CHEM 1331 1st Edition Exam 1 Study Guide Midterm 1 Diatomic elements Diatomic elements are elements that are naturally found in nature as molecules composed of two atoms There are seven diatomic elements You MUST know these Hydrogen H2 Nitrogen N2 Oxygen O2 Flourine F2 Chlorine Cl2 Iodide I2 Bromine Br2 Know how to find Neutrons protons electrons and mass If you have a neutral element you can find both the protons and electrons by looking at the Z number or atomic number A neutral element is an element without charge EX H is a neutral element H Is not neutral It is in fact a cation To find the neutrons you subtract the Z number from the element s mass Ex 12C has 6 neutrons because 12 mass 6 Z number 6 To find the mass you add the neutrons and Z number You need to Memorize the polyatomic ions and their charges The most commonly used ones are Acetate ammonium carbonate chlorate chlorite chromate cyanide dichromate nitrate nitride perchlorate phosphate phosphite sulfate sulfite bromate benzoate chromate permanganate and iodide Know how to find the average atomic mass of two isotopes To find the average atomic mass of two isotopes you need to multiply each of the isotopes mass by their respective abundance and add them together Ex Suppose the element Z contains only two isotopes 29Z atomic mass 28 93 u abundance 77 71 and 31Z atomic mass 30 91 u Calculate the atomic mass of Z that would appear in the periodic table with two decimal places Your first isotope has a mass of 28 93 and an abundance of 77 71 so you multiply 28 x 7771 which equals 22 481503 Then you need to find the abundance of the second isotope To find the abundance of the second isotope you assume that there is an abundance of 100 and subtract 77 71 which is the abundance of the first isotope Once you do that you will find that the second isotope has an abundance of 22 29 You then repeat the first step and multiply 30 91 atomic mass x 2229 which equals 6 889839 Add both values together to get your final answer which is 29 37 Note that I waited until the end to round up my answer Know how to find mass percent composition To find mass percent composition follow the example What are the mass of carbon and oxygen in carbon dioxide CO2 Mass Percent Solution Step 1 Find the mass of the individual atoms Look up the atomic masses for carbon and oxygen from the Periodic Table The atomic masses Are found to be C is 12 01 g mol O is 16 00 g mol Step 2 Find the number of grams of each component make up one mole of CO2 One mole of CO2 contains 1 mole of carbon atoms and 2 moles of oxygen atoms 12 01 g 1 mol of C 32 00 g 2 mole x 16 00 gram per mole of O The mass of one mole of CO2 is 12 01 g 32 00 g 44 01 g Step 3 Find the mass percent of each atom mass mass of component mass of total x 100 And the mass percentages of the elements are For carbon mass C mass of 1 mol of carbon mass of 1 mol of CO2 x 100 mass C 12 01 g 44 01 g x 100 mass C 27 29 For oxygen mass O mass of 1 mol of oxygen mass of 1 mol of CO 2 x 100 mass O 32 00 g 44 01 g x 100 mass O 72 71 Answer mass C 27 29 mass O 72 71 When doing mass percent calculations it is always a good idea to check to make sure your mass percents add up to 100 This will help catch any math errors 27 29 72 71 100 00 The answers add up to 100 which is what was expected Know how to find the empirical formula An empirical formula is a formula giving the proportions of the elements present in a compound but not the actual numbers or arrangement of atoms To learn how to find the empirical formula follow the example below A compound was analyzed and found to contain 13 5 g Ca 10 8 g O and 0 675 g H What is the empirical formula of the compound Start with the number of grams of each element given in the problem Convert the mass of each element to moles using the molar mass from the periodic table Divide each mole value by the smallest number of moles calculated Round to the nearest whole number This is the mole ratio of the elements and is represented by subscripts in the empirical formula You must know how to use scientific notation You should also know the basics on how to convert moles to grams Grams to moles Moles to molecules and Liters to moles Must know Avogadro s number 1 mole 6 02x1023 Also memorize the common formulas such as Molarity of moles of solute Liters of solution Molecules 6 02x1023 x Moles Grams Molar mass x moles Also memorize the element s charges