Question

This problem has been kicking my rear for a while now and I am just tired of looking at this thing... I would greatly appreciate the help!

What is the percent composition of compound that has a total mass of 63.9 g that contains 51.0 g Cu and 12.9 g O?

Answer 1

percent composition means what percentage by mass of the compound is a certain element in that compound

Answer 2

you have a total mass of the compound which is 63.9g and so it is a 100% of the compound
now how many percents is 51.0 g (of Cu)?
and how many is 12.9 g (of O)?
do basic math
\[63,9g \rightarrow 100%\]
\[51.0g \rightarrow ~?\]