 6.8

Phenolphthalein range --> 8.2 - 10.0

If this is a weak base, then we need to use a Bromocresol purple indicator, since pH is acidic... whereas phenolphtalein is for a final basic species

Q2.

if Ca(OH)2 is not compeltely dissolved, then the mass is not well accounted, not all the sample will be in solution

When calcualting moles in solution, they will be incomplete

So saturation value will be lower

If this is true

then

Molar solubility decreases

which decreases Ksp

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