Hydrates Chemistry Quantitative
 Analysis

Hydrates Even people who have never studied chemistry know that wa‐ ter is represented by the formula H2O. But how are the formu‐ las of chemical compounds established? One method involves determining the relative amount by weight of each element in the compound. For example, let’s say you raid an illegal drug lab and find a white powder that you suspect is cocaine. When you heat 1.00 g of the white powder, it decomposes into 0.40 g of carbon (C, M = 12.01 g/mol) and 0.60 g of water (H2O, M = 18.02 g/mol). The number of moles of carbon in the sample is ⎛ 0.40 g C⎞ ⎛ mol C ⎞ ⎜ ⎟⎜ ⎟ = 0.033 mol C ⎝ ⎠ ⎝ 12.01 g C⎠ and the number of moles of water in the sample is ⎛ 0.60 g H2O⎞ ⎛ mol H2O ⎞ ⎜ ⎟⎜ ⎟ = 0.033 mol H2O ⎝ ⎠ ⎝ 18.02 g H2O⎠ There is one mole of carbon per mole of water in the sample. Thus, the formula of the white powder could be CH2O, or C2H4O2, or C3H6O2, etc. Whatever the case, the white powder cannot possibly be cocaine, whose formula is C17H21NO4; the white powder may be mannose (C6H12O6), a sugar that drug dealers use to “cut” cocaine. In this experiment we determine the formula of a hydrate. Hydrates are salts that incorporate water in their crystal struc‐ ture. Simple hydrates have the generic formula MB·xH2O(s), where M represents a positively charged ion (e.g., Cu2+, Al3+ or Fe3+), B represents a negatively charged ion (e.g., NO3–, SO42–, or PO43–) and x represents the number of moles of water that combine with each mole of MB in the hydrate. Many hydrates are familiar substances, ranging from everyday building mate‐ rials such as gypsum (CaSO4·2H2O(s)) to rare and expensive gemstones like turquoise (CuAl6(PO4)4(OH)8·4H2O(s)). Procedure Quantitative analysis Wash a crucible with detergent and tap water and give it a final rinse with deionized water. Do not allow any water used to wash or rinse the crucible to run down the drain: collect it in a large beaker and, when the beaker is full, transfer its contents to one of the hazardous waste receptacles set up in the hoods. Figure 1‐1 Apparatus for heating a hydrate. The crucible, the Bunsen burner, the tripod and the wire triangle will be extremely hot after the heating period and will stay hot long after the flame is extinguished. Use tongs to handle the crucible; above all, do not touch any part of the appara‐ tus with your bare hands. Position the crucible inside a wire triangle supported by a tripod (see Figure 1‐1). Heat the crucible over a Bunsen burner, let the crucible cool for 5 min and weigh it to two decimal places. Do not attempt to weigh the crucible if it is hot: the reading will be inaccurate and you risk damaging the bal‐ ance. Do not touch the crucible with your bare hands after you have weighed it because the residue left by your fingerprints will change the crucible’s weight. Instead, use tongs to handle the crucible. When moving about the lab, carry the crucible in a beaker to minimize the likelihood of accidentally dropping the crucible. Heat the crucible a second time, let it cool for 5 min and reweigh it. If the two masses agree to within 0.05 g, the cruci‐ ble has attained constant weight and you may continue with the experiment (assume that the lower weight is the crucible’s true weight); otherwise, continue heating, cooling and weigh‐ ing the crucible until constant weight as we have defined it is attained. Once the crucible has attained constant weight, place a sample of hydrate whose mass ranges from 0.75 to 2.50 g in the crucible and record the mass of the crucible and sample to two decimal places. Be sure to record the identity of the hy‐ drate. Heat the crucible and hydrate sample over a Bunsen burner until the dehydration appears to be complete. Discon‐ tinue heating, let the crucible cool for 5 min, then weigh the crucible and its contents to two decimal places. The dehy‐ drated salt may be discarded in a trash can. Repeat the entire procedure using the same hydrate, but this time take a sample of significantly different mass. Yes, you must clean the crucible again with detergent and tap water and heat the crucible to constant weight as before. Find at least four other groups who dehydrated the same substance you did and exchange data with them. Get from them (1) the two masses of hydrate they heated and (2) the two masses of dehydrated salt remaining after heating. You should have a total of ten data items including two of your own. Make sure that the sample sizes of hydrate span the entire range of 0.75 to 2.50 g. Don’t point the test tube you’re heating at yourself or at others: the water may escape from the hydrate violently, ejecting hot material. Do not add water to the test tube while the test tube is still hot: the mixture may boil over and you may burn your hands. Qualitative observations Place copper(II) sulfate hydrate (CuSO4·xH2O(s)), nickel(II) sulfate hydrate (NiSO4·xH2O(s)) and cobalt(II) chloride hy‐ drate (CoCl2·xH2O(s)) in three separate 150‐mm (big) Pyrex test tubes to a depth of no more than 1 cm. Record in your notebook the appearance of each hydrate. Heat each test tube over a Bunsen burner for a few minutes, holding the test tube at an angle and moving it in and out of the flame. Record what happens to each hydrate as you heat it. Discontinue heating, let the test tube cool, then add a few drops of deionized water to each test tube and record your observations. Clean‐up The 150‐mm test tubes are not disposable. Try to rinse out the residue remaining in each test tube by adding a few milliliters of water. Be sure to catch the rinse water in a hazardous waste receptacle: do not dump it down the drain. Place the rinsed test tubes in the test tube tray in the hood. The baked‐on gunk may not wash out: that’s OK, just try to clean up the test tubes as best you can. Data analysis The goal here is to extract the value of x, the number of moles of water per mole of hydrate in MB·xH2O(s). Before x can be Table 1­1 Data collected for the dehydration of strontium chloride hydrate (SrCl2·xH2O(s)) Before heating Run 1 Run 2 Mass of crucible 27.29 g 27.51 g Mass of crucible + hydrate 28.56 g 30.00 g a Mass of hydrate 1.27 g 2.49 g After heating Run 1 Run 2 Mass of crucible + dehydrated salt 28.05 g 28.99 g a Mass of dehydrated salt 0.76 g 1.48 g Mass of water lost 0.51 g 1.01 g a This data item for your hydrate should be exchanged with at least four other students that dehydrated the same substance that you used. evaluated, we must calculate (1) the number of moles of dehy‐ drated salt remaining after heating and (2) the number of moles of water driven off during heating. Table 1‐1 shows sample raw data relating to the dehydra‐ tion of strontium(II) chloride hydrate (SrCl2·xH2O(s)). By con‐ sulting the periodic table, we compute the molar mass of SrCl2 (M = 158.52 g/mol) and that of water (M = 18.02 g/mol). If we assume that all the water is driven off during heating, that is, if we assume that the reaction SrCl2·xH2O(s) → SrCl2(s) + x H2O(g) goes to completion, the number of moles of dehydrated salt remaining after heating in Run 1 is found as follows: ⎛ 0.76 g salt ⎞ ⎛ mol salt ⎞ ⎜ ⎟⎜ ⎟ = 0.0048 mol salt ⎝ ⎠ ⎝ 158.52 g salt ⎠ Likewise, the number of moles of water that are driven off dur‐ ing the heating process in Run 1 is given by ⎛ 0.51 g H2O⎞ ⎛ mol H2O ⎞ ⎜ ⎟⎜ ⎟ = 0.028 mol H2O ⎝ ⎠ ⎝ 18.02 g H2O⎠ Carry out calculations of this sort on your own data and on the data gathered from other students, then plot all the data, plotting the moles of dehydrated salt remaining after heating on the horizontal axis and the moles of water lost on the verti‐ cal axis; include the origin {0,0} as a data point. Determine the slope m and the y‐intercept b of the best straight line through the data by the method of linear least‐ squares given in Appendix A “Statistical Treatment of Data” of this lab manual; include the origin {0,0} as a data pair in the calculations. Draw the least‐squares line through the data. Also calculate the standard error of estimate σm in the slope. Ap‐ pendix A gives directions for calculating m, b and σm using a calculator and using the Microsoft® Excel® spreadsheet appli‐ cation. You may use either method, but Excel® makes the task easier and faster. The value of x in the hydrate MB·xH2O(s) corresponds to whatever integer(s) lie within the interval m – σm and m + σm. On your plot, be sure to draw the two lines y = (m – σm)x + b and y = (m + σm)x + b that define the uncertainty inherent in the data. Your plot should resemble Figure 1‐2, especially in terms of clarity. Figure 1‐2 Determination of the number of moles of water per mole of SrCl2 in SrCl2∙xH2O(s). The slope m of the line determined by the method of least squares is 6.1 mol H2O/mol SrCl2 (solid line); the standard error of estimate σm in the slope is ±0.5 mol H2O/mol SrCl2 (dashed lines defining the uncertainty in the slope of the least‐squares line). Thus, the data indicate that x in SrCl2∙xH2O(s) lies between 6.1 – 0.5 = 5.6 and 6.1 + 0.5 = 6.6, that is, the data suggest the formula SrCl2∙6H2O(s).