Final answer:
The percent composition by mass of magnesium (Mg) in magnesium oxide (MgO) is 60.3%, and the composition of oxygen (O) is 39.7%, calculated from their molar masses and the molar mass of MgO.
Step-by-step explanation:
Percent Composition by Mass of O and Mg in MgO
To calculate the percent composition by mass of magnesium (Mg) and oxygen (O) in magnesium oxide (MgO), you need to know the molar mass of both Mg and O and the molar mass of MgO. The atomic mass of Mg is approximately 24.31 g/mol and that of O is approximately 16.00 g/mol. The molar mass of MgO is the sum of the molar masses of Mg and O, which is 24.31 g/mol + 16.00 g/mol = 40.31 g/mol.
To find the percent composition of Mg in MgO, divide the molar mass of Mg by the molar mass of MgO and multiply by 100:
(24.31 g/mol Mg / 40.31 g/mol MgO) × 100 = 60.3%
To find the percent composition of O in MgO, divide the molar mass of O by the molar mass of MgO and multiply by 100:
(16.00 g/mol O / 40.31 g/mol MgO) × 100 = 39.7%
Subtract to find the mass of oxygen in the compound. If you have a sample mass of MgO and the mass of Mg available, like in the provided exercise where a 20.00 g sample leaves 16.07 g of zinc after decomposition, subtract the mass of zinc from the total mass to get the mass of oxygen. Then, divide the respective masses by the total mass and multiply by 100 to find the percent by mass for each element.
In summary, for a sample of MgO, the mass of Mg and O are used along with the molar masses to calculate their percent compositions in the compound.