Percent Composition | Formula with Examples

What is the Percent Composition of a Compound tell you?

Percent composition is the percentage by mass of each element present in a compound. Assuming that we have a compound A_xB_y, what is the mass percentage of A and B? If we find the mass percentage of both A and B, you can easily express the percent composition of the compound A_xB_y. But the question is here how do we find the mass percentage of each element present in a compound.

How do you find the Mass Percent?

The mass percent of an element is the mass of an element in a given compound divided by the molecular mass of the compound multiplied by 100.

Mass % of an element = Mass of an element in a given compound / molecular mass × 100

How do we find the mass of an element?

Mass of an element = Number of atoms of that element × Atomic mass

By using the formula, we can easily calculate the percent composition. As we assume the compound A_xB_y, consider the atomic mass of A is M_A, the atomic mass of B is M_B, the number of atoms of A is xA, and the number of atoms of B is xB. First, we calculate the mass % of A,

Mass % of A = Mass of A in a given compound / molecular mass × 100

Mass % of A = (Number of atoms of A × Atomic mass) / molecular mass × 100

Mass % of A = (xA × M_A) / M_A X + M_B Y × 100

Similarly, we calculate the mass % of B,

Mass % of B = Mass of B in a given compound / molecular mass × 100

Mass % of B = (Number of atoms of B × Atomic mass) / molecular mass × 100

Mass % of B = (xB × M_B) / M_A X + M_B Y × 100

• The sum of mass % of A and mass % of B is 100.
• If we calculate the mass % of A, the mass % of B will be found by subtracting the mass % of A from 100.

Mass % of B = 100 – mass % of A

What is the Percent Composition of H₂O?

Water is an inorganic compound having hydrogen and oxygen atom. The molecular mass of a water molecule is 18. Now we calculate the percent composition of water.

Mass % of H = (Number of atoms of H × Atomic mass) / molecular mass × 100

Mass % of H = (2 × 1) / 18 × 100

Mass % of H = 11.11%

Mass % of O = (Number of atoms of O × Atomic mass) / molecular mass × 100

Mass % of O = (1 × 16) / 18 × 100

Mass % of O = 88.89%

The sum of mass % of H and mass % of O is 100. (11.11% + 88.89%)

How do you find the Percent Composition of Glucose?

The molecular formula of Glucose is C₆H₁₂O₆ having a molecular weight of 180. Now we calculate the percent composition of glucose.

Mass % of C = (Number of atoms of C × Atomic mass) / molecular mass × 100

Mass % of C = (6 × 12) / 180 × 100

Mass % of C = 40%

Mass % of H = (Number of atoms of H × Atomic mass) / molecular mass × 100

Mass % of H = (12 × 1) / 180 × 100

Mass % of H = 6.66%

Mass % of O = (Number of atoms of O × Atomic mass) / molecular mass × 100

Mass % of O = (6 × 16) / 180 × 100

Mass % of O = 53.33%

How do you calculate Urea Percent Composition?

The molecular formula of Urea is NH₂CONH₂ having a molecular mass of 60.

Mass % of N = (Number of atoms of N × Atomic mass) / molecular mass × 100

Mass % of N = (2 × 14) / 60 × 100

Mass % of N = 46.67%

Mass % of H = (Number of atoms of H × Atomic mass) / molecular mass × 100

Mass % of H = (4 × 1) / 60 × 100

Mass % of H = 6.67%

Mass % of C = (Number of atoms of C × Atomic mass) / molecular mass × 100

Mass % of C = (1 × 12) / 60 × 100

Mass % of C = 20%

Mass % of O = (Number of atoms of O × Atomic mass) / molecular mass × 100

Mass % of O = (1 × 16) / 60 × 100

Mass % of O = 26.67%

An oxide (Fe₂O_n) of Iron contains 70% Iron by mass. Calculate the value of n?

Molecular mass of Fe₂O_n = 56×2 + 16×n

Write the expression for mass % of Fe,

Mass % of Fe = Mass of Fe in Fe₂O_n / molecular mass × 100

Mass % of Fe = (Number of atoms of Fe × Atomic mass) / molecular mass × 100

Mass % of Fe = (56×2) / (56×2 + 16×n) × 100

70 = (56×2) / (56×2 + 16×n) × 100

n = 3

Hence the formula for Iron oxide is Fe₂O₃.

If mass % of Oxygen in divalent metal “M” carbonate (MCO₃) is 48%. Find the atomic mass of metal?

MCO₃ → M²⁺ + CO₃^2-

Write the expression for mass % of oxygen,

Mass % of O = Mass of O in MCO₃ / molecular mass × 100

Mass % of O = (Number of atoms of O × Atomic mass) / molecular mass × 100

Mass % of O = (2 × 16) / (x + 12×1 + 16×3) × 100

48 = (2 × 16) / (x + 12×1 + 16×3) × 100

n = 40

The atomic mass of metal “M” is 40.

Na₂SO₄.xH₂O has 50% H₂O by mass. Find the value of x?

Mass % of H₂O = Mass of H₂O in Na₂SO₄.xH₂O / molecular mass × 100

Mass % of H₂O = (Number of atoms of H₂O × Atomic mass) / molecular mass × 100

Mass % of H₂O = ( x × 18) / (2×23 + 32 + 4×16 +18x) × 100

(18x) / (2×23 + 32 + 4×16 +18x) × 100 = 50

(18x) / (2×23 + 32 + 4×16 +18x) × 2 = 1

36x = 78 + 64 + 18x

36x – 18x = 142

18x = 142

x = 7.9

x ≈ 8

The water molecules in given compound is 8.

Test Yourself

Polystyrene having the formula Br₃C₆H₃(C₃H₈)_n was prepared by heating styrene with tribromobenzoyl peroxide in the absence of air. If it was found to contain 10.46% of bromine by mass. Find the value of n?

Answer ≈ 19

If the percentage of water of crystallization in MgSO₄.xH₂O is 13%. What is the value of x?

Answer = 1

A hydride of Iron (III) thiocyanate Fe(SCN)₃.xH₂O was found to contain 19% water. What is the formula for the hydrate?

Answer = 3