% Comp and Empirical, Molecular Formulas

What is Percent Composition?

Percent composition expresses the relative amounts of the elements within a compound.    The different elements should eventually add up to 100%.

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How Can % Comp be Calculated?

Percent composition can be calculated two ways depending on how the information is presented. It can be calculated based on given mass data or based on its chemical formula. 

• Percent Composition, Molecular Formula, and Empirical Formula

% Comp Based on Given Mass

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When given the mass of the compound and the mass of at least one of the elements, the percent composition can be calculated.    The percent composition of a certain element in a compound can be found by dividing the mass of the individual element by the mass of the entire compound.   This will reveal what percent of the compound is taken up by that element. When there is only one unknown composition left, its percent composition can be found by subtracting the composition(s) of the other element(s) from 100%.

Caption: : The percent composition of the element's mass can be calculated by dividing the compound's mass by the mass of the individual element.

% Comp Based on Chemical Formula

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Percent composition can be calculated if the chemical formula is known. By using the molar masses of the elements (found on the periodic table), the molar mass of the entire compound can be found. [Make sure to pay attention to subscripts when calculating molar mass!]    To find the percent composition of the elements by molar mass, just divide the molar mass of the element by the molar mass of the entire compound.   This will reveal the amount of the individual element within that compound relative to the compound's other elements out of 100%. When there is only one unknown composition left, its percent composition can be found by subtracting the composition(s) of the other element(s) from 100%.

Caption: : To calculate the percent composition from molar mass, divide the molar mass of the entire compound by the molar mass of the individual element.

What is Empirical Formula?

• Percent Composition, Molecular Formula, and Empirical Formula

The empirical formula is the lowest whole-number ratio of the atoms of an element in a compound. 

How to Calculate Empirical Formula

When the percent composition of the compound is known, assume that 100g of the compound are present. Therefore, 54% = 54g etc.  ___________________________________________________________________ 1. Use the values of the composition of the elements and convert them to moles by dividing by that element's molar mass. 2. Divide both mole values by the smallest mole value achieved in step 1.  3. If the values still are not whole numbers, multiply each value by the smallest number achieved in step 2.  4. The whole-number mole values become the subscripts for each element in the empirical formula. 

Empirical Formula and Molecular Formula Introduction (Embed)

Caption: : Here is a helpful introduction video^^

How to Calculate Molecular Formula

When dealing with ionic compounds, the empirical formula is also the molecular formula. That is not the case, however, for covalent compounds. ___________________________________________________________________ 1. Find the molar mass of the empirical formula using the periodic table. You will be given the molar mass of the entire compound. 2. Divide the molar mass of the entire compound by the molar mass of the empirical formula.  3. Multiply the subscripts of the empirical formula by the number achieved from step 2 to find the molecular formula.

Calculating Molecular Formula from Empirical Formula (Embed)

Caption: : Here is a helpful video with examples^^