Lab 4 Formula of a Hydrate Online

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Feb 20, 2024

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1 Formula of a Hydrate Lab Online Name: __________________________ Please, do not forget to print or type your name out. Thanks Many compounds, particularly ionic compounds, react with water to produce compounds called hydrates. An excellent example is nickel(II) sulfate hexahydrate NiSO 4 · 6H 2 O. Because the nickel +2 cation Ni 2+ has lost its two valence electrons, each valence electron to two of the four oxygen atoms in the −2 sulfate anion SO 4 2− , a total of six water molecules will form six coordinate covalent bonds to the nickel +2 cation (Figure 1). This occurs not only because of the one of the two lone pairs of valence electrons on each oxygen atom in the water molecule can fill the empty valence electron shell of a Ni 2+ cation, but also because water is a polar molecule due to the unequal sharing of valence electrons in both single bonds of the water molecule. Oxygen being more electronegative than hydrogen, the oxygen atom in any water is molecule is slightly negative and therefore attracted to positively charged cations. Figure 1 Each oxygen atom in the six water molecules hydrating the Ni 2+ cation has formed a coordinate single covalent bond to the Ni 2+ cation. The oxygen atom of each water molecule places one of its two lone pairs of valence electrons into the empty valence electron shell of the Ni 2+ cation. To remove the 6 water molecules from one formula unit of nickel(II) sulfate hexahydrate, the ionic compound has to be heated to temperatures greater than the normal boiling point of water at 100ºC. When one heats a sample of this hydrate to remove the water molecules and obtain anhydrous nickel(II) sulfate there is a change in color (Figure 2), evidence of an endothermic chemical reaction taking place. Δ Heat + NiSO 4 · 6H 2 O (s) → NiSO 4 (s) + 6 H 2 O (g) NiSO 4 · 6H 2 O NiSO 4 Figure 2 Nickel(II) sulfate hexahydrate is a light blue and the anhydrous nickel(II) sulfate is nearly white or a cream color.

2 In this experiment, we will be heating Epsom salt which is hydrated magnesium sulfate MgSO 4 . Unlike nickel(II) sulfate hexahydrate, when Epsom salt is heated to temperatures above the boiling point of water there will be no change in color. Both anhydrous magnesium sulfate and hydrated magnesium sulfate are white or colorless. What does take place is a change in the texture of Epsom salt when it is dehydrated by heat. Epsom salt is comprised of white crystals that are semi-transparent to light. Anhydrous magnesium sulfate, on the other hand, is a fine white powder of very small crystals that are not transparent to light at all. Procedure First, measure the mass of a ceramic or porcelain evaporation dish (Figure 3) on the top loading balance that has initially been tared to 0.0 grams. In this experiment it is necessary to know the mass of the evaporation dish being used to dehydrate Epsom salt at high temperatures. Measure the mass of the evaporation dish and write it down where it is to be recorded in this report at A on page 4. The mass of the evaporation dish should be between 40.0 and 45.0 grams. No two evaporation dishes will have the exact same mass. Figure 3 Ceramic or porcelain evaporation dish used to dehydrate Epsom salt and the top loading balance Then with the evaporation dish on the top loading balance using a scoopula (Figure 4) add 3.0 grams of Epsom salt to the evaporation dish and record this value at B on page 4. If your evaporation dish has a mass of 43.0 grams, then while still weighing the empty evaporation dish on the top loading balance, then add Epsom salt to the evaporation dish until the mass is 46.0 grams. 46.0 grams – 43.0 grams = 3.0 grams. Record the mass of Epsom salt added on the data sheet at C on page 4. Then place the evaporation dish, containing the 3.0 grams of Epsom salt, on the center of the hotplate (Figure 4) and turn to maximum heat. It will take about 5 minutes before the hotplate attains maximum temperature, and so one will need to leave their sample on the hotplate to heat for about 25 minutes total. Five minutes for the hotplate to reach maximum heat and another 20 minutes for the 3.0 grams of Epsom salt to become completely anhydrous. Figure 4 The scoopula for obtaining 3.0 grams of Epsom salt and the hotplate for heating

3 After 25 minutes of constant heating, turn off the hotplate and then unplug the hotplate to be sure it is off. Avoid touching the hotplate surface for about 10 minutes. It will take more than 10 minutes for the very hot surface of the hotplate to cool down to room temperatures after heating the sample of Epsom salt. Then using crucible tongs (Figure 5) carefully remove the hot evaporation dish containing anhydrous magnesium sulfate and place the dish on square wire gauze (Figure 5) resting on the lab tabletop. Do not let the hot evaporation dish touch the top of the lab table. The lab tabletop is usually made of linoleum, which will melt and possibly catch fire when in contact with the porcelain dish being heated at maximum heat on the hotplate. Figure 5 Crucible tongs used to remove hot evaporation dish from the hotplate and square wire gauze for the evaporation dish to rest on why slowly cooling to room temperatures. After the anhydrous magnesium sulfate and the evaporation dish have cooled down to room temperatures, weigh them again on the top loading balance and record this new mass measurement on the lab handout data table at D on page. Note that the decrease in mass should be 1.5 grams, since about 50% of the mass of Epson salt is water and the other 50% mass is anhydrous magnesium sulfate MgSO 4 . Following the mathematical instructions on the next page, the data page, calculate the number moles of water per one mole of magnesium sulfate in Epsom salt, or the number of water molecules per one formula unit of magnesium sulfate.

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4 Data Sheet Replace question marks with calculated answers or write them near the question marks. See similar experiment performed online at https://www.youtube.com/watch?v=E1EZUIdmfg8 A Mass of evaporating dish is 45.0 grams B Mass of evaporating dish and hydrated salt is 48.0 grams C Mass of salt (hydrate) = B – A = 48.0 g – 45.0 g =? grams (2 significant figures) D Mass of evaporating dish and anhydrous salt after heating is 46.5 grams E Mass of salt (anhydrate) = D – A = 46.5 g – 45.0 g = ? grams (2 significant figures) F Mass of water lost = C – E = 3.0 g – 1.5 g = ? grams (2 significant figures) % water mass = F C × 100% = 1.5 3.0 × 100% = ? % ( 2 significant figures ) G moles water = F× 1 mole H 2 O 18.0 g H 2 O = 1.5 g H 2 O × 1 mole H 2 O 18.0 g H 2 O = ? ( 2 signficant figures ) H moles salt (anhydrate) = E× 1 mole MgSO 4 120.4 g MgSO 4 = 1.5 g MgSO 4 × 1 mole MgSO 4 120.4 g MgSO 4 = ? ( 2 sig. fig. ) Ratio of moles of water to moles of hydrate = G H = 0.083 0.012 = ? ( 2 significant figures ) Use nearest integer number below for G divided by H. Gmole H 2 O H mole MgSO 4 = Number of moles H 2 O 1 mole MgS O 4 = 0.083 0.012 = ? Hydrate Formula  MgSO 4 · ? H 2 O

5 Post Lab Questions 1. Describe copper(II) sulfate pentahydrate CuSO 4 · 5H 2 O before heating and after heating (anhydrous). Use Google Search to find answer. Type out answered color next to question mark. Δ Heat + CuSO 4 · 5H 2 O (s) → CuSO 4 (s) + 5 H 2 O (g) Color of CuSO 4 · 5H 2 O? Color of anhydrous CuSO 4 ? When performing Google Search, type in copper(II) sulfate pentahydrate to find the color of the hydrated ionic compound CuSO 4 · 5H 2 O. Type in anhydrous copper(II) sulfate on Google Search to find the color of the anhydrate salt CuSO 4 . Changed to bold print, circle with pen or pencil, or highlight in yellow for your answers in the following multiple-choice question. 2. Could you tell by looking at a sample of MgSO 4 if it was hydrated or anhydrous? Explain. a) No, since there is no change in color. b) Yes, due to the change in the texture. Note that the anhydrous form is fine white powder and the hydrated form is white crystalline. There is a change in the texture and that holds true for all compounds that have different colors for the anhydrous and hydrated forms. Google search on the Internet will be useful.

6 Pre-Lab Questions Changed to bold print, circle with pen or pencil, or highlight in yellow for your answers in the following multiple-choice question. 1. If you place a watch glass upside down on top of a sample of a hydrate, what would you expect to see when you heated the hydrate? a) Condensation of steam b) Formation of water droplets on the watch glass c) Both of these 2. Calculate the % concentration of water in CoCl 2 · 4H 2 O. Replace question marks with calculated answers or write them near the question marks. Using atomic weights rounded off to 3 significant figures obtained from the Periodic Table 1 Co = 58.9, 2 Cl = 2×35.5 = 71.0, and 4 H 2 O = 4×(2×1.0 + 16.0) = 72.0 Molar massof CoC l 2 · 4 H 2 O = 58.9 + 71.0 + 72.0 = ? grams 1 mole ( 4 sig .fig. ) %mass H 2 O = 72.0 201.9 × 100% = ? % ( 3 significant figures ) 3. What color (look up on web, book, etc.) is CoCl 2 · 6H 2 O? What is the formula and color of the anhydrate? Use Google Search to find color to type next to question mark. Δ Heat + CoCl 2 · 6H 2 O (s) → CoCl 2 (s) + 6 H 2 O (g) Color of CoCl 2 · 6H 2 O? Color of CoCl 2 ? Type in cobalt(II) chloride hexahydrate to find the color of CoCl 2 · 6H 2 O in Google Search

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7 Type in anhydrous cobalt(II) chloride to find the color of anhydrate CoCl 2 Pre-Lab Questions Continued Replace question marks with calculated answers or write them near the question marks. 4. A student heats a 3.16 g sample of NiCl 2 · ?H 2 0. After heating, the anhydrous salt weighs 1.73 g. a) How many moles of anhydrous salt remained after heating? Using atomic weights rounded off to 3 significant figures obtained from the Periodic Table 1 Ni = 58.7 and 2 Cl = 2×35.5 = 71.0 Molar massof NiC l 2 = 58.7 + 71.0 = ? grams 1 mole ( 4 sig .fig. ) Moleof NiCl 2 = 1.73 gNiC l 2 × 1 mole NiCl 2 129.7 g NiCl 2 = ?mole NiCl 2 ( 3 sig. fig. ) b) How many moles of water did the compound lose? Grams of H 2 O = 3.16 g − 1.73 g = ? g H 2 O ( 3 sig .fig. ) Moleof H 2 O = 1.43 g H 2 O× 1 mole H 2 O 18.0 g H 2 O = ? mole H 2 O ( 3 sig .fig. ) c) How many waters of hydration does the compound have? (give both calculated value and whole number value) 0.0794 mole H 2 O 0.0133 mole NiCl 2 = ? ( 3 significant figures ) Round off the above number to nearest integer is ?