Final answer:
To find the percent composition by mass of phosphorous acid, you calculate the molar mass of the compound, divide by the molar mass for each element, and translate that into percentages. Upon calculation, none of the student-provided options matches the correct percentages for hydrogen, phosphorus, or oxygen.
Step-by-step explanation:
To calculate the percent composition by mass of each element in phosphorous acid, first consider the chemical formula of phosphorous acid, which is H3PO3. This means there are 3 hydrogen (H) atoms, 1 phosphorus (P) atom, and 3 oxygen (O) atoms in one molecule of phosphorous acid.
To calculate the percent composition by mass:
- Find the molar mass of each element in the compound.
- Multiply the atoms of each element by their atomic mass.
- Add the individual masses to get the molar mass of the compound.
- Divide the mass of each element by the molar mass of the compound to get the fraction of the mass that each element contributes.
- Multiply by 100 to convert to a percentage.
Specifically, the molar mass of hydrogen is approximately 1 g/mol, phosphorus is approximately 31 g/mol, and oxygen is approximately 16 g/mol. The molar mass of phosphorous acid is:
3(1 g/mol H)+1(31 g/mol P)+3(16 g/mol O) = 3+31+48 = 82 g/mol
Thus, the percent compositions are:
- Hydrogen: (3 g / 82 g/mol) x 100% ≈ 3.66%
- Phosphorus: (31 g / 82 g/mol) x 100% ≈ 37.80%
- Oxygen: (48 g / 82 g/mol) x 100% ≈ 58.54%
None of the options given by the student are correct. Make sure that the percentages add up to 100%, which they do here.