Chalcopyrite, the principal ore of copper (Cu), contains 34.63 percent Cu by mass. A particular sample of chalcopyrite contains 321.5g of copper. Determine the mass of the sample of chalcopyrite in units of grams ?

##"928.4 g"##

You know that chalcopyrite is ##34.63%## copper by mass, which essentially means that every ##"100 g"## of chalcopyrite contain ##"34.63 g"## of copper.

That is what by mass tells you. That ##%## sign means that you're looking at ##"100 g"## of a compound.

So, you know that you get ##"34.63 g"## of copper for every ##"100 g"## of chalcopyrite, and that the sample contains ##"321.5 g"## of copper.

You can use the ore's percent concentration by mass as a conversion factor to help you calculate the mass of ore that would contain that many grams of copper

##321.5 color(red)(cancel(color(black)("g Cu"))) * "100 g ore"/(34.63color(red)(cancel(color(black)("g Cu")))) = color(green)(bar(ul(|color(white)(a/a)color(black)("928.4 g ore")color(white)(a/a)|)))##

The answer is rounded to four .