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 AxBy, 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 AxBy. 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

percent composition

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 AxBy, consider the atomic mass of A is MA, the atomic mass of B is MB, 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 × MA) / MA X + MB 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 × MB) / MA X + MB 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 (Fe2On) of Iron contains 70% Iron by mass. Calculate the value of n?

Molecular mass of Fe2On = 56×2 + 16×n

Write the expression for mass % of Fe,

Mass % of Fe = Mass of Fe in Fe2On / 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+ + CO32-

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

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