Hydrates are compounds with water molecules present in the crystal structure. You will be given an unknown metal sulfate hydrate (MSH) and you need to determine its identity using quantitative methods. The previous lab was qualitative in nature, meaning that there were no measurements to take or numerical data to record. This lab, however, requires careful attention to measurements and the collection of numerical data. In order to determine the identity of your MSH, you are going to determine the percent mass of water and the percent mass of sulfate in your sample. Then, you can compare your data to a list of the possible hydrates used and make an identification.
You may remember copper(II) sulfate pentahydrate from your Regents Chemistry lab. The hydrated form of this compound is bright blue in color. However, when it has been dehydrated, it is a white solid. Dehydration of a hydrate can be achieved simply by heating the substance. When heating a hydrate, a crucible is used. Crucibles are small porcelain vessels that can withstand very high temperatures. Heat the MSH gently at first to avoid splattering as the water escapes. After the MSH has been heated for a while, it can be heated more aggressively to ensure that it is dry. Use differences in mass to calculate the percent mass of water present in your MSH.
Determining the percent mass of sulfate in the MSH is more involved. You will make an aqueous solution of MSH and react it with an aqueous solution of barium chloride. A precipitate will form when the two solutions are combined. The precipitate will be collected, dried and weighed. Based on the mass of the precipitate collected and the percentage of the precipitate that is made up of sulfate (sulfate is an ion and cannot be isolated), the percent mass of sulfate present in the MSH can be calculated.
Once you know the percent mass of water and sulfate in your MSH you can determine the identity of your sample. You will need to calculate your percent error for water and sulfate as compared to the accepted values.