1 / 94

Stoichiometry: Basic Concepts and Molar Mass Calculations

This topic explores the basic concepts of stoichiometry, including the use of the mole and Avogadro's constant to relate macroscopic measurements to the number of particles in a substance. It also covers how to calculate the molar mass of elements and compounds.

moores
Download Presentation

Stoichiometry: Basic Concepts and Molar Mass Calculations

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Topic 16

  2. Table of Contents Topic 16 Topic 16: Stoichiometry Basic Concepts Additional Concepts

  3. Stoichiometry: Basic Concepts Topic 16 Stoichiometry • Using the methods of stoichiometry, we can measure the amounts of substances involved in chemical reactions and relate them to one another. • For example, a sample’s mass or volume can be converted to a count of the number of its particles, such as atoms, ions, or molecules.

  4. Stoichiometry: Basic Concepts Topic 16 Stoichiometry • Atoms are so tiny that an ordinary-sized sample of a substance contains so many of these submicroscopic particles that counting them by grouping them in thousands would be unmanageable. • Even grouping them by millions would not help.

  5. Stoichiometry: Basic Concepts Topic 16 Stoichiometry • The group or unit of measure used to count numbers of atoms, molecules, or formula units of substances is the mole (abbreviated mol). • The number of things in one mole is 6.02 x 1023. This big number has a short name: the Avogadro constant. • The most precise value of the Avogadro constant is 6.0221367 x 1023. For most purposes, rounding to 6.02 x 1023 is sufficient.

  6. Stoichiometry: Basic Concepts Topic 16 Molar Mass • Methanol is formed from CO2 gas and hydrogen gas according to the balanced chemical equation below.

  7. Stoichiometry: Basic Concepts Topic 16 Molar Mass • Suppose you wanted to produce 500 g of methanol. • How many grams of CO2 gas and H2 gas would you need? How many grams of water would be produced as a by-product? • Those are questions about the masses of reactants and products.

  8. Stoichiometry: Basic Concepts Topic 16 Molar Mass • But the balanced chemical equation shows that three molecules of hydrogen gas react with one molecule of carbon dioxide gas. • The equation relates molecules, not masses, of reactants and products.

  9. Stoichiometry: Basic Concepts Topic 16 Molar Mass • Like Avogadro, you need to relate the macroscopic measurements—the masses of carbon dioxide and hydrogen—to the number of molecules of methanol. • To find the mass of carbon dioxide and the mass of hydrogen needed to produce 500 g of methanol, you first need to know how many molecules of methanol are in 500 g of methanol.

  10. Stoichiometry: Basic Concepts Topic 16 Molar Mass of an Element • Average atomic masses of the elements are given on the periodic table. • For example, the average mass of one iron atom is 55.8 u, where u means “atomic mass units.”

  11. Stoichiometry: Basic Concepts Topic 16 Molar Mass of an Element • The atomic mass unit is defined so that the atomic mass of an atom of the most common carbon isotope is exactly 12 u, and the mass of 1 mol of the most common isotope of carbon atoms is exactly 12 g.

  12. Stoichiometry: Basic Concepts Topic 16 Molar Mass of an Element • The mass of 1 mol of a pure substance is called its molar mass.

  13. Stoichiometry: Basic Concepts Topic 16 Molar Mass of an Element • The molar mass is the mass in grams of the average atomic mass. • If an element exists as a molecule, remember that the particles in 1 mol of that element are themselves composed of atoms.

  14. Stoichiometry: Basic Concepts Topic 16 Molar Mass of an Element • For example, the element oxygen exists as molecules composed of two oxygen atoms, so a mole of oxygen molecules contains 2 mol of oxygen atoms. • Therefore, the molar mass of oxygen molecules is twice the molar mass of oxygen atoms: 2 x 16.00 g = 32.00 g.

  15. Stoichiometry: Basic Concepts Topic 16 Number of Atoms in a Sample of an Element • The mass of an iron bar is 16.8 g. How many Fe atoms are in the sample? • Use the periodic table to find the molar mass of iron. • Use the periodic table to find the molar mass of iron. The average mass of an iron atom is 55.8 u. • Then the mass of 1 mol of iron atoms is 55.8 g.

  16. Stoichiometry: Basic Concepts Topic 16 Number of Atoms in a Sample of an Element • To convert the mass of the iron bar to the number of moles of iron, use the mass of 1 mol of iron atoms as a conversion factor. • Now, use the number of atoms in a mole to find the number of iron atoms in the bar.

  17. Stoichiometry: Basic Concepts Topic 16 Number of Atoms in a Sample of an Element • Simplify the expression above.

  18. Stoichiometry: Basic Concepts Topic 16 Molar Mass of a Compound • Covalent compounds are composed of molecules, and ionic compounds are composed of formula units. • The molecular mass of a covalent compound is the mass in atomic mass units of one molecule. • Its molar mass is the mass in grams of 1 mol of its molecules.

  19. Stoichiometry: Basic Concepts Topic 16 Molar Mass of a Compound • The formula mass of an ionic compound is the mass in atomic mass units of one formula unit. • Its molar mass is the mass in grams of 1 mol of its formula units. • How to calculate the molar mass for ethanol, a covalent compound, and for calcium chloride, an ionic compound, is shown.

  20. Stoichiometry: Basic Concepts Topic 16 Molar Mass of a Compound • Ethanol, C2H6O, a covalent compound.

  21. Stoichiometry: Basic Concepts Topic 16 Molar Mass of a Compound • Calcium chloride, CaCl2, an ionic compound.

  22. Stoichiometry: Basic Concepts Topic 16 Number of Formula Units in a Sample of a Compound • The mass of a quantity of iron(III) oxide is 16.8 g. How many formula units are in the sample? • Use the periodic table to calculate the mass of one formula unit of Fe2O3.

  23. Stoichiometry: Basic Concepts Topic 16 Number of Formula Units in a Sample of a Compound • Therefore, the molar mass of Fe2O3 (rounded off) is 160 g.

  24. Stoichiometry: Basic Concepts Topic 16 Number of Formula Units in a Sample of a Compound • Now, multiply the number of moles of iron oxide by the number in a mole.

  25. Stoichiometry: Basic Concepts Topic 16 Mass of a Number of Moles of a Compound • What mass of water must be weighed to obtain 7.50 mol of H2O? • The molar mass of water is obtained from its molecular mass. • The molar mass of water is 18.0 g/mol.

  26. Stoichiometry: Basic Concepts Topic 16 Mass of a Number of Moles of a Compound • Use the molar mass to convert the number of moles to a mass measurement.

  27. Stoichiometry: Basic Concepts Topic 16 Mass of a Number of Moles of a Compound • The concept of molar mass makes it easy to determine the number of particles in a sample of a substance by simply measuring the mass of the sample. • The concept is also useful in relating masses of reactants and products in chemical reactions.

  28. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant • Ammonia gas is synthesized from nitrogen gas and hydrogen gas according to the balanced chemical equation below.

  29. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant • How many grams of hydrogen gas are required for 3.75 g of nitrogen gas to react completely? • Find the number of moles of N2 molecules by using the molar mass of nitrogen.

  30. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant • To find the mass of hydrogen needed, first find the number of moles of H2 molecules needed to react with all the moles of N2 molecules. • The balanced chemical equation shows that 3 mol of H2 molecules react with 1 mol of N2 molecules.

  31. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant • Multiply the number of moles of N2 molecules by this ratio. • The units in the expression above simplify to moles of H2 molecules.

  32. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant • To find the mass of hydrogen, multiply the number of moles of hydrogen molecules by the mass of 1 mol of H2 molecules, which is 2.00 g.

  33. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Product • What mass of ammonia is formed when 3.75 g of nitrogen gas react with hydrogen gas according to the balanced chemical equation below? • The amount of ammonia formed depends upon the number of nitrogen molecules present and the mole ratio of nitrogen and ammonia in the balanced chemical equation.

  34. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Product • The number of moles of nitrogen molecules is given by the expression below.

  35. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Product • To find the mass of ammonia produced, first find the number of moles of ammonia molecules that form from 3.75 g of nitrogen. • Use the mole ratio of ammonia molecules to nitrogen molecules to find the number of moles of ammonia formed.

  36. Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Product • Use the molar mass of ammonia, 17.0 g, to find the mass of ammonia formed.

  37. Stoichiometry: Basic Concepts Topic 16 Using Molar Volumes in Stoichiometric Problems • In terms of moles, Avogadro’s principle states that equal volumes of gases at the same temperature and pressure contain equal numbers of moles of gases. • The molar volume of a gas is the volume that a mole of a gas occupies at a pressure of one atmosphere (equal to 101 kPa) and a temperature of 0.00°C.

  38. Stoichiometry: Basic Concepts Topic 16 Using Molar Volumes in Stoichiometric Problems • Under these conditions of STP, the volume of 1 mol of any gas is 22.4 L. • Like the molar mass, the molar volume is used in stoichiometric calculations.

  39. Stoichiometry: Basic Concepts Topic 16 Using Molar Volume • In the space shuttle, exhaled carbon dioxide gas is removed from the air by passing it through canisters of lithium hydroxide. The following reaction takes place. • How many grams of lithium hydroxide are required to remove 500.0 L of carbon dioxide gas at 101 kPa pressure and 25.0°C?

  40. Stoichiometry: Basic Concepts Topic 16 Using Molar Volume • The volume of gas at 25°C must be converted to a volume at STP. • Now, find the number of moles of CO2 gas as below.

  41. Stoichiometry: Basic Concepts Topic 16 Using Molar Volume • The chemical equation shows that the ratio of moles of LiOH to CO2 is 2 to 1. • Therefore, the number of moles of lithium hydroxide is given by the expression below. • To convert the number of moles of LiOH to mass, use its molar mass, 23.9 g/mol.

  42. Stoichiometry: Basic Concepts Topic 16 Using Molar Volume

  43. Stoichiometry: Basic Concepts Topic 16 Ideal Gas Law • Exactly how the pressure P, volume V, temperature T, and number of particles n of gas are related is given by the ideal gas law shown here. PV = nRT

  44. Stoichiometry: Basic Concepts Topic 16 Ideal Gas Law • The value of the constant R can be determined using the definition of molar volume. • At STP, 1 mol of gas occupies 22.4 L. Therefore, when P = 101.3 kPa, V = 22.4 L, n = 1 mol, and T = 273.15 K, the equation for the ideal gas law can be shown as follows.

  45. Stoichiometry: Basic Concepts Topic 16 Ideal Gas Law • Now, we can solve for R.

  46. Stoichiometry: Basic Concepts Topic 16 Using the Ideal Gas Law • How many moles are contained in a 2.44-L sample of gas at 25.0°C and 202 kPa? • Solve the ideal gas law for n, the number of moles.

  47. Stoichiometry: Basic Concepts Topic 16 Using the Ideal Gas Law • First, find the volume that 2.44 L of a gas would occupy at STP.

  48. Stoichiometry: Basic Concepts Topic 16 Using the Ideal Gas Law • Then, find the number of moles in this volume. • 0.200 mol is close to the calculated value.

  49. Stoichiometry: Basic Concepts Topic 16 Determining Mass Percents • The formula for geraniol (the main compound that gives a rose its scent) is C10H18O.

  50. Stoichiometry: Basic Concepts Topic 16 Determining Mass Percents • The formula shows that geraniol is comprised of carbon, hydrogen, and oxygen. • Because all these elements are nonmetals, geraniol is probably covalent and comprised of molecules.

More Related