Problem calculating relative molecular mass [closed]








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I am starting be re-learn Chemistry after several years.


The Book I am using has a test question to calculate the relative atomic mass of:


$ (CH\_3CO)\_2O $


My take


Carbon C, $ A\_r = 56 $   

Hydrogen H, $ A\_r = 1 $   

Oxygen O, $ A\_r = 16 $


$ M\_r = ( 56 + 1 \times 3 + 56 + 16 ) \times 2 + 16 = 131 \times 2 + 16 = 278 $


This is nothing like the answer given in the answer key. Is there an error in my text book?


If not where am I going wrong?




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**Edit:** Problem was a misprint in my text book. See correct answer below




Solved. The problem was a typing error in the text book $ A\_r $ for carbon should of course be 12 not 56. This is given correctly in the periodic table shown on the back cover but was misprinted in the body of chapter 1 of my book.


Thanks to Zenix for pointing this out in comments.


$ M\_r $ of $ (CH\_3CO)\_2O $


Carbon C, $ A\_r = 12 $   

Hydrogen H, $ A\_r = 1 $   

Oxygen O, $ A\_r = 16 $


$ M\_r = ( 12 + 1 \times 3 + 12 + 16 ) \times 2 + 16 = 43 \times 2 + 16 = 102$


This matches the answer given in the answer key.


I have not studied chemistry in over 30 years which explains why 56 as a relative atomic mass for Carbon did not scream out as being wrong.


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