Major Points*We can count individual units by weighing if we know the average mass of the units. Thus, when we know the average mass of the atoms of an element as that element occurs in nature, we can calculate the number of atoms in any given sample of that element by weighing the sample.
*A mole is a unit of measure equal to 6.022x10^23, which is called Avogadro's number. One mole of any substance contains 6.022x10^23 units. *One mole of an element has a mass equal to the element's atomic mass expressed in grams. The molar mass of any compound is the mass (in grams) of 1 mole of the compound and is the sum of the masses of the component atoms. *Percent composition consists of the mass percent of each element in a compound *The empirical formula of a compound is the simplest whole-number ration of the atoms present in the compound; it can be derived from the percent composition of the compound. The molecular formula is the exact formula of the molecules present; it is always an integer multiple of the empirical formula. VideosMoles to grams conversion:
http://www.youtube.com/watch?v=NMdN1LtHuDA Grams to atoms: http://www.youtube.com/watch?v=kKSeGnM5jeQ Online ResourcesMolar Mass and Moles:
http://cnx.org/content/m38717/latest/ Moles: http://cnx.org/content/m42294/latest/ Activity |
Example Calculations1. Find the molar mass of H2O
18.0148g/1mol 2. Find the molar mass of N2H4 32.0456g/1mol 3. There are 6.022x10^23 atoms of aluminum. How many grams of aluminum are there? 26.98g of aluminum 4. You have 10.09 g of Neon. How many atoms are there? 3.011x10^23 atoms of Neon 5. How many moles are in 4.23kg of calcium chloride? 38.1 mol of calcium chloride 6. How many g are in 2.69x10^-2 mol of mercury (I) chloride? 12.7g of mercury (I) chloride 7. Give the percent composition for BaO. O=10.44% Ba=89.56% 8. A compound was analyzed and was found to contain the following percentages of the elements by mass: boron, 78.14%; hydrogen, 21.86%. Determine the empirical formula. BH3 9. A compound with the empirical formula C2H5O was found in a separate experiment to have a molar mass of approximately 90g. What is the molecular formula of the compound? C4H10O2 10.You have 20.4 mL of O2 gas. Find the number of molecules for the compound. 5.48x10^20 molecules of O2 |