Name: _____________________________ ( ) Class: _________ Date: __________

Percentage composition by mass of an element in a compound = A r of element x number of atoms in formula X 100% Relative molecular mass (M r) of compound

Worked Example 1The percentage composition by mass of nitrogen and hydrogen present in ammonia can be calculated from its chemical formula NH3.

Step 1:Relative molecular mass of ammonia = 1 x A r of N + 3 x A r of H

= 14 + 3= 17

Step 2:Percentage composition of nitrogen in ammonia = A r of nitrogen x number of nitrogen atoms in formula X 100% relative molecular mass (M r) of ammonia = 14 x1 x 100% 17= 82.4 %

Step 3: Percentage composition of hydrogen in a ammonia = A r of hydrogen x number of atoms in formula X 100% relative molecular mass (M r) of ammonia= 1 x 3 x 100% 17= 17.6 %

Worked Example 2Calculate the percentage composition by mass of hydrogen and oxygen present in hydrogen peroxide (H2O2)

Step 1:

Step 2:

Step 3:

1

Secondary 3 Express Chemistry (with SPA)Chapter 9: The MoleNotes 3 (Percentage Composition of Compound)

Worked Example 3Calculate the percentage of water in copper(II) sulphate crystals (CuSO4.5H2O).Step 1: Mr of copper(II) sulphate crystals =

Step 2: Mr of water =

Step 3: Percentage of water in CuSO4.5H2O

=

=

=

Mass of an element in a compound= Percentage composition by mass of an element in a compound x Mass of compound

Worked Example 4: Calculate the mass of calcium in 25g of calcium nitrate Ca(NO3)2

Step 1:

Step 2: =

=

=

Step 3:= =

Worked Example 5: Calculate the mass of water in 54.1g of FeCl3. 6H2OStep 1: Mr of FeCl3. 6H2O

Step 2: Mr of water, H2O

Step 3: Percentage composition by mass of water in FeCl3. 6H2O= Mr of water x No. of water molecules in formula X 100% M r of FeCl3. 6H2O

Step 4: Mass of water in 54.1g of FeCl3. 6H2O=

2

Practice Questions1. Calculate the percentage of: (a) copper in copper(II) carbonate, CuCO3

(b) nitrogen in zinc nitrate Zn(NO3)2

(c) water in CoCI2•6H2O

(d) water in SnCl2•2H2O -

3

2. Calculate the mass of

(a) chlorine in 27g of lead(II) chloride PbCl2

(b) aluminium in 36g of Al2(SO4)3

(c) water in 12.3g of MgSO4.7H2O

4

(d) water in 34.5g of CuSO4.5H2O

5