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Empirical Hydration Numbers

Determination of the Hydration Number of CuSO4 by Empirical Methods

The lab was designed to find the hydration number of copper II sulfate. To do this, we will calculate the mass percentage of water in the hydrous salt. Our procedure started with the hydrated salt and subjected it to heat to drive off the water. Our result was a 36.31% mass of water which translates into CuSO4 5.052 H2O with a 0.32% error.

Introduction

Empirical determination of various substances is important for research on new compounds as well as identifying an unknown compound. Determination of formulas can vary widely. Most methods rely on a reaction to convert the unknown substances to simpler known substances. The goal here is to find out how much of each element you have in moles, so simpler known substances are good. You can mass the decomposition products and find how many moles they have of each element. By knowing what you added to the reactant side, you can find out how many moles of each element were in the unknown compound. Since the ratio of atoms must stay the same for a compound, by examining the ratios of moles of each element, you can find an empirical formula for the compound. Further refinement can be done by determining its molecular mass and making sure that the empirical formula agrees with that number.

Our experiment involved finding the hydration number of CuSO4. This just involved removing the hydrated water to create an anhydrous form of CuSO4 which then could be massed. Thus we could find out how much water left the sample. We hypothesized that water would make up 36% of the mass of the salt.

Materials and Methods

Using a Bunsen burner and a dry Pasteur pipette, we heated the sample for several minutes and then let it cool. After massing it, we repeated the procedure until we could be relatively sure that most or all of the hydrate water had been removed. We made sure of this by trying to burn a second time then making sure that the masses were equal. Our group did this a second time to ensure that any significant random errors would be at least contradicted.

This lab did not have any extreme health hazards which would warrant protection in addition to our goggles and aprons. The CuSO4 was a dust hazard but was relatively stable. The lab dealt with hot equipment. Standard precautions and checks were taken to make sure equipment was cool before handling. The Bunsen burners were also a hazard. These were shut off when not in use to minimize risks.

The first sample, during handling between the two burns showed a 0.0022g loss of material. The second burn data thus was not analyzed. It was further noted that the glass of the dry pipette did bend a bit. Fortunately no melting occurred.

For some fun, we went for a third time using a watch glass, soap, and a kitchen appliance. The soap tended to poof up when heated.

Results


Sample           Empty Mass of    Loaded Mass    Mass After First  Mass after       
                 the Container                   Burn              Second Burn      

A-CuSO4          2.8577g          3.0200g        2.9595g                            

A2-small                                         2.9573g           2.9579g          
leakage                                                                             

B-CuSO4          3.1891g          3.5755g        3.4359g           3.4352g          

C-Soap           35.0860g         38.7153g       38.0825g          38.0725g         



By subtraction (documented in the Questions and Calculations Section) we can easily obtain the mass of the of the sample and the water lost through the two burns. We can now compute the percent of water in the salt. Through division and the molar mass, we can further find out the moles of the sample and the water lost. Further knowledge of the established formula for anhydrous copper II sulfate, allowed us to find the number of moles of the anhydrous salt. Now we were able to find the mole ratio between water and the anhydrous salt, thus finding the hydration number.

Our results came out with good precision. The percent water in sample A was 37.28% water by mass with a 3.5% error. It contained only 0.1018g of the anhydrous copper II sulfate which may explain the error because it is difficult to measure such a small amount of salt accurately. The hydration number for this sample was 5.266 . Sample B was better and had a 36.31% water by mass with a 0.8611% error. It had a 5.052 hydration number. This error can result just from the randomness of the environment.

The hydrate blue salt, during heating, would bubble and turn brown briefly. It would change to white and with further heat would temporarily take a greenish tint. The green tint would disappear after cooling.

Discussion

We found that copper II sulfate in its hydrous form takes 5 molecules of water. Moreover, the data reveals that the hydrous salt is roughly 36.31% water by mass. Our results closely matches the accepted values of the hydrate of copper II sulfate. Our data affirms our hypothesis that the hydrous copper II sulfate salt contains 36% water by mass.

The slight errors present in the experiment are not major. They could result from the presence of some water or finger oils on the walls of the pipette. The tinting that occurred during heating could possibly be the oxidation of copper because the ambient energy becomes high enough to allow the bonding. Because the temperature is not hot enough, this conjectured reaction would not be permanent and upon cooling, would return to the white anhydrous copper II sulfate salt.

Our data had a weird little glitch where sample A gained 0.0006g after a second burn. To check for subtle losses during the experiment you go back with the empirical formula (CuSO4 5H2O) and determine the number of moles that you started with. This should match up with the moles of anhydrous CuSO4 when you finish. However, for sample B, we see a loss of 6 10-6 mol (about 0.3% error).

The methods used could easily be applied to other salt hydrates such as the additional analysis of soap (C18H40O23H2O, empirically determined). Applications of this lab could be the use of this method to determine hydration and then accurately create stock solution of CuSO4.

Questions and Calculations

  1. SAMPLE A:
    Before: 3.0200g-2.8577g=0.1623g
    After 1st burn: 2.9595g-2.8577g=0.1018g (constant, second burn sample unusable due to material attrition during handling)
    SAMPLE B:
    Before: 3.5755g-3.1897g=0.3864g
    After 1st burn: 3.4359g-3.1897g=0.2468g
    After 2nd burn: 3.4352g-3.1897g=0.2461g (constant)
  2. SAMPLE A:
    Mass: 0.0605g / 0.1623 g = 37.28%
    Moles: 0.003361mol
    SAMPLE B:
    Mass: 0.1403g / 0.3864 g = 36.31%
    Moles: 0.007794mol
  3. Result: CuSO45H2O => hydration number
  4. The formulas are the same. The fractional difference in hydration number is just random fuzz and noise in the data collecting instruments and handling.