1.3: Elemental Composition of Pure Substances

Learning Targets/ Content Objectives:

I can understand that each substance is represented by a unique combination of atoms called the chemical formula.
I can explain the quantitative relationship between the elemental composition by mass and the empirical formula of a pure substance.
I can differentiate between the empirical formula and the molecular formula of a substance.
I can use the percent composition by mass of a pure substance to determine its empirical and molecular formulas.
I can use lab data for percent composition and percentage by mass of a substance to find its empirical formula.
I can determine the empirical formula of a hydrate using evaporated amount of water in a lab.
I can use combustion analysis data to determine the empirical formula of a carbon containing substance.

Vocabulary:

Empirical formula- molecular formula- percent by mass- hydrate- anhydrate- anhydrous compound- precipitate- combustion- combustion analysis- excess.

Combustion Analysis of a Hydrocarbon

Combustion analysis is a technique used to determine the empirical formula of a hydrocarbon or compounds that contain hydrogen, carbon and oxygen. It takes place by combusting (burning) the compound in the presence of excess oxygen and analyzing the amount of water vapor and carbon dioxide produced. Then, the amount of CO2 produced is used to find the number of moles of Carbon atoms in the compound and H2O produced is used to find the number of moles of Hydrogen atoms in the compound. atoms and then figure out the number of Oxygen atoms and find the empirical formula. Below are some examples on combustion analysis.

This video explains combustion analysis and explains how to find the empirical formula of a Hydrocarbon given combustion analysis data.

Check your understanding

12.915 g of an organic substance containing only carbon, hydrogen, and oxygen was burned in an atmosphere of excess oxygen. Subsequent analysis of the gaseous result yielded 18.942 g carbon dioxide and 7.749 g of water. Determine the empirical formula of the substance.

the empirical formula of the compound is CH2O

Here how you do the problem step by step

Gravimetric Analysis to Find the Formula of a Hydrate ( Anhydrous Solid. n H2O)

Gravimetric analysis is a method used to find the formula of a hydrate salt. The formula of a hydrate can be determined by dehydrating a known mass of the hydrate, then comparing the masses of the original hydrate and the resulting anhydrous solid. The mass of water evaporated is obtained by subtracting the mass of the anhydrous solid from the mass of the original hydrate

mH2O = m Hydrate − m Anhydrous Solid

After you find the mass of water, you convert the mass water to moles water using the molar mass as a conversion factor.

Finally, you divide the number of moles of water by the number of moles of the anhydrous solid

Determining the formula of a hydrate lab

Watch this Youtube video to understand how we can determine the formula of a hydrate in a lab. The video also shows how to perform the calculation after data collection from the lab.

Practice On Your Own/ Check for Understanding

The following data were collected from the gravimetric analysis of a Copper (II) Sulfate Hydrated salt:

mass of crucible and lid: 36.528 g

mass of crucible, lid and hydrated salt 42.342 g

mass of crucible, lid and anhydrous salt 40.244 g

Determine the formula of the hydrate.

mass H2O= 42.342-40.244= 2.098 grams H2O

mass CuSO4= 40.244- 36.528= 3.7157 g CuSO4

Convert grams to moles by dividing by the molar mass

2.098 g H2O/ 18.02 = 0.1164 mole H2O

2.778 g CuSO4 / 159.61 = 0.02328 moles CuSO4

Divide moles water by moles CuSO4

0.1164/0.02328= 5 therefore, the formula of the hydrated salt is CuSO4.5H2O and its name is Copper (II) Sulfate pentahydrate

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