Experiment 2: Hydrate Pre-Laboratory Assignment

Name: ________________________________ Lab Section: _________ EXPERIMENT 2: HYDRATE PRE‐LABORATORY ASSIGNMENT Score: ___ /9 (To be completed prior to lab, read the experiment before attempting) 1. A student obtains the following data: Mass of test tube: Mass of test tube and hydrate: Mass of test tube and anhydrous residue after heating: 27.29 g 28.56 g 28.05 g The laboratory instructor identifies the sample as strontium chloride hydrate. (SrCl2⋅nH2O) a) Calculate the mass percent of water in the hydrate? Answer: _______________________ b) Calculate the number of moles of water in the hydrate sample that were driven off by heating? Answer: _______________________ c) Calculate the mole ratio of water to salt in the hydrate, round to the nearest whole number. ("n" in SrCl2⋅nH2O) Answer: ______________________ d) Write the correct formula for the hydrate? Answer: _______________________ EXPERIMENT 2: HYDRATES Introduction: (You will work alone for this experiment) Experiments in chemistry often involve the measurements and observation of changes in macroscopic properties for a system. Analysis and interpretation of these macroscopic properties of matter provides information that leads to understanding at the molecular or atomic scale. In this experiment, you will relate the loss of mass of a hydrate sample upon heating to its molecular formula. Hydrates are salts with water physically incorporated into the crystal structure. Some hydrates represent familiar substances such as gypsum CaSO4 ∙ 2H2O(s) or Plaster of Paris CaSO4 ∙ ½ H2O. Simple hydrates have the general chemical formula MX ∙ nH2O(s) where “M” represents a cation (for example Ni+2, Cu+2, Fe+3), “X” represents an anion (Cl‐, SO4‐2, NO3‐, etc.), and “n” represents the number of moles of water associated with each mole of hydrate MX ∙ n H2O(s). Heating a hydrate removes the water that is locked inside at room temperature. The substance that remains after the water leaves is called the “anhydrous salt.” (meaning without water) For example, CoCl2 in its anhydrous form is a blue compound. When CoCl2 is hydrated, forming CoCl2 ∙ 6H2O, the salt has a pink appearance. CoCl2 is routinely used as a humidity indicator found in small packets stored with electronic equipment to ensure no exposure to water or significant humidity. The chemical reaction illustrating the dehydration process is: CoCl2 ∙6 H2O(s) → CoCl2(s) + 6 H2O(g) Notice that one mole of the hydrate releases 6 moles of water. The process of dehydrating a hydrate can provide us with information about the chemical formula of the hydrate. More specifically, if we weigh a hydrated compound before dehydration and after dehydration, we can determine the mass of water loss. The mass of water lost can be converted into moles of water released. The molar ratio of moles of water released to moles of dehydrated compound provides us with moles of water per moles of the hydrated compound, namely the factor “n”. In this laboratory experiment, you will determine the molecular formula of a known hydrate and the percent composition of a hydrate in a mixture of a hydrate and an inert salt. Mass Percent of a hydrate in a mixture = grams of hydrate in the sample × 100 total mass of the sample Experiment Objectives: 1. Experimentally determine the chemical formula of a known hydrate MX ∙ n H2O(s) given the identity of the anhydrous salt MX. 2. Experimentally determine the percent composition by mass of hydrate in a mixture containing unknown proportions of a hydrate and a non‐hydrate salt. 3. Understand and be able to utilize the relationships between mass, moles, and molecular structure in chemical calculations. Experimental Procedure: 1. Percent water in a hydrate • Weigh a clean and dry 20 x 150mm test tube on an analytical balance to ± 0.0001g. (There are clean test tubes in drawers at the front of the lab if you need one) • Weigh approximately two grams ( 1.8 to 2.2g) of a hydrate sample (labeled I, II or III), record the mass on your data sheet. (± 0.0001g) - Note: The mass doesn't have to be exactly two grams. Just record your value. - Be sure to record the sample number! • Grasp the test tube with your test tube holder and heat the test tube using a Bunsen burner for 5‐6 minutes. Move the test tube around to heat the entire sample and drive of the ensuing water vapor that condenses at the test tube opening. Make sure the test tube opening is not facing you or anybody near you. - Note: Overheating the test tube will cause the glass to melt or break. - Over heating may cause the salt to oxidize and/or decompose. - Avoid overheating your metal test tube holder, it will get hot! • Allow the test tube to cool to the touch (at least 5 minutes). Weigh the test tube and dehydrated salt on the same analytical balance. (± 0.0001g) • Record the mass on your data sheet and calculate the mass percent of water lost. • Heat the test tube a second time and measure the mass of the test tube and dehydrated salt on the same analytical balance. • Record the mass on your data sheet and calculate the percent of water lost. - Note: If the change in mass after your second heating is less than ± 0.010g, ask your instructor for the identify of your anhydrous salt. - You may need to heat your test tube a third time if you have not sufficiently dehydrated your sample. • Heat your test tube a third time if you were told to do so. • After all the water has been displaced, calculate the formula of your hydrated salt. 2. Percent composition of a hydrate in a mixture • Obtain an unknown mixture of BaCl2⋅2H2O and a non‐hydrated salt (such as NaCl) from your laboratory instructor. Record the sample number on your data sheet. • Repeat the procedure from above to dehydrate your salt Note: Your instructor will not tell you when you have completely dehydrated your salt. • Using the same guidelines from part one; determine the percent mass loss and percent composition of your known hydrate in your mixture. Exp. 2 Data & Results Name: ________________________________ Lab Section: _________ Data Part I: (To be completed and signed by you instructor before leaving lab) Mass of the clean dry test tube: _________________g Mass of test tube and hydrate: _________________g Mass of test tube and anhydrous residue after first heating: _________________g Mass of test tube and anhydrous residue after second heating: _________________g Mass after 1st heating − mass after 2nd heating: (must be < ± 0.010g) _________________g Mass of test tube and anhydrous residue after 3rd heating: _________________g (if necessary) Total mass lost after last heating: (use the last mass) _________________g Name of anhydrous salt (supplied by instructor after heating) ____________________________ Formula of the anhydrous salt: _________________ Calculations and Results Part I Mass of unknown sample: _________________g Mass of water removed: _________________g Percent water in unknown: (report this to instructor) _________________% Moles H2O in your sample: _________________mol (Show calculation below for credit) Mass of anhydrous salt: Moles of anhydrous salt: n= _________________g (Show calculation for credit) moles H2O : moles anhydrous salt Formula of your unknown hydrate: _________________mol _________________ _________________ Instructor (date & initial): _______________________ Data Part II Mixture number: _________________ Mass of the clean dry test tube: _________________g Mass of test tube and hydrated mixture: _________________g Mass of test tube and anhydrous residue after 1st heating: _________________g Mass of test tube and anhydrous residue after 2nd heating: _________________g Mass after 1st heating − mass after 2nd heating: (must be < ± 0.010g) _________________g Mass of test tube and anhydrous residue after 3rd heating : _________________g (if necessary) Mass of water lost: _________________g (use the last mass) Percent mass loss: (use the last mass) _________________% Calculations and Results Part II Mass of mixture before heating: _________________g Mass of water removed: _________________g Moles of H2O in sample: (Show calculation for credit) _________________mol Moles of BaCl2 ∙ 2H2O in your sample: (Show calculation for credit) _________________mol Grams of BaCl2 ∙ 2H2O in your sample: (Show calculation for credit) (244.26 g/mol) _________________g Percent by mass BaCl2 ∙2H2O in your original sample: (Show calculation) _________________% Instructor (date & initial): _______________________ Data Summary page: (Place this page on top of your data pages) Name: ________________________________ Lab Section: _________ Score: _________ /26 Part 1: Unknown Hydrate Sample Sample Number: (I, II or III) ___________ Name of anhydrous salt ________________________ Formula of the anhydrous salt: ________________________ % water lost in sample: ________________________ Formula of your unknown hydrate: ________________________ Chemical equation showing the dehydration reaction for your hydrate: _________________________________________________________________________ Part 2: Hydrate Sample Mixture Mixture number: _________________ Percent by mass BaCl2 ∙2H2O in your original sample: _________________% Instructor Comments: