A sample of a compound that has a mass of 0.432g is analyzed. The sample is found to be made up of oxygen and fluorine. Given that the sample contains 0.128 of oxygen, calculate the percentage composition of the compound. Since you know the mass of oxygen (0.128g), and you know the total mass (0.432g), then you know the amount of fluorine. Subtract the oxygen from the total:
0.432 - 0.128 = 0.304g F
Now convert these to moles:
0.128g O(1 mole O / 16g O) = 0.008 moles O
0.304g F(1mole F / 19g F) = 0.016 moles F
Therefore, the total moles of the compund is 0.024 moles. Now divide th parts from the whole and multiply by 100 to find the percentage.
0.008 / 0.024 = 0.333 O
0.016 / 0.024 = 0.666 F
Therefore, the compound is 33.3% O and 66.7% F.
Make sure to take molar ratios, because the elements have different masses.
Hope that helps!