Chemistry Worksheet



Homework: Average Atomic Mass

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Calculate the average atomic masses. Use the atomic mass for significant digits.

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1. What is the atomic mass of hafnium if, out of every 100 atoms, 5 have a mass of 176, 19 have a mass of 177.0, 27 have a mass of 178.0, 14 have a mass of 179.0, and 35 have a mass of 180.0?

178.55amu; use three significant digits: 179amu

2. The element germanium has five naturally occurring isotopes. The relative abundance and atomic mass of each isotope is given in the chart below. Use the data provided to determine the average atomic mass of germanium.

|Isotope |Atomic mass |Natural abundance (atom %) |

| |(amu) | |

|70Ge |69.9 |20.84 |

|72Ge |71.9 |27.54 |

|73Ge |72.9 |7.73 |

|74Ge |73.9 |36.28 |

|76Ge |75.9 |7.61 |

72.6amu

3. Complete the chart below for the element silicon, which has three naturally occurring isotopes:

|Isotope |Atomic mass (amu) |Natural abundance (atom %) |

|28Si |28.0 |92.2 |

|29Si |29.0 |? |

|30Si |?? |3.1 |

The average atomic mass of silicon is 28.09amu.

%29Si = 4.7% mass = 29.4amu

4. Calculate the relative abundance of each isotope of iridium. The average atomic mass of iridium is 192.22amu

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Isotope mass (u) relative abundance

Ir-191 191.0 ? 39.00%

Ir-193 193.0 ? 61.00%

5. Copper has two naturally occurring isotopes. 63Cu has an isotopic mass of 62.93amu and 65Cu has an isotopic mass of 64.93amu. Determine the relative abundance of each isotope if the average atomic mass of copper is 63.55amu.

63Cu = 69.00% 65Cu = 31.00%

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