THE LAWS LAB LAW OF CONSERVATION OF MASS, LAW OF DEFINITE PROPORTIONS, LAW OF MULTIPLE PROPORTIONS

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Dominic Harvey
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1 THE LAWS LAB LAW OF CONSERVATION OF MASS, LAW OF DEFINITE PROPORTIONS, LAW OF MULTIPLE PROPORTIONS PRELAB Please answer the following questions on a separate piece of paper using complete sentences. 1. State the law of conservation of mass. 2. State the law of definite proportions. 3. State the law of multiple proportions. 4. Using the periodic table list the symbol and name of each element compromising the compounds in the 2 reactions below. (Ex. Na is sodium) 5. Find the chemical formulas for the following: sodium carbonate, calcium chloride, sulfuric acid and calcium carbonate. Introduction Lavoisier stated that matter can neither be created nor destroyed by a chemical change. This very important principle is known as the Law of Conservation of Mass. This law applies to ordinary chemical reactions (as opposed to nuclear reactions where matter can be changed to energy). During a chemical reaction, the atoms of one or more substances, known as the reactants, undergo some rearrangements. The result of these rearrangements is the formation of new substances, known as products. All of the original atoms are still present. It is because of the law of conservation of mass that we are able to write balanced chemical equations. Such equations make it possible to predict the masses of reactants and products that will be involved in a chemical reaction. In this experiment, aqueous solutions of three different compounds will be used to produce two separate and distinct chemical reactions. The changes that will occur during the reactions will be readily observable. Balanced chemical equations for the two reactions are: Reaction1: Na 2 CO 3 (aq) + CaCl 2 (aq) Reaction 2: CaCO 3 (aq) +H 2 SO 4 (aq) 2 NaCl (aq) + CaCO 3 (s) CaSO 4 (s) + H 2 O (l) + CO 2 (g) The combined masses of the three solutions (and their containers) will be measured before and after each reaction has occurred. This experiment should give you a better understanding of the law of conservation of mass and its importance in chemistry. Later, it was observed by Proust that compounds contain the same ratio of elements no matter the size of the sample. A third law, proposed by Dalton, states that if two or more elements chemically combine to form a multiple compounds, the ratio of masses

2 of the second element when compared to the first element, must be a small whole number. This is known as the law of multiple proportions. These two laws will be demonstrated in this lab using models of compounds. Objectives To determine if mass is conserved after two reactions have been completed. To model the law of definite proportions and multiple proportions. Materials PART I: Balance, 50ml Erlenmeyer flask, 2 small test tubes and stoppers to fit, 10 ml of sodium carbonate solution, 3ml of calcium chloride solution, 3ml of 3M (3molar) sulfuric acid. Procedure Preparing the solutions 1. Pour 10ml of the sodium carbonate solution into the 50ml flask and stopper it. 2. Pour the 3ml of calcium chloride into a test tube and stopper it. Label your test tube. 3. Pour 3ml of dilute sulfuric acid into a second test tube and stopper that. Label the tube. Make sure that the exteriors of the tubes are dry. 4. Place the stoppered flask and tubes on the balance and record the total mass. Reaction1 1. Carefully pour the calcium chloride solution into the flask. 2. Swirl gently to mix and note any changes that occur. 3. Stopper the flask and test tube. 4. Determine and record the total mass of the flask and two test tubes. Reaction 2 1. Carefully pour the acid into the flask. 2. Swirl gently to mix. 3. Continue to swirl until all evidence of a chemical reaction has disappeared. 4. If the flask is not at room temperature, wait a few minutes until it is. 5. Stopper the flask and the test tubes. 6. Determine the total mass of the flask and the two test tubes. Data and Observations Total mass of flask, test tubes, solutions and stoppers before mixing Total mass after reaction 1 (include all glassware and stoppers) Total mass after reaction 2 (include all glassware and stoppers) Theoretical mass after reaction2 = initial mass Data and unit

3 % error Observations of reaction1: Observations of reaction2: Calculations Calculate the percent error of your data using the equation l theoretical experimental l X 100 theoretical where the theoretical value is the total mass of all the chemicals and containers before mixing. PART II: Materials Compound models Balance Procedure Definite Proportions 1. Observe the compound model and write a possible chemical formula to show the relative number of atoms in your compound. Sketch a molecule of your compound to scale. Name your compound using covalent compound naming rules. 2. Decompose your compound. Write a balanced reaction to show the exact number of molecules reacted and the exact number of atoms that are formed as products of the decomposition reaction. 3. Find the total mass of the one type of atom in your compound. Find the total mass of the other type of atom in your compound. 4. Find the ratio of the masses of the atoms in your compound. Find another group in the room that has the same ratio. What can be said about their compound? Multiple Proportions 5. Using the atoms in your bag, create as many molecules of a different compound as you possibly can. You do not have to use every atom, set aside extras. Sketch a molecule of your compound to scale. Name your compound using covalent compound naming rules. 6. Decompose your compound. Write a balanced reaction to show the exact number of molecules reacted and the exact number of atoms that are formed as products of the decomposition reaction. 7. Find the total mass of one type of atom in your compound. Find the total mass of the other type of atom in your compound. 8. Find the ratio of the masses of the atoms in your compound. Go back to the group with the same atoms as yours. Did they create the same new compound? What is their ratio of masses? Compare your new ratio to their new ratio and to the original ratio for you first compound, can you find a small whole number ratio?

4 9. Reform the original compound (see step 1 sketch) using all of the atoms in your sample and return the sample for future use. Summary Please discuss your qualitative observations, your % error, and whether or not your data demonstrated that mass is conserved. Account for any losses in mass and suggest ways to account for such errors in the future. Discuss how your model compounds demonstrated the law of definite proportions, multiple proportions, and conservation of mass. Post Lab Questions 1. Look at the reactions in the introduction section, what can you say about the number of atoms of each element on each side of the reaction? Can the same be said for your decomposition reactions in part II of the lab? 2. Define precipitate. When was a precipitate observed in the lab? How could you separate the precipitate if you needed to collect it? Find two methods of separation. 3. What evidence of a chemical change was observed during reaction 2? 4. When you burn a log in a fire place, the resulting ashes have a mass less than that of the original log. Account for the difference in mass. 5. Before Antoine Lavoisier quantified his data which led to the law of conservation of mass, the phlogiston theory existed as a reason for the change in mass in burning materials (like the log in question #4). What is phlogiston? Who proposed the existence of phlogiston? 6. If 25.0 grams of H 2 (g) react with O 2 (g) to form ml of H 2 O, what is the mass of O 2 gas needed? Write the chemical reaction that occurs including phases and balance the reaction with coefficients. 7. A sample of water was analyzed and found to contain grams of oxygen gas and 2.02 g of hydrogen gas. Find the ratio of O to H in a water molecule. A sample of colorless liquid (X) was analyzed and found to contain grams of oxygen and 8.08 grams of hydrogen gas. Could (X) be water? Show calculations to support your claims. If not water, suggest another formula. 8. Another colorless liquid was analyzed. Sample (Y) contained grams of oxygen gas and 4.04 grams of hydrogen gas. Could (Y) be water? Show calculations to support your claims. If not water, suggest another formula.

5 9. Three compounds contain the elements sulfur, S and fluorine, F. Show how the following data supports the law of multiple proportions. Compound A: g F for every g S Compound B: g F for every g S Compound C: g F for every g S

O) and carbon dioxide gas (CO 2

O) and carbon dioxide gas (CO 2 Have you ever tried to balance a book on the top of your head while walking across a room? How do you compensate if you begin to feel the books sliding to one side? You might lean in the opposite direction