Avogadro s Number: The Mole Lab

Transcription

1 Avogadro s Number: The Mole Lab Name Period The purpose of this lab is to practice measuring mass, to calculate molar mass, and to use mass, molar mass, and Avogadro s number to calculate the number of moles and number of atoms in a particular sample. About the same time Dalton was publishing his Atomic Theory, Amedeo Avogadro proposed that the volume of a sample of any gas was proportional to the number of particles in the sample of the gas. Jean Perrin, a French physicist, built on work by Johann Joseph Loschmidt, who determined the average diameter of molecules in air, determined the constant relating gas volume and number of particles to be X particles in 22.4 Liters of gas, and named this number of particles 1 mole. Loschmidt, who won the Nobel Prize in Chemistry in 1926 for his work, proposed naming his constant after Avogadro. A mole is defined as Avogadro s number of representative particles. The appropriate name for a representative particle is determined by the identity of the substance. Representative particles of ionic substances (metal- nonmetal) would be appropriately named formula units. Representative particles of covalently bonded, molecular substances (nonmetal- nonmetal) are correctly termed molecules. Representative particles of a substance that is an element are labeled atoms. It is not possible in a human lifetime to count to 6.02 x even if many friends and all their friends help. Chemists use the concept of the mole to calculate the number of particles from the mass of the substance. This idea is referred to as counting by weighing. Conversion factors from the equalities, below. Equality: 1 mole of (representative particles) = x (representative particles) Conversion Factors: Write sample conversion factors, 2 each, for the conversion of each of the 3 types of representative particles from moles to representative particles, in the space below. There will be 6 conversion factors, one in each box. In the Lab: Starting with measured mass of a substance Because particles are too small (and too numerous!) to count, start with the mass of each substance and figure out a way to calculate the number of particles. Because atoms of each element have different numbers of each subatomic particle (protons and neutrons; the mass of electrons is too small to be significant), the masses of atoms of each element are different. The mass of 1 mole, or x atoms of any element, has been found to be equal to the atomic mass of the element (Where can you find this information? ) Equality to know: 1 mole of any substance = the atomic mass, in grams, of that substance Use this equality to write equalities, then conversion factors, for Carbon, Sulfur, and Iron in the space below. 1 mole of carbon = grams carbon 1 mole sulfur = grams sulfur 1 mole iron = grams iron Conversion Factors:

2 A sample calculation would look like this, starting from a mass of 5.00 grams of carbon and finding first the moles of carbon and then the number of representative particles (in this case, atoms why?) in 2 steps: 5.00 g C X 1 mole C = moles C g C moles C X x atoms C = 2.51 X atoms C 1 mole C Write the calculation with both conversion factors in the same problem, using dimensional analysis skills: Add a paragraph, including an example and explanation, telling how to calculate molar mass. When you finish the calculations, check your work with another lab group. Materials: Balances and samples are at each lab station. Safety: The bottles are plastic and sealed. Procedure: 1. Go to a station you may start anywhere. Be sure to record the data for that station in the correct row of your data table. 2. Using information written on the container label, fill in the chemical formula and name of the substance in the appropriate row of Data Table The mass of the empty sample bottle is conveniently written on the bottom of the bottle. Without opening the bottle, record the mass of the empty sample bottle in Data Table 1, in the appropriate row. 4. Use the balance at the station to obtain the mass of the sample bottle and contents. Record the mass of the bottle and its contents in Data Table Record other information about the substance in Data Table Complete all measurements with your partner, then return to your seats to write your calculations. If you want to work on calculations as you complete each station or after you have completed a few stations, you may do so.

3 Bottle Number Chemical Formula Chemical Name of Substance empty sample bottle substance and bottle Other information

4 Calculations Table 1: Include only the answers. Actual calculations to be shown in the 3- step process, in Data Analysis. Bottle Number 1 Calculation 1: Substance in Bottle (g) Calculation 2: molar mass (g/mol) Calculation 3: Moles of Substance Term for the Representative Particle Calculation 4: Number of Representative Particles. BONUS: Number of Oxygen atoms

5 Avogadro s Number: The Mole Lab Data Analysis: 1. Write out your work for each calculation. In doing conversion factor calculations, only 2 steps are needed (there is no equation.) State what you are finding and then show your work including all units and the substance. Observe significant digit rules. Ex. Calculate the moles of carbon in 5.00 grams carbon: 5.00 g C X 1 mole C = moles C g C Write your answers for questions 2-8 in your lab book, in complete sentences. 2. Give 3 examples of representative particles. 3. Explain how to write and use a conversion factor for converting between representative particles and moles. 4. Explain how to write and use a conversion factor for converting between moles of an element and grams of that element. 5. Explain how to write and use a conversion factor for converting between moles of a compound and grams of that compound. What must you calculate first? 6. What skills and learning did you need to complete this activity? 7. Are there any skills you need help with, or just need to practice more? Which ones? 8. Explain why this learning is important. Conclusion: State the purpose of the lab. Write a set of steps to use in calculating the number of particles in a sample of an element or a compound. Explain clearly how you can use mass to count particles that are too small to see.