03.09 Molar Mass of Compounds

Empirical Formulas

When scientists conduct experiments to determine the composition of a compound, they must determine the mass of each element present based on the amount of each product produced in the reactions. Empirical and molecular formulas for organic compounds containing only carbon and hydrogen or carbon, hydrogen, and oxygen can be determined by a process called combustion analysis.

  • Steps
  • Practice

Let’s examine the steps that chemists follow to determine the mass of each element present in a compound.

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Use the steps you have learned to solve the following question: Cumene is a carbon and hydrogen compound that, when 47.6 g is combusted in excess oxygen, 156.8 g carbon dioxide and 42.8 g of water are produced. What is the empirical formula of the compound?

Check your answer:

Hydrates

Determining the Molar Mass of a Hydrate

As you learned in an earlier lesson, hydrates are crystals that contain water molecules inside their structure. The formula for a hydrate includes a certain number of water molecules in order to represent the ratio of ionic compound to water molecules.

The molar mass of a hydrate is determined in a similar way to how the molar mass of any other compound is calculated. Remember that the dot in the formula does not represent a multiplication sign; it only indicates that the formula is a ratio.

  • Example
  • Practice One
  • Practice Two

CuSO4 · 5H2O

To find the molar mass of a hydrate, first determine the molar mass of the ionic compound, and then add the total molar mass of the water molecules present in the formula.

  1. 1 mol CuSO4 + 5 mol H2O = 1 mol copper (II) sulfate pentahydrate
  2. 1 mol Cu + 1 mol S + 4 mol O + 5(2 mol H + 1 mol O) = molar mass of copper (II) sulfate pentahydrate
  3. 63.546 g/mol Cu + 32.065 g/mol S + 4(15.999 g/mol O) + 5[2(1.0079 g/mol H)] + 15.999 g/mol O
  4. 159.607 g/mol CuSO4 + 5(18.015 g/mol H2O)
  5. 249.681 g/mol CuSO4 · 5H2O

Determine the molar mass of the following hydrate: MgSO4 · 7H2O

Check your answer:

Determine the molar mass of the following hydrate: Na2B4O7 · 10H2O

Check your answer:

Determining the Formula of a Hydrate

As with any compound, chemists must determine the formula of a hydrate that has been discovered or created by analyzing it in the laboratory. To determine the amount of water present in the compound, you can heat the hydrate until all of the water has evaporated. The difference in the mass before and after the water was evaporated will determine the moles of water present in the compound, allowing you to determine the formula of the hydrate

  • Step One
  • Step Two
  • Step Three
  • Step Four
  • Step Five
  • Step Six
  • Step Seven

Step One: Measure the mass of the sample of hydrate.

A clean, empty crucible has a mass of 12.785 g. A sample of hydrated MgSO4 salt is added to the crucible, making the total mass of the crucible and hydrate 15.996 g. Subtraction of the mass of the empty crucible from the mass of the hydrate and crucible gives the mass of the MgSO4 hydrate. (15.996 g – 12.785 g = 3.211 g hydrate).

Step Two: Heat the hydrate until all of the water has evaporated.

The crucible and hydrate are heated above a Bunsen burner for a total of 15 minutes to ensure that all of the water evaporates.

Step Three: Measure the mass of the dehydrated ionic compound again.

The mass of the crucible and dehydrated ionic compound is ____.

Subtracting the mass of the empty crucible from the mass of the crucible and dehydrated MgSO4 gives you the mass of the dehydrated compound, 1.570 g MgSO4.

Step Four: Subtract the mass of the dehydrated compound from the mass of the hydrate to determine the mass of water that was present in the hydrate.

3.211 g MgSO4 hydrate – 1.570 g dehydrated MgSO4 = 1.641 g H2O present in the sample of hydrate

Step Five: Convert the mass of water to moles of water.

1.641 g H2O × fraction: one mol H subscript two O over eighteen point zero one five g H subscript two O = 0.0911 mol H2O

Step Six: Convert the mass of dehydrated compound to moles of the compound.

1.570 g MgSO4 × fraction: one mole MgSO subscript four over one hundred twenty point three six six g MgSO subscript four = 0.0130 mol MgSO4

Step Seven: Divide by the lowest mole value to get a whole number mole ratio.

0.0911 mol H2O ÷ 0.0130 = 7 mol H2O

0.0130 mol MgSO4 ÷ 0.0130 = 1 mol MgSO4

Formula of the Hydrate: MgSO4 · 7H2O

Virtual Lab

In this lab, you will be determining the formula of a copper sulfate hydrate by heating the sample until all of the water evaporates.

Procedure:

  1. Measure the mass of the clean, empty crucible, record the mass.
  2. Fill the crucible about 1/2 to 2/3 full with the hydrate (3 to 5 scoops), record the mass.
  3. Heat the crucible and hydrate above a Bunsen burner for at least ten minutes to make sure that all of the water evaporates.
  4. Let the crucible cool and then mass the crucible with dehydrated solid inside, record the mass.
  5. Calculate the formula of the hydrate.

You will complete a formal lab report for this assignment. Many of the lab assessments you have completed thus far in the course guided you through many of the parts of a general lab report, but this time you will type up the lab report yourself and submit it to your instructor. Refer to the outline for the title of each required section of this lab report, as well as a brief description of what should be included in each section. Be sure to review the rubric before submitting your lab report.

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Assessment

It is time to complete your assignment for this lesson. Before you try this assignment, you should be able to name compounds and write chemical formulas from the names of compounds.

If you do not feel comfortable with these topics, go back and review the lesson or contact your instructor before attempting your assignment.

Assignment
3.09 Molar Mass of Compounds Honors

  1. Complete all tasks and readings that are outlined in the lesson.
  2. Complete the virtual lab.
  3. Complete and submit assignment "3.09 Molar Mass of Compounds Honors."