AP Chemistry Formula Sheet 2014, Lab Reports of Chemistry

Download AP Chemistry Formula Sheet 2014 and more Lab Reports Chemistry in PDF only on Docsity! Bellevue College | CHEM& 161 Chemical Formula Detective: Determining the empirical formula of a hydrate Background Different elements can form chemical bonds to create compounds. For example, sodium and chlorine combine to form sodium chloride, NaCl. In the chemical formula NaCl, there is a 1:1 ratio of sodium ions:chloride ions. However, not all compounds form in a 1:1 ratio of their constituent elements. If they did, John Dalton would have been correct in 1803 when he proposed the chemical formula of water as HO∗. Of course, we now know that the correct chemical formula of water is H2O, in which there is a 2:1 ratio of hydrogen atoms to oxygen atoms. Since a mole is Avogadro’s number of atoms, H2O is also a 2:1 ratio of moles of hydrogen to moles of oxygen. Thus, the atom ratio is equivalent to the mole ratio (not a mass ratio) in a given chemical formula. As chemistry students you have learned how to predict chemical formulas of ionic compounds based on periodic trends and nomenclature rules, but it hasn’t always been that way. For hundreds of years, the chemical composition of compounds was studied experimentally, and the results generalized into the nomenclature rules used today. These rules allow the accurate prediction of chemical formulas for many ionic compounds without doing any experimentation. For example, the nomenclature rules can be used to correctly predict the formula of magnesium iodide as MgI2 rather than MgI. The curious student of chemistry will wonder how such a prediction could be verified by experimental means. Your task is to determine the chemical formula of an unknown copper chloride hydrate by experiment. An ionic hydrate is an ionic compound that has water molecules trapped within its crystal lattice (refer to the index/glossary of your textbook for more information). For example, Epsom salt (MgSO4·7H2O) is a heptahydrate of magnesium sulfate: within one mole of magnesium sulfate heptahydrate there are seven moles of water. This water can be driven off by heat to form the anhydrous (dehydrated) ionic compound, magnesium sulfate (MgSO4). The chemical formula of your unknown copper chloride hydrate is in the general form of CuxCly·zH2O. Your objective is to determine what the actual formula is (what are the integers x, y, and z?) You will be required to make careful mass measurements and make calculations based on these. The Overall Strategy ∗ John Dalton (1766-1844) made an assumption that when only one compound was formed from two elements, they did so in the simplest ratio, 1:1. (Water was the only known compound formed from hydrogen and oxygen at the time. Hydrogen peroxide, H2O2, was not discovered until 1815.) Since the mass ratio of oxygen to hydrogen in water is 8:1, he assigned the mass of hydrogen (the lightest element) to be 1 and , assuming the formula HO, assigned the value 8 to oxygen. The correct formula of water and the relative atomic mass of oxygen as 16 was a puzzle that would not be solved for another fifty years, despite evidence on the combining volumes of hydrogen and oxygen gas in a 2:1 ratio. Avodgadro’s hypothesis would later be used to interpret this evidence correctly. Chemical Formula Detective Bellevue College | CHEM& 161 Page 2 of 9 The formula will be determined by careful mass measurements. Remember, you are starting with CuxCly·zH2O. You will decompose this into several components, taking mass measurements along the way. The first step is to gently dehydrate a known mass of your sample. The resulting dehydrated sample will be weighed to determine the amount of water lost (this is the zH2O part). The dehydrated copper chloride (now just CuxCly) will be made into a solution, dissolving the sample into water, making a mixture of copper ions and chloride ions. The copper ions will be reduced∗ to copper metal, which will be collected, dried, and weighed (now just Cux). The remaining task is to determine the mass of chloride∗∗ in the compound, which can easily be done by mass difference. (The masses of the initial sample, water lost, and copper were determined in the previous steps.) These steps should give you enough data to figure out the chemical formula of the unknown copper chloride hydrate. A flow chart for today’s experiment: ∗ Reduction of copper means that copper ions gain electrons to form copper metal. These electrons will be provided by the oxidation (loss of electrons) of an aluminum wire in the solution. ∗∗ The mass of chloride (Cl- ion) is being determined. The difference between the mass of chlorine (Cl) and chloride (Cl- ion) is negligible. (Why?) CuxCly·z H2O (hydrate crystal) (goal = find x, y, z) Remove water CuxCly (anhydrous crystal) Reduction Cu (reduced copper) Chemical Formula Detective Bellevue College | CHEM& 161 Page 5 of 9 Data Mass of crucible Mass of crucible + CuxCly•zH2O Color of CuxCly•zH2O crystals Mass of crucible + CuxCly Color of CuxCly solid Initial color of CuxCly solution (before adding Al) Final color of CuxCly solution (after reaction with Al) Mass of watchglass + filter paper Mass of watchglass + filter paper + dried copper Results mass (g) moles CuxCly•zH2O CuxCly water released copper chlorine Empirical formula of the copper chloride hydrate: ______________________ Report Sheets Name________________________ Chemical Formula Detective Lab partner_______________ Section______ Chemical Formula Detective Bellevue College | CHEM& 161 Page 6 of 9 Post Lab Questions 1. a) Ask your instructor for the true (correct) formula of the unknown hydrate: b) Write the systematic (use nomenclature rules) name for this compound. c) Compare this formula with the one you obtained. Are they the same or different? 2. What colors did you observe for the following? a) Copper chloride hydrate______________ b) Copper chloride dehydrated solid______________ c) Copper chloride aqueous solution before reaction with aluminum______________ d) Copper chloride aqueous solution after reaction with aluminum______________ 3. In this experiment, you worked with all types of matter: elements, compounds, homogeneous mixtures, heterogeneous mixtures. Give examples of each type that were encountered in this lab. a) Element(s) ____________________ b) Compound(s) ____________________ c) Homogeneous mixture(s) _____________________ d) Heterogeneous mixture(s) _____________________ 4. a) What ions are present in the correct copper chloride hydrate? Give the sign (+ or –) and magnitude of their charges. b) Does water exist in the crystal lattice predominately as molecules of H2O or as ions of H+ and OH-? Chemical Formula Detective Bellevue College | CHEM& 161 Page 7 of 9 5. What effect would each of the following situations have on the calculation of the number of moles of copper in this experiment? Would the moles of copper increase, or decrease, or stay the same? Give a very brief explanation for each. a) You removed the aluminum wire while the mixture was still blue or contained bubbles. b) You added twice as much aluminum wire as necessary to the copper chloride. (Hint: Is aluminum a limiting reactant or in excess?) c) You couldn’t scrape all of the copper off the aluminum wire. d) The dehydration was not complete because your crystals were still green. You did not reheat to complete the dehydration. 6. List at least two reasonable sources of error in this experiment. DO NOT list human error (e.g. spilling chemicals), miscalculations, or significant figures/rounding errors.