Reading: Empirical Formula Introduction
Think back to math class, when you were first learning about fractions. Think about the fraction 64/800. Is that the best way to represent the fraction? Would 8/100 be better? Or 4/50? Oh, 2/25. They are all equivalent, but 2/25 is the reduced form. Justify this statement.
In chemistry, the same line of thinking is not true. Glucose will always exist as C6H12O6, it will never exist as CH2O (although now does it make sense why it’s called a carbohydrate?).
Because the subscripts represent actual atoms that are bonded together, it’s not possible to “reduce” a chemical formula. But CH2O is called the empirical formula of glucose – it simply represents the ratio of elements present in the compound. The formula C6H12O6 is called the molecular formula – it represents the actual atoms in the compound.
Sometimes when chemists do an analysis of an unknown sample, they are able to detect the molar ratios of elements in the sample, which allows them to find the empirical formula. They need to gather more information to determine the molecular formula. Here’s an example:
A chemist analyzed a white powder and found the sample contained 66.0 g of carbon, 87.9 g of oxygen, and 11.1 g hydrogen. What is the empirical formula of the compound?
Further tests indicated that this white powder is NOT glucose. But it was determined the molar mass of the sample is 180 g/mol. What is the empirical formula? More ratios are needed to solve this problem.
After further analysis, it was determined that this white powder is a different form of sugar called fructose – the sugar found in fruit. It has the same empirical formula and molecular formula as glucose, but its structure is different. Glucose and fructose are isomers of each other.
Another helpful way to express data for composition of compounds is percent by mass. This is no different than finding the percent of anything else – you take the part you’re interested in and divide it by the whole thing. So in this case, we’re dealing with atomic and molecular masses.
UNIT 5: CHEMICAL REACTIONS
SECTION 5C: THE MOLE
1. What is a mole?
a. Reading: Intro to Mole
b. Infographic: Mole
c. Developing Skills: Molar Mass
d. Reading: Measure of a Mole
e. AACT Lab: Calculating Moles
2. Why do chemists use it?
a. Reading: Empirical Formula
b. Reading: % Composition
c. Developing Skills: % Composition
d. AACT Activity: Empirical Formula End of Section Questions