## 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_{x}B_{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_{x}B_{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_{x}B_{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 H_{2}O?

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 C_{6}H_{12}O_{6} 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 NH_{2}CONH_{2} 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 (Fe_{2}O_{n}) of Iron contains 70% Iron by mass. Calculate the value of n?

Molecular mass of Fe_{2}O_{n} = 56×2 + 16×n

Write the expression for mass % of Fe,

Mass % of Fe = Mass of Fe in Fe_{2}O_{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 Fe_{2}O_{3}.

### If mass % of Oxygen in divalent metal “M” carbonate (MCO_{3}) is 48%. Find the atomic mass of metal?

MCO_{3} **→** M^{2+} + CO_{3}^{2-}

Write the expression for mass % of oxygen,

Mass % of O = Mass of O in MCO_{3} / 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.

### Na_{2}SO_{4}.xH_{2}O has 50% H_{2}O by mass. Find the value of x?

Mass % of H_{2}O = Mass of H_{2}O in Na_{2}SO_{4}.xH_{2}O / molecular mass × 100

Mass % of H_{2}O = (Number of atoms of H_{2}O × Atomic mass) / molecular mass × 100

Mass % of H_{2}O = ( 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 Br_{3}C_{6}H_{3}(C_{3}H_{8})_{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 MgSO_{4}.xH_{2}O is 13%. What is the value of x?

**Answer** **= 1**

### A hydride of Iron (III) thiocyanate Fe(SCN)_{3}.xH_{2}O was found to contain 19% water. What is the formula for the hydrate?

**Answer** **= 3**