Empirical, Molecular and Percent Comp.

Empirical, Molecular and Percent Comp.

Analyze quantitatively the composition of a substance (empirical formula, molecular formula, percent composition, and hydrates).

Molecular Formulas

give the kind and number of atoms of each element present in a molecular compound.

Calculate the empirical formula mass (EFM), which is simply the molar mass represented by the empirical formula. Divide the molar mass of the compound by the empirical formula mass. The result should be a whole number or very close to a whole number. Multiply all the subscripts in the empirical formula by the whole number found in step 2. The result is the molecular formula.

Multiple Choice

If the empirical formula of a compound is known, what is needed to determine the molecular formula?

• Coordination numbers

• Molar Mass

• Density

• Molecular Geometry

The empirical formula of a compound of boron and hydrogen is BH₃. Its molar mass is 27.7 g/mol. Determine the molecular formula of the compound.

Step 1:List the known quantities and plan the problem.

Step 2:Plug in

Step 3:Calculate.

Multiple Choice

A compound has an empirical formula of CH and a molar mass of 39g/mol. What is its molecular formula?

• C4H4

• C3H3

• C2H2

• CH

Empirical Formulas

shows the lowest whole-number ratio of the elements in a compound.

Steps to find the Empirical Formula

• Assume a 100 g sample of the compound so that the given percentages can be directly converted into grams.

• Use each element’s molar mass to convert the grams of each element to moles.

• In order to find a whole-number ratio, divide the moles of each element by whichever of the moles from step 2 is the smallest.

• If all the moles at this point are whole numbers (or very close), the empirical formula can be written with the moles as the subscript of each element.

In some cases, one or more of the moles calculated in step 3 will not be whole numbers.  Multiply each of the moles by the smallest whole number that will convert each into a whole number.  Write the empirical formula.

A compound of iron and oxygen is analyzed and found to contain 69.94% iron and 30.06% oxygen. Find the empirical formula of the compound.

1.List the known quantities and plan the problem.

2.Assume its a 100 g sample!

3.Plug in and calculate!

Multiple Choice

A formula with the lowest whole # ratio of elements in a compound is called

• Molecular Formula
• Chemical Formula
• Empirical Formula 
• Distance Formula

Multiple Choice

You are given the following percentages: 40.05% S and 59.95% O. Find the empirical formula for these elements.

• SO
• SO2
• SO3
• SO4

Percent Composition

percent by mass of each element in a compound

A certain newly synthesized compound is known to contain the elements zinc and oxygen. When a 20.00 g sample of the sample is decomposed, 16.07 g of zinc remains. Determine the percent composition of the compound.

Step 1:List the known quantities and plan the problem.

Step 2:Calculate.

Step 3:Does this make sense?

Multiple Choice

What is the percent composition by mass of oxygen in the compound MgSO₄ (gram formula mass = 120 g/mol)?

• 20%
• 27%
• 46%
• 53%

Multiple Choice

What is the percent composition by mass of magnesium in the compound MgSO₄ (gram formula mass = 120 g/mol)?

• 20%
• 27%
• 46%
• 53%

Multiple Choice

What is the percent composition by mass of sulfur in the compound MgSO₄ (gram formula mass = 120 g/mol)?

• 20%
• 27%
• 46%
• 53%

Hydrate

A hydrate is a compound that has one or more water molecules bound to each formula unit.