We can calculate the molecular mass of lactose as follows:
#12 color(red)(cancel(color(black)("atoms C"))) × "12.01 u"/(1 color(red)(cancel(color(black)("atom C")))) = "144.12 u"#
#22 color(red)(cancel(color(black)("atoms H"))) × "1.008 u"/(1 color(red)(cancel(color(black)("atom H")))) =color(white)(l) "22.18 u"#
#11 color(red)(cancel(color(black)("atoms O"))) × "16.00 u"/(1 color(red)(cancel(color(black)("atom O")))) = "176.00 u"#
#"TOTAL"color(white)(mmmmmmmmll) = "342.30 u"#
The mass percentages of each element are:
#"% C" = (144.12 color(red)(cancel(color(black)("u"))))/(342.30 color(red)(cancel(color(black)("u")))) × 100 % = 42.10 %#
#"% H" = (22.18 color(red)(cancel(color(black)("u"))))/(342.30 color(red)(cancel(color(black)("u")))) × 100 % =color(white)(ll) 6.48 %#
#"% O" = (176.00 color(red)(cancel(color(black)("u"))))/(342.30 color(red)(cancel(color(black)("u")))) × 100 % =color(white)(l) 51.42 %#
The composition of lactose is #"42.10 % C, 6.48 % H"#, and #"51.42 % O"#.
