# Percent Composition

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General Chemistry

In the topic of Percent Composition, it's crucial to understand empirical and molecular formulas. A compound's empirical formula is the simplest whole number ratio of the elements it contains, while the molecular formula is the actual number of atoms of each element in one molecule. The law of constant composition states that a pure sample of a compound will always contain the same elements in the same proportion by mass.

Percent composition is the percentage of a compound mass that is made up of a particular element. It can be calculated by multiplying that element's molar mass by the number of atoms of that element in the compound, then dividing by the compound's total molar mass and multiplying by 100%. Percent composition problems often involve finding the molecular formula when given the compound's molar mass and either the percent composition or the empirical formula.

Lesson Outline

<ul> <li>Introduction to percent composition, empirical and molecular formulas <ul> <li>Explanation of empirical formula and example with glucose (empirical formula: CH2O)</li> <li>Empirical formula does not show number of atoms in one molecule, only the ratio</li> <li>Molecular formula is a multiple of empirical formula, actual number of atoms in molecule (for glucose, molecular formla is C6H12O6)</li> </ul> </li> <li>Law of constant composition <ul> <li>Explanation of the law (a pure sample of a compound will always contain the same elements in the same proportion by mass) and its importance in understanding percent composition</li> <li>Only applies to pure samples</li> </ul> </li> <li>Calculating percent composition <ul> <li>Definition of percent composition: the percentage of a compound mass that is made up of a particular element</li> <li>Calculating percent composition using molar mass and number of atoms in the molecule</li> </ul> </li> <li>Percent composition problems <ul> <li>Finding molecular formula given percent composition and compound's molar mass</li> <ol> <li>Find out how much of compound's mass is made up of each element</li> <li>Determine number of atoms of each element in one molecule to make up the calculated masses</li> <li>Find subscripts in the molecular equation using the numbers obtained</li> </ol> <li>Finding molecular formula if given empirical formula and compound's molar mass</li> <ol> <li>Find the molar mass of the empirical formula</li> <li>Divide the compound's molar mass by the molar mass of empirical formula</li> <li>Multiply each subscript in empirical formula by the obtained number</li> </ol> </ul> </li> </ul> Don't stop here!

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FAQs

## What is the relationship between the empirical formula and the molecular formula when determining percent composition?

The empirical formula represents the simplest ratio of elements in a compound, while the molecular formula represents the actual number of atoms of each element in a molecule. Percent composition can be calculated using either the empirical or molecular formula, as it represents the percentage by mass of each element in a compound. In general, an empirical formula is used for obtaining percent composition when the molecular formula is unknown.

## How does the law of constant composition relate to percent composition in chemical compounds?

The law of constant composition states that a pure chemical compound always contains the same proportion of elements by mass. Percent composition is a direct application of this law, as it indicates the percentage by mass of each element in a compound. In any given sample of a pure compound, regardless of its size or origin, the percent composition of the elements will always be the same, illustrating the consistency expressed by the law of constant composition.

## How do you calculate the molar mass of a compound using the periodic table and molecular formula?

To calculate the molar mass of a compound, first identify the molecular formula of the compound. Then, use the periodic table to determine the atomic mass of each element involved. Multiply the atomic mass of each element by the number of atoms present in the compound and sum the results. The final value represents the molar mass of the compound, typically expressed in grams per mole (g/mol).

## How can you derive a compound's empirical formula from a pure sample with known compound mass?

To derive a compound's empirical formula from a pure sample with known compound mass, start by determining the mass of each individual element present in the sample. Using the periodic table, convert these masses to moles by dividing the mass of each element by its atomic mass. Next, divide the moles of each element by the smallest number of moles calculated among the elements to find the simplest whole-number ratio between the elements. The resulting ratio indicates the empirical formula of the compound, which represents the simplest ratio of elements in the compound.