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## Amu, Atomic Mass, Mole, they’re relative

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**Amu, Atomic Mass, Mole, they’re relative**but neatly related!!!**In the early days**how much does an element weigh? how much does an atom weigh? Just take a stab… “I decree that hydrogen will have a standard mass of 2!” Stanislao Cannizzaro (1826–1910),**Using Hydrogen as the relative standard**• too small – • too much error • Think bigger – Oxygen sixteen! • it forms chemical compounds with many other elements, • lowest whole number and still have hydrogen > 1**Discovery of oxygen 17 and 18 – oops!!**• no problem for the chemists • (1/16th of the average mass of oxygen) • Physicists, however, dealing with atoms required • a unit that distinguished between isotopes. • (1/16th of the mass of an atom of a particular isotope of oxygen)**In 1956, Alfred Nier (at the bar in the Hotel Krasnapolski**in Amsterdam) suggested that the atomic weight scale be based on carbon-12. • Physicists could care less • Chemists would have to adjust all their atomic • masses by 42 ppm! • Six years later chemists and physicists agreed • on C12**Amadeo Avogadro**• Italian physics professor • his law – “equal volumes of different gases at the same temperature and pressure contain equal numbers of atoms or molecules.”**Equal Volumes of gases**O2 N2 Cl2 T = 0oC P = 1 atmosphere**Equal Number of Particles**O2 N2 Cl2**but mass is not the same!**O2 N2 Cl2 2 x 16g 2 x 14g 2 x 35g**Definition!**A sample of any element with a mass equal to that element's atomic mass in grams contains one mole of atoms (6.02 x 1023 atoms) it’s also called the gram atomic mass or gam**Practice:**• One mole of chromium weighs ______ grams. • One mole of chromium contains _______ atoms. • The gam of chromium is _________. • 0.5M of chromium weighs ________ grams. • 0.5M of chromium contains __________ atoms**Definition!**A sample of any compound with a mass equal to that compound’s molecular mass (in grams) contains one mole of molecules (6.02 x 1023 molecules) It’s also called the gram molecular mass or gmm.**Practice:**• One mole of water weighs ______ grams. • One mole of water contains _______ molecules. • One mole of water contains ______ atoms of H • One mole of water contains ______ atoms of O • The gmm of water is ________ • 0.2M of water weighs ________ grams. • 0.2M of water contains __________ molecules • 0.2M of water contains ______ grams of H • 0.2M of water contains ______ molecules of H**Definition!**A sample of any ioniccompound with a mass equal to that compound’s unit formula mass (in grams) contains one mole of formula units (6.02 x 1023 molecules) It’s also called the gram formula mass or gfm.**Practice:**• How many moles are there in 320 grams of CoCl2? • How many grams are there in 4.2M Ca(NO3) 2 ? • How many molecules are there in 4.2M Ca(NO3) 2 ? • 2.45 x 1023 molecules of CH4 are ______ moles?**Practice:**• One mole of table salt weighs ______ grams. • One mole of table salt contains _______ formula units. • One mole of table salt contains ______ atoms of Na • One mole of table salt contains ______ atoms of Cl • 0.75M of NaCl weighs ________ grams. • 0.75M of NaCl contains __________ formula units**In Summary**• Mass is a way to count numbers of particles • A mole represents 6.02 x 1023 particles, be they • atoms, molecules, formula units or whateva • The mass of 1 mole of a substance is = • 6.02 x 1023 particles x mass of one particle • for elementsgam = gram atomic mass • for molecular compounds gmm = gram molecular mass • for ionic compounds gfm = gram formula mass**Volume of 1 Mole of Gas**STP = 0oC and 1 atmosphere One Mole of any gas occupies 22.4 liters at STP**Practice – Part I:**• One mole of gaseous oxygen weighs ______ grams. • One mole of gaseous oxygen contains _______ molecules • One mole of gaseous oxygen occupies_______ liters • Practice – Part II: • 0.75M of gaseous oxygen weighs ________ grams. • 0.75M of gaseous oxygen contains __________ molecules. • 0.75M of gaseous oxygen occupies _______ liters at STP.