Percent Composition of a Hydrate

Transcription

1 Chem 110 Lab Percent Composition of a Hydrate Clark College Percent composition will be discussed in your text, lecture and in lab. This concept is often used to determine how many grams of an element might be produced when a compound is decomposed, or how many grams of an element is necessary to produce a given quantity of compound (in grams). By using the mole relationship to get mass/number conversion factors, it is possible to determine the number of moles of water present per mole of copper sulfate. LEARNING OBJECTIVES After completing this experiment, you should feel comfortable with: The identity, location and use of more laboratory equipment. Experimentally determining the mole amounts of species in a compound. Using the electronic balances Synthesizing data to determine the formula of a compound. A. Definition Hydrates are compounds having water incorporated within the crystal structure in specific whole number ratios. The bonds holding the water are weak bonds and are easily broken when heated. BaCl2 2 H 2O BaCl2 + 2 H 2O hydrate solid + water Today you will determine both the percent of water present in a hydrated copper (II) sulfate (CuSO 4 n H 2O) and n, the actual number of moles of water molecules present per mole of CuSO 4. B. Calculating Percent Composition Percent literally means parts per 100. Since we rarely work with exactly 100 items, we usually must calculate what the value would be. This is accomplished quite easily by use of the equation shown below. Part % = x 100 Total To calculate the percent composition (by mass) of each element in a compound: 1. Determine the compound s molar mass (or formula mass) by adding up the atomic masses of each element and multiplying that mass by the number of atoms of that element. Use the periodic table for the atomic mass. 2. Divide the total mass of each element by the compound s total mass. 3. Multiply that fractional mass by 100. Percent Composition of a Hydrate F09 AEM Page 1 of 7

2 Example I: Find the mass percent of oxygen in one mole of H 3PO 4. Solution: Mass H : 3 mole H Mass P :1mole P Mass O : 4 mole O Total Molar mass of 1.01g = 3.03g H 1mol H 30.97g = 30.97g P 1mol P 16.00g = 64.00g O 1mol O compound = 98.00g Mass O %O = 100% Total mass 64.00g = 100% 98.00g = 65.31% O Now calculate the % by mass of H and also the % by mass of P in H 3PO 4 Percent Composition of a Hydrate F09 AEM Page 2 of 7

3 The percent composition values can be used to calculate the number of grams of an element in a given mass of compound. Example II: Calculate the number of grams of oxygen that would be present in a 4.00 gram sample of phosphoric acid, H 3PO 4. Solution: From the previous example, 65.31% of phosphoric acid (H 3PO 4) is oxygen by mass. This means that there are grams of O in 100. grams of the compound phosphoric acid. Therefore: 65.31g O 4.00g H 3 PO 4 = 100.0g H 3PO g O Finding the mass percent water in a hydrate is very similar. The only difference is the replacement of the element mass by the mass of water (H 2O). Example III: The formula of a hydrate of barium chloride is BaCl 2 2H 2O. Find the mass percent water in this hydrate. Solution: Step 1: Determine the molar mass of BaCl 2H2O 2 Step 2: Divide the mass of water in the compound by the total mass of the compound and then multiply by 100 to determine the % by mass of the waters in the compound Percent Composition of a Hydrate F09 AEM Page 3 of 7

4 C. Experimental Considerations In this experiment, you will be heating a hydrate of copper (II) sulfate (CuSO 4 nh 2O) to drive off the water. Masses are taken before heating to determine the mass of the original sample (the hydrate) and after heating to determine the mass of copper (II) sulfate (CuSO 4) remaining. The difference between these two masses is equal to the mass of the water lost. Heating time and temperature are critically important for this experiment. If not enough heat is applied, some water will remain attached to the copper sulfate producing a low calculated mass percent water for the hydrate. If too much heat is applied, the anhydrous copper (II) sulfate (CuSO 4), which has a grayish white color, can be decomposed to copper (II) sulfide, a black colored compound. D. Experimental Procedure After you have washed the crucible and lid, use only tongs (not your hands) to handle them 1. Place a clean, empty crucible with lid in a clay triangle on a ring stand (as shown in diagram). Tilt the lid so that it is slightly ajar, then heat strongly (bottom of crucible should turn red) for about 3 minutes. Turn off the Bunsen burner, and use tongs to close the lid so that water from the air does not get inside the dry crucible. Allow the crucible and lid to cool (this should take 5 minutes). 2. Using tongs, transfer the crucible and lid (still closed) to a wire gauze and carry them to the balance. It s okay if you need to remove the lid momentarily to transfer the crucible & lid separately to the wire gauze. Mass the crucible and lid (together) carefully to the nearest 0.01g and record the mass in the Data Table provided. 3. Add approximately g of the copper (II) sulfate hydrate to the crucible and mass the crucible with the hydrate and the lid again to the nearest 0.01g. Record the mass in the Data Table. 4. With the lid slightly ajar, heat the crucible gently (crucible should NOT glow red; use the top of the outer flame, not the inner flame) for about 12 minutes. Turn off the Bunsen burner and use tongs to close the lid. Allow the crucible with sample and closed lid to cool for 5 minutes, then mass to the nearest 0.01g, recording the mass in the Data Table. During the time you are cooling the sample and crucible you can be working on the Exercise questions at the end of this experiment. 5. Reheat the sample for 2-3 minutes (with lid slightly ajar), cool (with lid closed) and mass again. This should be repeated until the successive masses are constant within 0.03g. When the masses are constant, record the lowest mass as the final mass. Waste Disposal: Place compound remaining in your crucible in the jar labeled Copper Sulfate Collection. Percent Composition of a Hydrate F09 AEM Page 4 of 7

5 DATA TABLE I Mass of empty, dry crucible and lid (after heating & cooling) Mass of lid, crucible and hydrate (before heating the sample) Mass of hydrate (original sample) Description of original sample Mass of crucible, lid, and dehydrated sample: after first heating & cooling after second heating & cooling 4. final mass: last mass taken Observation of solid after heating Mass of the water driven off Mass percent water in the hydrate (show calculations below) E. Calculations 1. The actual mass percent of water in the hydrated copper (II) sulfate compound should have been 36.1%. Compare this value to the experimental percentage you obtained. Percent Composition of a Hydrate F09 AEM Page 5 of 7

6 2. If you had a g sample of hydrated copper (II) sulfate that is 36.1% water by mass: a. How many grams of water would that correspond to? b. and if the remaining sample was copper (II) sulfate (CuSO 4), how many grams would you have? c. How many moles of copper (II) sulfate (CuSO 4) and how many moles of water is this? Hint: Convert using molecular mass (g/mol). d. Examine the formula for the hydrate: CuSO 4 n H 2O. Notice that n is the molar ratio of water to copper sulfate. Find the numerical value for n in this sample (use your numbers from part c above). Hint: divide the moles of water by the moles of CuSO 4 Follow up Questions 1. Why shouldn t you touch your crucible with your hands? 2. Why shouldn t you heat the crucible with the sample in it too much/at too high of heat? Percent Composition of a Hydrate F09 AEM Page 6 of 7

7 3. Determine the mass percent of each element present in CaCO 3. Percent Composition of a Hydrate F09 AEM Page 7 of 7