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Understanding Nitrogen's Role in Plant Biology and Chemical Quantities

This chapter explores the importance of nitrogen for plants, focusing on its bioavailability and different forms such as ammonia and nitrate. It discusses how fertilizers like NH3 and NH4NO3 contribute to nitrogen needs, and emphasizes the relationship between the chemical formulas and the amount of nitrogen they deliver. Additionally, the chapter delves into chemical measurement concepts such as relative masses, the mole, and how these principles apply to counting atoms and molecules in various substances, as well as practical examples of mass measurement.

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Understanding Nitrogen's Role in Plant Biology and Chemical Quantities

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  1. Chapter 5 Quantities in Chemistry

  2. Setting the Stage - Bioavailability of Nitrogen • Plants need nitrogen as either ammonia or nitrate in order to use it for biosynthesis • While air is 80% nitrogen, it is in the form of the relatively inert gas N2, hence plants cannot use it directly Malone and Dolter - Basic Concepts of Chemistry 9e

  3. Type and Efficacy of Nitrogen Sources • Main types of fertilizer are ammonia gas (NH3) and ammonium nitrate (NH4NO3) • 100 kg of NH3 delivers 82 kg of N • 100 kg of NH4NO3 only delivers 35 kg of N • The mass of N delivered is related to the formula of the compound and the relative masses of each element in the formula Malone and Dolter - Basic Concepts of Chemistry 9e

  4. Setting a Goal – Part AThe Measurement of Masses of Elements and Compounds • You will become proficient at working with the units of moles, mass, and numbers of atoms and molecules, and at converting between each of these Malone and Dolter - Basic Concepts of Chemistry 9e

  5. Objective for Section 5-1 • Calculate the masses of equivalent numbers of atoms of different elements Malone and Dolter - Basic Concepts of Chemistry 9e

  6. 5-1 - Relative Masses of Elements • When the masses of samples of any two elements are in the same ratio as that of their atomic masses, the samples have the same number of atoms • We can therefore use the atomic masses and the masses of samples of chemical substances to “count” the number of atoms or molecules Malone and Dolter - Basic Concepts of Chemistry 9e

  7. The Mass of an Atom • Recall that an atom has an unbelievably small mass • 12C is used as the standard and is assigned a mass of exactly 12 amu. • Other isotopes are present in natural samples (i. e. C has an overall atomic mass of 12.01) so the periodic table lists masses that are the weighed average mass of the natural sample Malone and Dolter - Basic Concepts of Chemistry 9e

  8. Relative Masses of the Elements • The amu has no practical value in a laboratory situation: 12.01 amu = 1.994  10-23 g • The best balance made can detect no less than 10-5 g, so we have to scale up the masses to something we can measure Malone and Dolter - Basic Concepts of Chemistry 9e

  9. Counting by Weighing Bolts Nuts • Hardware stores often count by weighing • If we want 175 bolts and assume that the mass of an average bolt is 10.5 g, then the mass of bolts will be: Malone and Dolter - Basic Concepts of Chemistry 9e

  10. Counting by Weighing • Nuts, being smaller, will have a lower average mass (2.25 g) • What weight of nuts will provide a nut for each bolt? Malone and Dolter - Basic Concepts of Chemistry 9e

  11. Counting by weighing • By using ratios of the average masses of bolts and nuts, or the masses of a fixed number of items, we can get equivalent numbers of bolts and nuts: Malone and Dolter - Basic Concepts of Chemistry 9e

  12. From Hardware to Atoms • We need to know the relative numbers of atoms of different elements present and the relative masses of the individual atoms • A 4He atom has a mass of 4.00 amu and a 12C atom has a mass of 12.00 amu. • If 4He and 12C are present in a 4.00:12.00 mass ratio, regardless of the units of mass, the number of atoms is the same Malone and Dolter - Basic Concepts of Chemistry 9e

  13. Objective for Section 5-2 • Define the mole and relate this unit to numbers of atoms and to the atomic masses of the elements Malone and Dolter - Basic Concepts of Chemistry 9e

  14. 5-2 The Mole and the Molar Mass of Elements • A mole is: • The number of atoms in exactly 12 g (12.00 recurring) of 12C • Avogadro’s number of atoms(6.022  1023) • The number of atoms in one atomic mass of an element expressed in grams Malone and Dolter - Basic Concepts of Chemistry 9e

  15. Avogadro Amedeo Avogadro, a pioneer in the investigation of quantitative aspects of chemistry Malone and Dolter - Basic Concepts of Chemistry 9e

  16. The Mole • One mole of an element implies • The atomic mass expressed in grams, it is different for each element • It contains Avogadro’s number (6.022 x 1023) of atoms, which is the same for all elements • A conversion factor between mass and numbers of things (allows us to count atoms by weighing) Malone and Dolter - Basic Concepts of Chemistry 9e

  17. Mass and Number of Things • For our purposes, assume that oranges are identical in mass; 12 have a mass of 2.71 kg • We can do the same with atomic mass Malone and Dolter - Basic Concepts of Chemistry 9e

  18. Moles of Elements • Here is 1 mole each of copper, iron, mercury and sulphur Malone and Dolter - Basic Concepts of Chemistry 9e

  19. Objective for Section 5-3 • Perform calculations involving masses, moles, and numbers of molecules or formula units for compounds Malone and Dolter - Basic Concepts of Chemistry 9e

  20. 5-3 The Molar Mass of Compounds • Molecular compounds • chemical formula represents a discrete molecular unit (e. g. CO2) • Ionic compounds • chemical formula represents a formula unit (the whole number ratio of cations to anions; e. g. K2SO4) Malone and Dolter - Basic Concepts of Chemistry 9e

  21. Calculation of Formula Weight • The sum of the atomic masses of all the atoms in a molecule • This is often referred to as the molecular weight • Consider CO21 C (12.01 amu) + 2 O (2  16.00 amu) = 44.01 amu • One molar mass of a compound contains Avogadro’s number of molecules Malone and Dolter - Basic Concepts of Chemistry 9e

  22. Calculation of Formula Weight • Second example, using a salt, Fe2(SO4)3 Malone and Dolter - Basic Concepts of Chemistry 9e

  23. Hydrates • Some ionic compounds can have water molecules attached within the structure • These compounds are termed hydrates and have properties distinct from the unhydrated form • The formula weight of a hydrate includes the mass of the water molecules Malone and Dolter - Basic Concepts of Chemistry 9e

  24. Hydrates • Examples • CuSO4 - [copper(II) sulfate] - a pale green solid • CuSO4•5H2O - [copper(II) sulfate pentahydrate - a dark blue solid • Often, the waters of hydration can be removed by heating Malone and Dolter - Basic Concepts of Chemistry 9e

  25. The Molar Mass of a Compound • The mass of one mole (6.022 × 1023 molecules or formula units) is referred to as the molar mass of the compound • It is the formula weight expressed in grams • For example, 44.0 g of CO2 is the molar mass of CO2 and is the mass of 6.022 × 1023 molecules of CO2 Malone and Dolter - Basic Concepts of Chemistry 9e

  26. Moles of Compounds • One mole of copper sulfate pentahydrate, sodium chloride, sodium chromate and water Malone and Dolter - Basic Concepts of Chemistry 9e

  27. Summary Chart for Part A (1) Malone and Dolter - Basic Concepts of Chemistry 9e

  28. Summary Chart for Part A (2) Malone and Dolter - Basic Concepts of Chemistry 9e

  29. Setting a Goal – Part BThe Component Elements of Compounds • You will learn about the relationship between the formula of a compound and its elemental composition Malone and Dolter - Basic Concepts of Chemistry 9e

  30. Objective for Section 5-4 • Given the formula of a compound, determine the mole, mass, and percent composition of its elements Malone and Dolter - Basic Concepts of Chemistry 9e

  31. 5-4 The Composition of Compounds Table 5-2 relates one mole of a compound (H2SO4) to all its component parts. All of these relationships can be used to construct conversion factors between the compound and its elements. Malone and Dolter - Basic Concepts of Chemistry 9e

  32. Table 5-2 Malone and Dolter - Basic Concepts of Chemistry 9e

  33. The Mole Composition of a Compound • Mole composition is the number of moles of each of the elements that make up 1 mole of the compound • CO2 – one mole of C and two moles of O • H2SO4 – one mole of S, two moles of H, and four moles of O Malone and Dolter - Basic Concepts of Chemistry 9e

  34. The Mass Composition of a Compound • Mass composition is the mass of each element in the compound • CO2 – 12.01 g of C and 32.00 g of O • H2SO4 – 2.016 g of H, 32.07 g of S, and 64.00 g of O Malone and Dolter - Basic Concepts of Chemistry 9e

  35. Percent Composition of a Compound • mass of each element per 100 mass units of compound • in 100 g of NH3, there are 82.2 g of N • therefore, the mass percentage of N is 82.2% N Malone and Dolter - Basic Concepts of Chemistry 9e

  36. CO2 • Calculation of % composition of carbon dioxide requires determining the number of grams of each element (C and O) in one mole Malone and Dolter - Basic Concepts of Chemistry 9e

  37. Objectives for Section 5-5 • Use percent or mass composition to determine the empirical formula of a compound • Given the molar mass of a compound and its empirical formula, determine its molecular formula Malone and Dolter - Basic Concepts of Chemistry 9e

  38. 5-5 Empirical and Molecular Formulas • Empirical formula - simplest whole number ratio of atoms in the compound • Procedure to find empirical formula from % composition data • Convert percent composition to an actual mass • Convert mass to moles of each element • Find the whole number ratio of the moles of different elements Malone and Dolter - Basic Concepts of Chemistry 9e

  39. Empirical Formula of Laughing Gas • Contains 63.6% N and 36.4% O • Assume 100 g of substance, so you have 63.6 g of N and 36.4 g of O • Calculation gives an empirical formula of N2O Malone and Dolter - Basic Concepts of Chemistry 9e

  40. Molecular Formula • The actual number of each atom in a formula unit • Consider acetylene and benzene • both have the empirical formula CH, but different molecular formulas: • acetylene is actually C2H2 • benzene is actually C6H6 Malone and Dolter - Basic Concepts of Chemistry 9e

  41. Molecular Formula Determination • Needs the molecular mass, which must be determined from an independent measurement (e.g. via mass spectrometry) • First determine the mass of the empirical formula Malone and Dolter - Basic Concepts of Chemistry 9e

  42. Molecular Formula Determination…contd. • Divide the empirical formula mass into the molecular mass • The resulting number (which should be a small whole number or close to it) is the number of times the empirical formula unit appears in the molecular formula Malone and Dolter - Basic Concepts of Chemistry 9e

  43. Acetylene and Benzene • The empirical formula mass for both substances is 12.0 g + 1.0 g = 13.0 g • The actual molar mass of acetylene is 26.0 g, so the empirical formula mass divides into the actual mass two times - C2H2 • Benzene’s actual molar mass is 78.0 g, so the empirical formula mass divides into the actual mass six times - C6H6 Malone and Dolter - Basic Concepts of Chemistry 9e

  44. Acetylene and Benzene: Same Empirical Formula, but DifferentMolecular Formulas These are structural formulas Malone and Dolter - Basic Concepts of Chemistry 9e

  45. Summary Chart for Part B (1) Malone and Dolter - Basic Concepts of Chemistry 9e

  46. Summary Chart for Part B (2) Malone and Dolter - Basic Concepts of Chemistry 9e

  47. Summary of Types of Formula Malone and Dolter - Basic Concepts of Chemistry 9e

  48. Worked Example Nicotine is a compound containing C, H and N only. Its molar mass is 162 g. A 1.50 g sample of nicotine is found to contain 1.11 g of C. Analysis of another sample indicates that nicotine has 8.70% by mass of H. Determine the molecular formula of nicotine. Solution. We should convert the data to masses or %, and find the mass or % of N by difference. Malone and Dolter - Basic Concepts of Chemistry 9e

  49. Worked Example Continued Malone and Dolter - Basic Concepts of Chemistry 9e

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