To get the percent by mass of a mixture you divide the mass of each component by the total mass and multiply by 100.
##Percent by mass = mass of component/total mass 100 %##
EXAMPLE:
A mixture contains 1.203 g ##CaCO_3## and 1.797 g ##NaCl##. What is the percent by mass of each component?
Solution
##Total mass = (1.203 + 1.797) g = 3.000 g##
##% by mass CaCO_3 = 1.203 g/3.000 g 100 % = 40.10 %##
##% by mass NaCl = 1.797 g/3.000 g 100 % = 59.90 %##