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Calculations with Elements and Compounds

Calculations with Elements and Compounds. The Mole Concept. Pisgah High School M. Jones. Rev. 051111 050212. Moles?. No. Not real moles, or Whack-a-moles, or even the moles on your back. Chemistry moles. What is a mole?. It’s a way of measuring atoms and molecules.

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Calculations with Elements and Compounds

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  1. Calculations with Elements and Compounds The Mole Concept Pisgah High School M. Jones Rev. 051111 050212

  2. Moles?

  3. No. Not real moles, or Whack-a-moles, or even the moles on your back. Chemistry moles

  4. What is a mole? It’s a way of measuring atoms and molecules. The “mole concept” is a lot like the “dozen concept.”

  5. It doesn’t make any difference what you are counting. A dozen is 12. A dozen is always 12.

  6. It doesn’t make any difference what you are counting. A dozen is 12. A dozen is always 12. A dozen donuts.

  7. It doesn’t make any difference what you are counting. A dozen is 12. A dozen is always 12. A dozen donuts. A dozen pencils.

  8. It doesn’t make any difference what you are counting. A dozen is 12. A dozen is always 12. A dozen donuts. A dozen pencils. A dozen Volkswagons.

  9. A dozen is always 12.

  10. Like we can count by dozens, or multiples of 12, we can also count by multiples of a much larger number, which we call Avogadro’s number.

  11. Avogadro’s number is much larger… Avogadro’s number is 6.022 x 1023. So how did chemists come up with such a huge number?

  12. It began with Amedeo Avogadro about a 150 years ago. He said that for equal volumes of gases at constant temperature and pressure, the masses of the gases are proportional to their atomic weights.

  13. 19.6 g 44.0 g = = 28.0 g 12.5 g Avogadro’s law 10.0 liters of CO2 weighs 19.6 grams 10.0 liters of N2 weighs 12.5 grams 10.0 liters of Xe weighs 58.6 grams 1.57 1.57 Ratio of the masses of equal volumes of CO2 and N2 Ratio of the molar masses of CO2 and N2

  14. 0.335 0.335 19.6 g Ratio of the masses of equal volumes of CO2 and Xe Ratio of the molar masses of CO2 and Xe 44.0 g = = 58.6 g 131.3 g Avogadro’s law 10.0 liters of CO2 weighs 19.6 grams 10.0 liters of N2 weighs 12.5 grams 10.0 liters of Xe weighs 58.6 grams

  15. See! I told you so. He said that for equal volumes of gases at constant temperature and pressure, the masses of the gases are proportional to their atomic weights.

  16. But Avogadro never determined the value we know as Avogadro’s number. I wish I knew what my number was. It was called Avogadro’s number as a tribute to his pioneering work.

  17. After Avogadro’s death, Stanislao Cannizzaro published an explanation of Avogadro’s law. Equal volumes of gases at the same temperature and pressure have equal numbers of particles.

  18. Cannizzaro was able to convince chemists of the value of standardized atomic weights. Loschmidt (1865) was the first to determine the number of atoms or molecules in a fixed volume of gas.

  19. Jean Perrin was the first to use the term Avogadro’s number in 1909 to describe the number of atoms in the atomic weight of an element. He was the first to determine a value of Avogadro’s number which is close to the present-day value of 6.022 x 1023.

  20. Atomic Mass We can relate Avogadro’s number to the amount of an element equal to its atomic mass measured in grams.

  21. Atomic Mass This used to be called the “gram atomic weight”. Atomic weights are now called atomic masses. Atomic masses are given in units of “atomic mass units”, or amu. The amu is also called the dalton.

  22. Atomic Mass The “atomic mass unit” (amu) is based on the mass of the carbon-12 isotope. It was decided that an atom of C-12 be assigned a mass of exactly 12.0000 amu

  23. Atomic Mass This way, hydrogen,the lightest element (H) would have a mass very close to 1. The average atomic mass of hydrogen is 1.0079 amu.

  24. Atomic Mass Instead of 12.0000 amu of carbon-12, which is too small to even see, we consider 12.0000 grams of C-12. How many atoms are in 12.0000 g of C-12?

  25. x Atomic Mass How many atoms are in 12.000 g of carbon-12? A C-12 atom has a mass of 1.993 x 10-23 g 1 C atom C atom 12.000 g C 1.993 x 10-23 g C = 6.022 x 1023 C atoms

  26. Atomic Mass The mole is the “official” metric unit for the “quantity of matter”. A mole is the quantity of an element which has a mass equal to the atomic mass expressed in grams.

  27. The word “mole” can be abbreviated as “mol”.

  28. So, what is a mole? It is a word that describes a very large number of atoms. 6.022 x 1023 atoms It comes from the Latin word moles which means “quantity” 6.022 x 1023 is also called Avogadro’s Number.

  29. A mole is … 6.022 x 1023 atoms, molecules, electrons or ions … – any particle. Can you count to a mole? Since they are so small, we must count atoms indirectly.

  30. Suppose you wanted to know the number of marbles in a large jar. Weigh a few marbles, weigh all the marbles, use a conversion factor.

  31. Suppose … 2472.47 g x = This is a conversion factor Like this: 10 marbles = 86.45 grams Avg. marble = 8.645 grams All the marbles = 2472.47 g 1 marble 286 marbles 8.645 g

  32. We can count a large number of particles by weighing them.

  33. We can count atoms if we know the mass of an Avogadro's number of atoms.

  34. An Avogadro's number of atoms has a mass equal to the molar mass. 6.022 x 1023 atoms of sodium have a mass of 23.0 g 6.022 x 1023 atoms of iron have a mass of 55.8 g 6.022 x 1023 atoms of aluminum have a mass of 27.0 g

  35. Q. How many atoms are in 50.0 grams of potassium? x = We know that 1 mole of K has a mass of 39.1 grams; and 1 mole is 6.02 x 1023 atoms. 6.02 x 1023 atoms K 50.0 g K 39.1 g K 7.70 x 1023 atoms K

  36. The molar massof an element is the mass of 6.022 x 1023 atomsor1 moleof that element.

  37. FYI: We now use “molar mass” instead of.. gram atomic weight, gram molecular weight, gram formula weight.

  38. The molar mass of an element is usually rounded to one decimal place: sodium (Na) 23.0 g/mol iron (Fe) 55.8 g/mol chlorine (Cl) 35.5 g/mol phosphorous (P) 31.0 g/mol

  39. What do the subscripts in a formula tell us? The subscripts give the number of moles of each element in one mole of the compound. 1 mole of Fe2O3contains … 2 moles of iron (Fe),and … 3 moles of oxygen (O)

  40. How many moles of each kind of element are in one mole of … CaCl2 1 mole Ca and 2 moles Cl C12H22O11 12 moles C, 22 moles H, 11 moles O Fe3(PO4)2 3 moles Fe, 2 moles P, 8 moles O

  41. Calculating molar masses of compounds … Sum of the molar masses of the elements Molar mass of the compound = The molar mass of an element its atomic mass

  42. Calculate the molar mass of water: (2 x 1.0) + (1 x 16.0) = Molar mass of H2O = 18.0 g/mol 2 hydrogen atoms 1 oxygen atom

  43. General Equation for Molar Mass Molar mass = S (atomic masses) For each element, multiply it’s atomic mass by its subscript, then add them all together.

  44. What is the molar mass of calcium chlorate? Start with the formula: Ca(ClO3)2 1 mol Ca,2 mol Cl,6 mol O (1 x 40.1)+(2 x 35.5)+(6 x 16.0) = 207.1 g/mol

  45. What are the molar masses of each of the following? 1. HCl 2. Fe2O3 3. H2SO4 4. Ca3(PO4)2 5. (NH4)2CO3 36.5 g/mol 159.6 g/mol 98.1 g/mol 310.3 g/mol 96.0 g/mol

  46. Molar Conversions Use conversion factors to convert between moles, grams, molecules and the volumes of gases at STP. 1 mol = 1 molar mass 1 mol = 6.022 x 1023 particles 1 mol = 22.4 L of any gas at STP

  47. Convert the following: 1. 17.5 grams of iron metal to moles. 0.314 moles Fe 2. 0.0362 moles of CaCl2 to grams. 4.02 g CaCl2 3. 5.45 x 1024 molecules of SO3 to moles. 9.05 moles SO3

  48. Detailed solutions:

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