a) If the beaker was heated too vigorously and solution splattered out, the mass percent of the NaCl (salt) would be less than the actual because some of it was lost when it splattered out of the beaker. .
b) If the recovered sand wasn't completely dry it would still contain some of the solution that was removed from it and would therefore cause a higher mass content.
c) There wasn't a specified amount of HCL to use but you needed to use enough so that all of the calcium chloride is removed from the seashell and sand mixture. If you only used half of the recommended amount, then more than likely you didn't remove all of the calcium chloride from the mixture and will therefore have a higher % of sand and lower % of seashells than their actual mass percent. .
d) Once again, there wasn't a specified amount of potassium carbonate to use but you needed to use enough so that a precipitate was formed and if you used double the recommended volume, you definitely made sure a precipitate was formed. Therefore, the actual mass percent more than likely stayed the same because all of the excess potassium carbonate would have been filtrated out of the solution. .
3. Since benzoic acid is the only solid soluble in cold water, you should dissolve the mixture in cold water and then filtrate the remaining mixture through gravity filtration. Now, you should use a hot plate to evaporate the water from the benzoic acid solution. Let the benzoic acid solution cool to room temperature and then weigh it. Next, you can separate the sodium sulfate from the mixture by dissolving it in hot water. When it's completely dissolved, use gravity filtration to separate the two and then evaporate the water from the sodium sulfate mixture using a hot plate. Finally, you can weigh the sodium sulfate solution. To separate the remaining two solids, use suction filtration and add 3M HCL to the mixture until the reaction is complete and you will have a remaining solid of zinc hydroxide that you will need to let dry before weighing and a liquid solution of HCL and magnesium hydroxide.