The purpose of the lab and the information reported is to allow one to infer as to what the unidentified substance from the experiment. This lab is useful for teaching students to identify hydrates through finding percentage of mass in terms of H2O.
Introduction
A hydrate is a substance that forms as a reaction that occurs in water solutions. While these compounds appear to be dry, large amounts of water are released when they are heating. These molecules bond to the molecules or ions that make up the compound to form a hydrate. The solid that is left over from the reaction is an anhydrous salt. By comparing the mass before and after heating, the ratio of water to anhydrous salt can be found. Through this, the particular hydrate that was used can be determined.
Materials and Methods
Correct replication of this experiment requires following precise instructions and obtaining multiple materials. This experiment was performed in a chemistry lab at DuPont Manual High School on Monday, February 25th from 2:30PM-3:00PM. The first step of this experiment is to make sure you follow all the proper safety precautions. This includes wearing safety goggles and a lab apron. It is also important to tie back loose hair and clothing. These preparations include setting up a Bunsen burner, pipe-stem triangle, ring stand, and iron stand in a rig that will allow for the hydrate to be suspended over the fire in a crucible. The iron ring is attached to the ring stand and the pipe-stem triangle is placed on the iron ring. The burner is then placed below the iron ring so that the crucible can be placed on the ring and be heated by the open flame. Next, light the burner using a standard mechanical lighter and get the flames going. In order to properly weigh the crucible, place it with its cover on over the hottest part of the flame for three minutes. After the heating, the crucible was given an additional three minutes to cool on the heat resistant pad. Always transport the crucible with heat resistant tongs in order to not burn anything. The crucible should then be weighed on the scale to the closest .01 grams and the mass should be recorded on the date table.
Next, place two grams of the hydrate being experimented with in the crucible, and then weigh it to make sure the correct mount is added. The possible hydrates that are distributed include: magnesium sulfate heptahydrate (MgSO4 7H2O), manganese (II) sulfate monohydrate (MnSO4 H2O), copper (II) sulfate pentahydrate (CuSO4 5 H2O), calcium sulfate dihydrate (CaSO4 2 H2O), and sodium carbonate monohydrate (Na2CO3 H2O). With two grams of the hydrate inside the crucible, heat the crucible over the burner with the lid tilted to the side so evaporation of the hydrate to escape. The hydrate needs to be heated until any audible crackling or popping has stopped inside the crucible. Then use the tongs to move the crucible onto the heat-resistant pad and let it cool for one minute. After it has been allowed to cool, weigh the crucible once again on the scale and record the given mass. After this weighing, heat the crucible for around three more minutes, weigh it, and if the mass is unchanged (or very similar) then all of the hydrate has been evaporated and heating is complete.
Results
After completing the information, the identity of the mystery element has been discovered to be Copper (II) sulfate pentahydrate (CuSO4 5H2O) through careful studying of the information listed below.
It is important to note that the mass of the crucible includes the mass of the lid.
Before the substance was heated, it resembled small, blue, crystal-like pebbles. After it was heated, the substance looked more like a finer grain of salt. Said substance was dull and opaque with a starch white color. Through analysis of the data mentioned above, we can see that .74 grams of water evaporated during the first eating. In order to find the percentage of water in the hydrate, the molar mass must be found first.
This data gives all the information that is required to find out what the mystery hydrate was. By dividing the grams of water lost from the crucible by the mass of the hydrate, the percent of the hydrate’s mass that was water can be seen. In this particular instance, 37% of the mass of the hydrate was water. In the chart it can be seen that copper (II) sulfate pentahydrate with 36.1% of water contains the closest percentage to the results of this experiment. Therefore copper (II) sulfate pentahydrate was the mystery substance.
Discussion
A hydrate can easily be identified through heating it and comparing it against a list of hydrates. This is made possible by the fact that individual hydrates contain a certain percentage of water in respect to their total weight, and when heated this percentage can be found by comparing a before-heat mass to a after-heat mass. This process gives chemists a method of determining which hydrate is present in any identifiable reaction. Due to the fact that this before the execution for the lab, the results were as expected. It is possible that improper experimentation occurred which led to the result of 37% instead of the 36.1% that copper (II) sulfate pentahydrate is supposed to contain. While this is a very small difference, it still does not match the theoretical yields of the hydrate, so it is important that that is mentioned. This could have been caused by the presence of residue in the crucible that was not removed in its cleaning. This residue could have somehow inadvertantly affected the data of the experiment. This experiment shows that the heating of a chemical can cause decomposition, as opposed to the synthesis or combustion that is usually seen. It is important to realize that evaporation is a perfect example of decomposition. In the experiment, copper (II) sulfate pentahydrate was exposed to heat, which causes H2O and CuSO4 to separate, or decompose, and resulted in CuSO4 being left at the bottom of the crucible without its H2O counterpart. This lab is very good at expanding one’s knowledge of how a hydrate works.
Resources
1. Water In A Hydrate Lab handout
2. Notes taken in class
