Using Chemical Formulas

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  • Good Morning. We have a short class so lets get started with the lesson. The first concept is calculating formula/molar mass.  When doing this, there are three steps. 1. Use the chemical formula to determine the number of each type of atom present in the compound. 2. Multiply the atomic weight of each element by the number of atoms of that element present in the compound. 3. Add it all together and put the units of grams or mole after the number. An example is Na2. You take 23.0 times 2 and get a total of 46.0.
  • The next concept is how to calculate the number of molecules, formula units, or ions in chemical compounds. Well, the way to do this is to convert the molar mass between mass in grams and amount in moles of a chemical compound. Many of you know grams is a form of measurement. Molar mass can be used as a conversion factor for converting grams to moles and moles to grams.
  • The next concept of percentage has a little to do with math. Percent composition is the term used to describe the percent by mass of each element in a compound. It is typically found using the molar mass values for both the elements in the compound and the compound itself. An example is H2O or water. Take 1.01 times 2=2.02 g over mole. O is 16 times 1=g over mole. Whn added you get 18.02 g over mole.
  • Now based on the lesson and the information you know, try the problems on this worksheet and we will go over the answers momentarily. .... Ok lets go over the answers. Q1: The molar mass of an element is the mass of one... A1: Mole of the element. Q2: what is the sum of the atomic masses of all the atoms in a formula for a compound? A2: formula mass. Q3: What is the formula mass of magnesium, chloride? A3: 95.211 amu
  • Q4; The molar mass of No2 is 46.01 g/mol. How many moles of NO2 are present in 114.95 g. A4: 2.498 mol. Q5: How many Mg2+ ions are found in 1 mol of MgO? A5: 6.02 * 10²³. Q6: What is the percentage composition of Cs4? A6: 13.6% C, 86.4%F. Q7: The percentage composition of sulfur in SO2 is about 50%, What is the percentage of oxygen in this compound? A7: 50%. 
  • Q8: The molar mass of MgL2 is... A8: The sum of the masses of 1 mol of Mg and 2 mol of I. Q9: The molar mass of Cs2 is 76.14 g/mol. how many grams of Cs2 are present in 10 mol? A9: 761.4 g. Q10: What is the percentage composition of oxygen in H2O? A10: 88.8%. BING! Have a good day.
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