Determine the percent by mass of S in Al2(SO4)3

Question

Determine the percent by mass of S in Al2(SO4)3

Answer 1

When asked to find mass percent, what you are being asked to do is determine "how much of a larger compound is made up of a certain element" (in this case sulfur). In the compound Al₂(SO₄)₃ there are 2 Al atoms, 3 S atoms, and 12 O atoms. Therefore, what you are calculating is what percentage of the total mass of the compound is made up from the mass of the three sulfur atoms.

Al: (mass of 26.99 * 2) = 53.98

S: (mass of 32.06 * 3) = 96.18

O: (mass of 16 * 12) = 192

The equation for mass percent is as follows.

Mass % = (mass of element of interest / Mass of entire compound) * 100%

Mass % = (mass of S / (mass of Al + mass of S + mass of O)) * 100%

Mass % = (96.18 / (53.98 + 96.18 + 192)) * 100%

Mass % = 28.11%

Answer 2

Hello, Kiyan,

We need the molecular weights of both the Al₂(SO₄)₃ and sulfur, S.

Al₂(SO₄)₃ = 342.2 amu

S = 32 amu

We have 3 sulfurs per molecule, so the total contribution of sulfur to the 342.2 is 3 x 32 = 96 amu. The percentage is thus (96amu)/(342.2 amu) = 0.28 or 28%.

Bob