Question: An oxide of iron contains 70.0% iron and 30.0% oxygen by mass. Find its empirical formula. (Ar: Fe = 56, O = 16)Step 1: divide each percentage by the relative atomic mass. Using percentages is the same as taking a 100 g sample, so the numbers are masses in grams.Fe: 70.0 ÷ 56 = 1.25 O: 30.0 ÷ 16 = 1.875 Step 2: divide every value by the smallest.Fe: 1.25 ÷ 1.25 = 1 O: 1.875 ÷ 1.25 = 1.5 Step 3: convert to whole numbers. A ratio of 1 : 1.5 is not a whole number ratio, and it must not be rounded to 1 : 2. Multiply both numbers by 2.Fe : O = 2 : 3 Answer: the empirical formula is Fe2O3.Tips• Only round a value if it is within about 0.1 of a whole number, for example 1.98 becomes 2.• Learn the common fractions: a ratio ending in .5 means multiply by 2, one ending in .33 or .67 means multiply by 3, and one ending in .25 or .75 means multiply by 4.• Keep at least three significant figures in the mole values so that rounding errors do not change the ratio.Check: Fe2O3 has Mr = 160, and (2 × 56) ÷ 160 × 100% = 70.0% iron, which matches the data.
Example Question - 1
Read the text
Read the text and highlight anything you think is important. When you go on, the text is hidden and you answer from memory.
Specification: Edexcel Chemistry (8CH0), AS-level, Topic 5: Formulae, Equations and Amounts of Substance. Topic: Amount of Substance and Calculations.
Key terms in this lesson
- empirical formula
- The simplest whole number ratio of atoms of each element in a compound.
More in Amount of Substance and Calculations
- Empirical Formula
- Percentage Composition by Mass
- Combustion Analysis
- Molecular Formula
- Example Question - 2
- Example Question - 3
- Example Question - 4
- Avogadro's Law
All 39 lessons in Amount of Substance and Calculations · All Edexcel AS-level Chemistry topics