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Atomic Mass

Atomic Mass. Atomic Mass – the mass of one atom of an element (unit: u) Ex. Carbon is 12.01 u; Hydrogen is 1.01 u. The Mole. A mole is 6.02 x 10 23 entities (atoms or molecules) of something (element or compound). The number 6.02 x 10 23 is known as Avogadro’s constant (N A ).

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Atomic Mass

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  1. Atomic Mass Atomic Mass – the mass of one atom of an element (unit: u) Ex. Carbon is 12.01 u; Hydrogen is 1.01 u

  2. The Mole A mole is 6.02 x 1023 entities (atoms or molecules) of something (element or compound). The number 6.02 x 1023 is known as Avogadro’s constant (NA). 6.02 x 1023 carbon atoms weigh a total of 12.01 g. 1 mol of magnesium atoms (6.02 x 1023) weighs 24.3 g. Notice something????

  3. Molar Mass Molar Mass – the mass of one mole of a chemical entity (unit: g/mol) M = m n Remember to use a capital M for molar mass and a lower-case m for mass!

  4. Molar Mass What is the molar mass for: a) NaCl b) CaCl2 M = 22.99 g/mol + 35.45 g/mol M = 58.44 g.mol M = 40.08 g/mol + 35.45(2) g/mol M = 110.98 g.mol

  5. Performing calculations… m = mass (unit: g) n = mole (unit: mol) M = molar mass (unit: g/mol)

  6. Example 1 How many grams of NaHCO3 are in 0.673 moles? m = n x M = 0.673 mol x 84.01 g/mol = 56.54 g

  7. Example 2 How many moles of KMnO4 are there if you have 250 g of it? n = m / M = 250 g / 158.04 g/mol = 1.58 mol

  8. Example 3 Calculate the molar mass of an element, if 2.220 mol has a mass of 26.66 g. Identify the element. M = m / n = 26.66 g / 2.220 mol = 12.01 g/mol Element is Carbon

  9. Counting the number of atoms from mass How many atoms of silver are in a 185.3 g lump of pure silver? Two steps: 1. Calculate amount of n (find mol) 2. Calculate # of X atoms (find NX) Formula: NX = n(N) where N = 6.0 x 1023

  10. Counting the number of atoms from mass • Calculate amount of Ag (find mol) • Calculate # of Ag atoms (find NAg) • nAg = m / M • = 185.3 g / 107.87 g/mol • = 1.72 mol # of Ag atoms = 1.72 mol x 6.02 x 1023 atoms 1 mol = 1.04 x 1024 atoms

  11. Example 4 Count the number of nickel (Ni) atoms there are in one 5-cent coin, which weighs 5.0 grams. • nNi = m / M • = 5.0 g / 58.69 g/mol • = 0.085 mol # of Ni atoms = 0.085 mol x 6.02 x 1023 atoms 1 mol = 5.12 x 1022 atoms

  12. Example 5 How many atoms of nitrogen are there in 1.26 kg of nitrogen gas? • nN2 = m / M • = 1260 g / 28.02 g/mol • = 44.97 mol # of N atoms = 44.97mol x 6.02 x 1023 molecules N2 x 2 atoms N 1 mol 1 molecule N2 = 5.41 x 1025 atoms

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