Formula to Percent Composition

Question

1. The percent by mass of silver in AgCl is ____%
2. The percent by mass of oxygen in C2H6O is ____%. (4 sig figs)
3. Calculate the percentage composition for SO2 .

Mass percentage of sulfur = %

Mass percentage of oxygen = %

Answer 1

Ag = 107.9 amu

Cl = 35.5 amu

AgCl = 143.4 g/mole

%Ag (by mass) = 107.9/143.4 (x100%) = 75.24%

C = 12.01 x 2 = 24.02

H = 1.008 x 6 = 6.048

O = 15.99 x 1 = 15.99

C₂H₆O = 46.06 g/mol

%O (by mass) = 15.99/46.06 (x100%) = 34.72%

For SO₂:

S = 32.1

O = 15.99 x 2 = 31.98

SO₂ = 64.08 g/mol

%S = 32.1/64.08 (x100%) = 50.09%

$O = 31.98/64.08 (x100%) = 49.91%

Answer 2

Percent mass is considered

(mass of part)/(mass of whole) * 100%

You need to take into account the masses that make up the compound for each. If you consider 1 mole of each compound, it will make this simple.

1 mole AgCl is made up of 1 mole of Ag and 1 mole of Cl

1 mole of Ag = 107.87 g

1 mole of Cl = 35.45 g

so, 1 mole of AgCl = mass of Ag + mass of Cl = 107.87 + 35.45 = 143.32 g

Percent mass of silver is: (mass of silver)/(mass of compound) * 100%

107.87/143.32 * 100% = 75.3%

A similar approach to each other question, taking into account the number of each element.

1 mole of C₂H₆O is made up of:

2 moles of C = 2 * 12.01g/mol = 24.02 g

6 moles of H = 6 * 1.008 g/mol = 6.048 g

1 mole of O = 1 * 16 g/mol = 16

Total mass of C₂H₆O = mass of C + mass of H + mass of O = 24.02 + 6.048 + 16 = 46.068 g

Percent mass of oxygen is: (mass of oxygen)/(mass of compound) * 100%

16/46.068 * 100% = 34.73%

Use the same approach for the last one to obtain percent mass of each element.

They all must add up to 100%, so if you find one, you can subtract from 100% to get the other.