# A sample of a substance containing only magnesium and chlorine was tested in the laboratory and was found to be composed of 74.5% chlorine by mass. If the total mass of the sample was 190.2 grams, what was the mass of the magnesium?

$\text{Mass of magnesium}$ $=$ 25.5%xx190.2*g=48.5*g
In a binary compound, $X Y$, if %X is given, then, clearly, %Y=100-X% with respect to mass.
So we take that figure and multiply it by the mass of the sample to give under $50 \cdot g$.