Chemistry Worksheet: Average Atomic Mass Per



Show your work for each of the following problems.

Calculation of average atomic mass is not a straight average but a weighted average based upon the percent isotopic ratio of the element. The following general formula can be used in all cases.

 

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What is the average atomic mass of the following isotopic mixtures? Show your work!!

1) 22.00% of 159.3 amu; 78.00% of 161.2 amu. 

 

2) 42.10 amu - 93.50%; 48.30 amu - 6.200%; 50.40 amu - 0.3000%

3)  75.0 % 133Cs with a mass of 132.97 amu, 20.0 % 132Cs with a mass of 131.98 amu, and 5.00 % 134Cs with a mass of 133.982 amu

4) 80.00 % 127I with a mass of 126.963 amu, 17.00 % 126I with a mass of 125.984 amu, 3.00 % 128I with a mass of 127.992 amu

5) 50.00 % 197Au with a mass of 196.798 amu, 50.00 % 198Au with a mass of 197.882 amu

6)  15.35 % 55Fe with a mass of 54.997 amu, 84.65 % 56Fe with a mass of 55.989 amu

7)  99.00 % 1H with a mass of 1.0003 amu, 0.8000 % 2H with a mass of 2.0074 amu, 0.2000 % 3H with a mass of 3.0121 amu

8) Find the average atomic mass for Li if 7.50% of Li atoms are 6Li with a mass of 6.0151223 amu and 92.5% are 7Li with a mass of 7.0160041 amu.

9) Find the average atomic mass for B if 19.9% of B atoms are 10B with a mass of 10.0129371 amu and 80.1% are 11B with a mass of 11.0093055 amu.

10) Find the average atomic mass for Cl if 75.78% of Cl atoms are 35Cl with a mass of 34.96885271 amu and 24.22% are 37Cl with a mass of 36.96590260 amu.

11 Find the average atomic mass for Mg if 78.99% of Mg atoms are 24Mg with a mass of 23.9850419 amu, 10.00% are 25Mg with a mass of 24.9858370 amu, and 11.01% are 26Mg with a mass of 25.9825930 amu.

12) There are 2 isotopes of copper that occur naturally; 63Cu and 65Cu. The 63Cu atoms which occur 77.93 % of the time have a mass of 62.929601 amu and the 63Cu atoms have a mass of 64.927794 amu.

13) There are 2 isotopes of gallium that occur naturally; 69Ga and 71Ga. The 69Ga atoms which occur 19.32% of the time have a mass of 68.925581 amu and the 71Ga atoms have a mass of 70.924707 amu.

14) Rubidium has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb which has an atomic mass of 84.974 amu is 72.2% and the abundance of 87Rb which has an atomic mass of 86.973 amu is 27.8%, what is the average atomic mass of rubidium?

15) Uranium has three common isotopes. If the abundance of 234U which has an atomic mass of 233.878 amu is 0.01%, the abundance of 235U which has an atomic mass of 234.892 amu is 0.71%, and the abundance of 238U which has an atomic mass of 237.911 amu is 99.28%, what is the average atomic mass of uranium?

16) Titanium has five common isotopes: 46Ti (8.00%, 45.983 amu), 47Ti (7.80%, 46.988 amu), 48Ti (73.4%, 47.910 amu), 49Ti (5.50%, 48.923 amu), 50Ti (5.30%, 49.942 amu). What is the average atomic mass of titanium?

17) What is the average atomic mass of silver if 51.84% of silver atoms have a mass of 106.905 amu and 48.16% of them have a mass of 108.905 amu?

18) Find the average atomic mass of krypton using the data below:

|Isotope Mass (AMU) |Percentage Abundance (%) |

|77.920 |0.350 |

|79.916 |2.27 |

|81.913 |11.56 |

|82.914 |11.55 |

|83.912 |56.90 |

|85.911 |17.37 |

19) Calculate the average atomic mass of chromium, given the following percent abundances and isotopic masses.

|Isotope Mass (AMU) |Percentage Abundance (%) |

| | |

|49.946 |4.345% |

|51.941 |83.789% |

|52.941 |9.501% |

|53.939 |2.365% |

20) Calculate the average atomic mass of indium if 4.3% of its atoms are indium-113 with an atomic mass of 112.941 amu and 95.7% of its atoms are indium-115 with an atomic mass of 114.943 amu.

21) Explain why atoms have different isotopes. In other words, how is it that helium can exist in three different forms?

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Average Atomic Mass = Mass1 X % Isotope1 + Mass2 X % Isotope2 + (etc.)

100 100

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