The percent composition of chromium in BaCrO4 is 20.5 percent. The percent composition is determined by dividing the mass of the desired component by the mass of the entire molecule and multiplying by 100 percent.
The masses are determined using the molar masses found on a periodic table. BaCrO4 has 1 mole of barium (137.3 g), 1 mole of chromium (52.0 g) and 4 moles of oxygen (4 x 16.0 g = 64.0 g). The total mass is all the masses added together (137.3 g + 52.0 g + 64.0 g = 253.3 g) The mass of just chromium is then divided by the total mass (52.0 g/253.3 g = 0.205). A decimal can be converted to percent by multiplying by 100 (0.205 x 100% = 20.5%).
This method can be used to determine the percent composition of a molecule of any mass.