A hydrate is a compound that contains water. After the water is removed, the solid that remains is called the anhydrous salt, or anhydrate for short. Our goal for this lab was to first determine the percent of water in the compound, then find out the molar ratio of water to anhydrous salt, and finally identify the unknown hydrate. It is necessary that the dish is heated before finding its mass because compounds are formed in heats and large amounts of water are released. The dish must be cooled before measuring its mass so a) you don’t burn yourself, and b) so you can get an accurate determination of the compounds mass.
MATERIALS AND METHODS
In order to conduct this lab, many materials were used. The items used were chemical splash goggles and an apron for safety purposes, a pipe-stem triangle, a ring stand, an iron ring, a lab burner, marches, a crucible with a cover so the water can evaporate, crucible tongs to not burn ourselves, a heat-resistant pad to put the crucible on when it was hot, a laboratory balance, a spatula, and an unknown hydrated salt. The instructions instructed to wear our goggles at all times during the lab and the crucible was to be moved only with the tongs because it was hot.
In order to conduct this lab, there are many procedures to follow. First and foremost, put your goggles and lab apron on. The burner is then lit and a dry, clean crucible with its cover on is added the hottest part of the flame for 3 minutes. After the time elapses, remove the crucible and place it on a heat-resistant pad. Then measure and record the mass of the cool crucible. After this is completed, add 2 grams of the unknown hydrate to the crucible. Then measure and record the mass of the crucible, cover, and hydrate and place it back on to the burner with the cover tilted slightly so water vapor can escape. The unknown solution should pop and spatter; once it stops, remove it from the heat. If the edges of the solution inside appear to be turning brown, turn the heat higher and hear the solution again slower this time for five more minutes. After letting it cool for one minute, measure and record the mass of the crucible. Reheat everything, let it cool, and record the mass again. The mass should be within 0.02 g of the last recorded mass, if it isn’t, remeasure and record the mass again. After performing these procedures, dispose the product and clean up the area and wash hands before leaving.
Upon conducting this experiment, many trials were performed in order to get the correct results. Some mistakes that were made could have been a) the cover of the crucible wasn’t tilted enough to let water vapor escape, or b) the heat on the burner wasn’t high enough.
Before the unknown hydrate in the experiment was heated, it looked like small pebbles of some sort of white rock. After it was heated, the substance didn’t look that much different, but you could tell that something had changed slightly. It then resembled a more fine grain appearance and was duller looking. By looking at the data, it is shown that .33 grams of water evaporated out of the crucible when it was heated. Once the molar mass of the salt is found, the percentage of water in the hydrate can be found by observing the data on this table.
From observing the molar masses, salt masses, and percentages of water in the unknown hydrates, I am able to figure out that the unknown hydrate was CuSO4 5 H2O. By dividing the grams of water lost from the crucible by the weight of the hydrate, I can see the percentage of the hydrate that was made up of water. Because .33 is closest to 36.1%, it is concluded that my unknown hydrate was copper (II) sulfate pentahydrate.
It is easy to identify a hydrate by heating it and comparing it to a list of hydrates. Certain hydrates contain percentages of water that compare with their total weight. When the hydrates are heated, you can find this total percentage by comparing the mass before you heat it to the mass after you heat it.
Even though the results to this experiment did not specifically identify which unknown hydrate was used, it was clear as to which one was given to use for this experiment. Although my results came out as 33% and CuSO4 5 H2O was 36.1%, it was evident that copper (II) sulfate pentahydrate was the unknown hydrate.
The little error that was made could have happened because of residue that was left in the crucible. The residue that was left over could have affected the weight of the hydrate and affected the data. By conducting this lab, I now have even further knowledge about hydrates and anhydrates. It was very interesting to conduct and see how hydrates work.
1. Water in a hydrate lab handout
2. Notes taken in class