Calculating Average Atomic Mass:



Calculating Average Atomic Mass:

EXAMPLE: What is the average atomic mass of Boron if it exists as 19.90% 10B (10.013 amu) and 80.10% 11B (11.009 amu)?

SOLVE: Multiply the mass by the %. This is called the mass contribution. The sum of the mass contributions is the average atomic mass but you have to divide the sum by 100! Sig figs come from the mass which is a measured amount.

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Practice: A mystery metal has four isotopes. Calculate the average atomic mass and tell the identity of this metal.

|Isotope name |Isotope abundance |Isotope mass |mass contribution (work) |mass contribution (answer)|

| | |(amu) | | |

|Istp-54 |5.90% |53.94 | | |

|Istp-56 |91.72% |55.93 | | |

|Istp-57 |2.10% |56.94 | | |

|Istp-58 |0.280% |57.93 | | |

Average atomic mass ________________________________

2.The made-up element unobtanium (Uo) has three isotopic forms.

|Isotope |natural abundance |atomic mass |mass contribution (work) |mass contribution |

| | |amu | | |

|Uo-220 |32.32 % |220.0 | | |

|Uo-221 | |221.0 | | |

|Uo-224 |47.18 % |224.0 | | |

a) If Uo were an element on the periodic table we have on earth, what atomic mass would be displayed for Uo?

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b) In a sample of 325 g of Uo, how much of each isotope exists?

|Isotope |natural abundance |work |amount |

|Uo-220 |32.32 % | | |

| | | | |

|Uo-221 | | | |

| | | | |

|Uo-224 |47.18 % | | |

| | | | |

2) A 883 g sample of sulfur is composed of three different isotopes.

|Isotope |amount in |amount of |amount of neutrons|atomic mass |percent abundance |mass contribution |

| |sample |protons | |amu | | |

|S-32 |737.7 g | | |31.977 | | |

|S-39 |27.3 g | | |38.976 | | |

|S-33 | | | |32.974 | | |

a) Which isotope is the most abundant? _____________________________________

b) Fill in the table for amount of protons and neutrons. Then solve for percent abundance and mass contribution and average atomic mass of sulfur Show your work below for percent and atomic mass.

average atomic mass: ____________________________

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