A compound has a molecular mass of #"180 u"#, contains six carbon atoms and six oxygen atoms, and an unknown element, #"Z"#. What is the unknown element and what is the chemical formula of the compound?

1 Answer
Oct 9, 2017

The chemical formula is #"C"_6"H"_12"O"_6"#, which a hexose such as glucose, fructose, and galactose.

Explanation:

Determine relative atomic mass of the #6# atoms of carbon #("C")# and the #6# atoms of oxygen #("O")#, and add the results.

#"C":##6xx"12 u"="72 u"#

#"O":##6xx"16 u"="96 u"#

Sum of the relative atomic masses of #"C"# and #"O"#.

#"72 u +96 u =168 u"#

Subtract #"168 u"# from #"180 u"#. The result is the total mass of #"Z"# in the compound.

Mass of #"Z"##=##"180 u"-"168 u"="12 u"#

Relative atomic mass of #"Z" = (color(red)cancel(color(black)(12)) "u")/color(red)cancel(color(black)(12))="1 u"#

We divide by #12# because the relative mass of a substance is relative to carbon-12.

The element with the relative atomic mass of #"1 u"# is hydrogen #("H")#.

The chemical formula is #"C"_6"H"_12"O"_6"#.