LAB 5: Percent Composition and Empirical Formula of a Hydrate, Lab Reports of Chemistry

Download LAB 5: Percent Composition and Empirical Formula of a Hydrate and more Lab Reports Chemistry in PDF only on Docsity! LAB 5: Percent Composition and Empirical Formula of a Hydrate INTRODUCTION: A hydrated salt is one in which a fixed number of water molecules is crystallized with each formula unit of the salt. The number of associated water molecules is referred to as the water of crystallization. For example, barium chloride dehydrate ( BaCl2 . 2H2O ) has two water molecules of crystallization, whereas the salt cobalt chloride hexahydrate ( CoCl2 . 6 H2O ) has six water molecules of crystallization. Most common hydrates have waters of crystallization ranging from two to twelve. Upon heating, a hydrate decomposes, releasing the water as steam leaving a dry salt called an anhydrous salt. The percent composition of a compound can be calculate by dividing the mass of the substance interested in by the total mass of the compound and multiplying the answer by 100 to convert it into a percentage. The theoretical percentage of water in a hydrate is found by comparing the mass of the water of crystallization to the mass of the entire hydrated salt, expressed as a percentage. This is only possible if the formula of the hydrate is known. The experimental percentage of water in a hydrate is found by comparing the mass of water driven off (evaporated) in the lab to the total mass of the original hydrated compound measured in the lab, expressed as a percentage. The mass of water driven off is calculated by the indirect technique called weighing by difference. The original mass of the hydrate is compared to the final mass of the anhydrous salt. The difference in mass must be the mass of the water driven off. The proficiency of your laboratory technique or the percentage of error is readily calculated by comparing the experimental percentage of water to the theoretical percentage of water. The following mathematical equation is used to calculate the percent error of an experiment. Experimental – Theoretical % Error = ------------------------------------------ x 100 Theoretical A negative percentage error indicates the answer obtained was less than the maximum possible, while a positive percentage error indicates the answer obtained was greater than the maximum yield possible. The empirical formula of a hydrate can be determined from the percent composition data for a hydrate or from the actual masses of the anhydrous salt and water measured in the lab. PURPOSE: The purpose of this lab is to calculate the percentage of water in an unknown hydrate, and thereby determine the empirical formula of the hydrated salt. MATERIALS REQUIRED: Unknown hydrate s: 250 ml beaker BaCl2 . ___ H2O Watch glass Ca(NO3) 2 . ___ H2O Hotplate CoCl2 . ___ H2O PROCEDURES: Part A: Percentage of Water in an Unknown Hydrate 1. Weigh a clean, dry 250 ml beaker. 2. Add about 1.2-1.8 g of the hydrate to the beaker and record the mass accurately. 3. Place the beaker on a hotplate. Heat the hydrate gently to avoid spattering the salt. You will observe moisture forming on the sides of the beaker as you heat the hydrate. Continue heating until all the moisture has evaporated. The salt will change from a crystalline form to a powder form. 4. Turn off the hotplate and allow the beaker to cool for 10 minutes. Carefully transfer the beaker to the balance. Find the mass of the beaker and the anhydrous salt. 5. Empty the beaker contents into a paper towel and discard in the waste basket. 6. Calculate the percentage of water in the unknown hydrate by dividing the mass of water driven off by heating by the original mass of the hydrate. Now multiply by 100 to convert the answer to a percentage. Part B: Percentage Error in Experimental Technique 1. Check with the instructor to confirm the percentage of water in the unknown hydrate. 2. Calculate the experimental error by subtracting the theoretical percentage of water from the experimental percentage of water, and dividing this answer by the theoretical percentage of water. Now multiply by 100 to get the percentage error. Part C: Empirical Formula of the Hydrate 1. Using the theoretical percentage of water for your hydrate, change the percentages to grams. This is allowed because percentage means “per 100” and if there was a 100 gram sample used, the percentage composition would equal the weight in grams. 2. Convert the grams of anhydrous salt and grams of water to moles. The answers will rarely be whole numbers; however molecular formulas must use whole numbers. To get a whole number ratio, divide all the number of moles by the smallest number of moles. This will generally make them whole numbers. 3. Check your answer for the molecular formula by looking at the label on the chemical container provided by the instructor QUESTIONS: 1. Calculate the theoretical percent of water in nickel chloride hexahydrate given the molecular formula for this compound is NiCl2 . 6 H2O. 2. Calculate the experimental percent of water in a hydrate given that the hydrate has a mass of 1.632 g before heating and a mass of 1.008 g after heating. Recall that the salt that remains after heating is called an anhydrous salt. 3. Calculate the empirical formula for the hydrated form of calcium sulfate (CaSO4) given that it contains 20.9% water. 4. The hydrated form of chromium (III) nitrate [ Cr(NO3)3 ] is determined to contain 59.5% anhydrous salt. Calculate the empirical formula for the hydrated form of this salt.