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Student can determine the empirical and molecular formulas of a compound from mass percent data.
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Formula for finding mass percentage of elements.
Want to check the purity of a substance or figure out its exact makeup? You need percentage composition.
It tells us the mass percentage of each element present in a compound.
The Formula:
Distinguish the simplest ratio from the exact atom count.
Chemical formulas tell us what's inside a molecule, but they come in two distinct types.
1. Empirical Formula: Represents the simplest whole number ratio of various atoms present in a compound.
Think of it as the basic recipe ratio. For example, the empirical formula of glucose is .
Problem 1.2: Finding empirical and molecular formula.
Problem. A compound contains 4.07% hydrogen, 24.27% carbon and 71.65% chlorine. Its molar mass is 98.96 g. What are its empirical and molecular formulas?
Step 1. Conversion of mass per cent to grams
Assume a 100 g sample of the compound. This directly turns percentages into masses:
Exercise 1.3: Empirical formula of an iron oxide.
To find the empirical formula of the iron oxide, we first assume a 100 g sample. This means we have 69.9 g of iron and 30.1 g of oxygen. We convert these masses to moles using the atomic masses.
For iron (55.85 g/mol), this calculation gives approximately moles of Fe. For oxygen (16.00 g/mol), it gives about 1.88 moles of O.
Next, we divide each mole value by the smallest number of moles, which is . This gives a molar ratio of 1 for Fe and 1.5 for O.
Since 1.5 is not a whole number, we must multiply both numbers by to get the simplest whole number ratio. This provides the final empirical formula of Fe2O3.
Check understanding of converting fractional mole ratios to whole numbers.
During an empirical formula calculation, dividing the moles of each element by the smallest mole value yields a ratio of to . What is the correct empirical formula?