Molecular Formula Calculations

Unit 5 Section 3: Determining Empirical and Molecular Formulas

Multiple Choice

What is the definition of molar mass?

• the number of grams per one mole of a substance

• the whole number ratio that is a multiple of a chemical formula

• the simplest, whole number ratio of a chemical formula

• 6.02 x 10^23 particles per one mole of a substance

Multiple Choice

What is the molar mass of SrS?

• 87.62 g/mol

• 119.69 g/mol

• 151.76 g/mol

• 32.07 g/mol

Fill in the Blanks

Calculate the molar mass for Al(OH)₃ Round to a whole Number

Multiple Choice

What is the percent by mass of oxygen in MgO?

• 20%
• 40%
• 50%
• 60%

Lesson Objectives

• Use Percent Composition to find an Empirical formula

• Use Empirical Formulas to find Molecular Formula

Empirical Formulas

• Sometimes two compounds can have the same elements, but different amounts of each element

  • Example: H₂O = Water, H₂O₂=Hydrogen Peroxide

• Molecular Formula: the total number of atoms in a molecule

• Empirical Formula: The lowest whole-number ratio of the atoms/moles of elements in a compound.

  • It can be useful in a lab setting for identification, but doesn't indicate the properties of substances

  • For Hydrogen Peroxide: the empirical formula is 1 : 1

Empirical Formulas Sample Problem 1

• Butane has the chemical formula of C₄H₁₀ What is the ratio of Carbon to hydrogen

  • There are 4 carbon and 10

    • 4:10

      • This ratio can be simplified by dividing all numbers by a common factor

Dropdown

Butane has the chemical formula of C₄H₁₀. The ratio of the elements in this formula are ​

Select answer

carbon : ​

Select answer

Hydrogen.

Dropdown

The empirical formula of water (H₂O) is H​

Select answer

: O​

Select answer

.

Dropdown

Acetylene (C₂H₂) is a flammable gas used in welders' torches. It has an empirical formula of ​ C

Select answer

: H

Select answer

Dropdown

Phenolphthalein has the chemical formula C₂₀H₁₄O₄. Its Empirical formula would be ​ C

Select answer

: H

Select answer

:​ O

Select answer

Fill in the Blanks

The molecular formula for acetylene is C₂H₂. The molecular formula for benzene is C₆H₆. What is the empirical formula for both

Multiple Choice

What is the empirical formula of a substance with the molecular formula X₂₀Y₁₅?

• X₁₀Y₁₅

• X₅Y₃

• X₄Y₃

• X₂₀Y₁₅

Using Percent Composition & Empirical Formulas Sample Problem 1

• Example: you have a sample that is 78.1% B and 21.9% H

  • ​Start by assuming you have 100.0 g of your compound

Fill in the Blanks

If you have a 100-gram sample and 78.1 % is Boron, how many grams of Boron do you have?

Fill in the Blanks

If you have a 100 gram sample and 21.9 % is hydrogen, how many grams of hydrogen do you have?

Calculating an Empirical Formula

• Example: you have a sample that is 78.1% B and 21.9% H

  • ​78.1% -> 78.1 g,

  • ​21.9% -> 21.9 g

• ​Convert the mass composition of each element to moles by dividing by the molar mass

  • NOTE: Empirical forces are one of the few places you CAN NOT Round the mass to a whole number

Calculating an Empirical Formula

Practice Problem 2:

• ​Quantitative analysis shows that a compound contains 32.38% sodium, 22.65% sulfur, and 44.99% Oxygen. Find the empirical formula

  • ​First step: go from percentage composition to mass composition

    • ​32.38% Na = 32.38 g Na

    • ​22.65% S = 22.65 g S

    • ​44.99% O = 44.99 g O

  • ​Second step: convert mass to moles

Practice Problem 2

• ​Quantitative analysis shows that a compound contains 32.38% sodium, 22.65% sulfur, and 44.99% Oxygen. Find the empirical formula

  • ​First step: go from percentage composition to mass composition

    • ​32.38% Na = 32.38 g Na

    • ​22.65% S = 22.65 g S

    • ​44.99% O = 44.99 g O

  • ​Second step: convert mass to moles

Fill in the Blanks

Convert 32.38 grams Na to moles. Round to 2 Decimal Places

Fill in the Blanks

Convert 22.65 grams S to moles. Round to 2 Decimal Places

Fill in the Blanks

Convert 44.99 grams O to moles. Round to 2 Decimal Places

Practice Problem 2:

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

How many molecules are in 2.5 mol of NaCl?

• 1.5 x 10²⁴ molecules

• 1.5 molecules

• 4.15 molecules

• 1.5 x 10²³ molecules

Multiple Choice

Which pair has the same empirical formula?

• NaCrO₄ and Na₂Cr₂O₇

• C₂H₄O₂ and C₆H₁₂O₆

• C₃H₆O₃ and C₂H₆O₂

• CH₄ and C₂H₆

Molecular Formulas

• ​The empirical formula is not necessarily the true formula of a substance

  • to find the molecular formula, you need to multiply the empirical formula by some value x

    • x is the Relationship between the molecular formula mass and the empirical formula mass and ​ can be equal to one

  • ​you can use the formula mass of a compound to determine the value of x

Finding Molecular Formulas Sample Problem 1

• ​The empirical formula of a compound containing Phosphorus and Oxygen was found to be P₂O₅. Experimentation shows the molar mass of the compound to be 283.89 g/mol. What is the compounds molecular formula

  • ​What do we know

    • Empirical Formula: P₂O₅

    • Empirical Formula Mass: ?

    • Molecular Formula: ?

    • molecular formula mass=molar mass= 283.89 g/mol.

    • ​x: ?

  • Step 1: ​Find the empirical formula mass by using the empirical formula

Fill in the Blanks

What is the empirical formula mass of P₂O₅? Round to 2 Decimal Places

Finding Molecular Formulas Sample Problem 1

Finding Molecular Formulas Sample Problem 1

• ​Find the molecular formula

  • ​x(empirical formula)=2(P₂O₅)= P₄O₁₀

Finding Molecular Formulas Sample Problem 2

• ​The empirical formula for trichloroisocyanuric acid is OCNCl. The molar mass for this compound is 232.41. Find the molecular formula

  • ​Organize what information we are given

    • ​Empirical Formula: OCNCl

    • ​Empirical Formula Mass: ?

    • ​Molecular Formula: ?

    • ​Molecular Formula Mass: 232.41

    • ​x: ?

Multiple Choice

What is the first piece of information we need to solve for?

• Empirical Formula

• Empirical Formula Mass of OCNCl

• Molecular Formula

• Molecular Formula Mass

• x

Fill in the Blanks

Calculate the Empirical formula mass of OCNCl. Round to two decimal places.

Finding Molecular Formulas Sample Problem 2

Experience Chemistry | Lesson 5.3.3

Fill in the Blanks

Solve for x. Round to the nearest whole number

Finding Molecular Formulas Sample Problem 2