# Chemistry I: Using Chemical Formulas. Formula Mass The sum of the average atomic masses of all elements in the compound. Units are amu.

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1 Chemistry I: Using Chemical Formulas Formula Mass The sum of the average atomic masses of all elements in the compound. Units are amu. Molar Mass - The mass in grams of 1 mole of a substance. Substance can be atoms, molecules, formula units, or ions. Units are. Molecular Mass The mass in grams of 1 mole of a molecular substance. Units are. Only used for molecules. Molecular Weight Old fashioned term that just won t die. Means the same as Molar Mass. Units are. Calculate the Molar Mass of the following compounds: H 2 O NaCl C 6 H 12 O HC 2 H 3 O CuSO 4 5H 2 O

2 Molar Mass can be used as a conversion factor to determine the number of of a substance from a given mass, or the mass of a substance from a given number of. To find Mass in grams from : USE THE GRID TO CANCEL UNITS!!!! # of (mass on periodic table) grams = mass in grams 1 mole Example 1. What is the mass of 5.00 of Nitrogen gas? 5.00 mol N g N 2 = 1.40 x10 2 g N 2 1 mol N 2 To find Moles from mass in grams: # of grams 1 mole = (mass on periodic table) grams Example 2. How many of Ammonium Dichromate are contained in 5.50 grams of the substance? 5.50 g (NH 4 ) 2 Cr 2 O 7 1 mol (NH 4 ) 2 Cr 2 O 7 = mol (NH 4 ) 2 Cr 2 O g (NH 4 ) 2 Cr 2 O 7 2

3 Counting Particles in Compounds Consider a mole sample of CH 4 Calculate the mass,, and molecules of the compound, and the mass,, and atoms of each element in a given amount of a compound. 1. Moles of Compound Mass of Compound: Find the Molar Mass: 1(12.01) + 4(1.10) = CH 4 = 12.3 g CH 4 2. Moles of Compound Moles of Elements in the compound: Use the subscripts in the formula! Carbon: = mole C Hydrogen: = 3.00 mole H 3. Moles of Element in the Compound Mass of Elements in compound: Use Molar Mass of the Element: Grams Carbon: = 9.01 gc Grams Hydrogen: = 3.03 g Hydrogen Notice that the mass of the individual elements must add up to the mass of the compound. (It s close, but off.01 gram because of sig fig rounding ) 3

4 4. Moles of each element in compound Atoms of each element in compound: Use Avogadro s Number: x atoms = 1 mole of atoms Atoms Carbon: = 4.52 E 23 atoms Carbon Atoms Hydrogen: = 1.81 E 24 atoms Hydrogen 5. Moles of Molecules of the compound Molecules of the compound Use Avogadro s Number: x molecules = 1 mole of molecules = 4.52 E 23 CH 4 molecules 1.20 H 2 O = Grams of Compound Mass Mole Atom Conversion Practice Fill in the following table 2 3 Moles of Grams of each each element element 21.6 g 2.40 mole H 1.20 mole O 2.42 g H 19.2 go 4 Atoms of each element 5 Molecules of compound 1.44 x x x H 2 SO 4 = g 3.50 mol H 1.75 mol S 7.00mol O 3.54gH 56.1gS 112g O 2.11x10 24 H 1.05 x10 24 S 4.21 x10 24 O 1.05 x

5 5.00 N 2 O 4 = BCl 3 = C 12 H 22 O 11 = g 10.0 mol N 20.0 mol O 73.2g mol B 1.88 mol Cl g 18.0 mol C 33.0 mol H 16.5 mol O 140 g N 320 g O 6.76g B 66.5 gcl gc 33.3 g H 264.gC 6.02 x x x x x x x x x x10 23 molecules Finding % Composition of a Formula (units= g element/100 g compound) Example: Find the % Composition of Fe 2 O 3 1. Start with 1 mole of the compound: In 1 mole Fe 2 O 3 (formula units) there are 2 mole Fe (atoms) and 3 mole O (atoms) 2. Determine the of each element in the compound: 2 mole Fe 3 mole O 3. Convert the of each element to grams : 2 mole Fe g Fe = g Fe 3 mole O g O = g O 1 mole Fe 1 mole O 4. Add the mass of each element to determine the Molar Mass of the compound: 5

6 111.7 g Fe g O = g Fe 2 O 3 5. Divide the mass of each element in the compound by the Molar Mass of the compound, and multiply by 100. (111.7 g Fe/159.7 g Fe 2 O 3 ) x 100 = 69.94% Fe (48.00 g O/159.7 g Fe 2 O 3 ) x 100 = 30.06% O 6. Check to make sure the % s add up to 100. Do sample problem J on page 243. Calculation of Simplest Formula Simplest Formula or Empirical Formula: Lowest whole number ratio of atoms or ions in a compound. For an ionic compound, the simplest formula is the only formula used. For a molecular compound, we must also calculate the molecular formula, which may or may not be the simplest formula. Example: Calculate the Simplest formula of a compound which has the following % composition: % Cr, 38.07% O, 26.58% K To Calculate Simplest formula from % composition: Remember units of % composition are g element/ 100 g compound. 1. Assume you have 100 g of the compound. 2. Use the % composition to calculate the # of grams of each element in the 100 grams of compound: In 100 g of the above compound, there are: g Cr, 38.07g O, g K. 6

7 3. Convert the masses of each element to using Molar Mass:.6798 Cr O.6798 K 4. Divide each # of by the smallest number, to obtain a mole ratio. This mole ratio is the ratio of atoms in the compound, if it is a whole number ratio, it is the simplest formula. K Cr O If you do not obtain a whole number ratio after step 4, multiply the ratio by small whole numbers, starting with 2,3,4,etc. until you obtain a whole number ratio. (K Cr O 3.5 ) x2 = K 2 Cr 2 O 7 Do sample problem M on page 247 Molecular Formulas Used for molecular compounds only. Simplest formula is just a RATIO of the atoms in a molecule. A molecule usually has more atoms than is indicated by the smallest whole number ratio. Molecular Formula the exact number of each type of atom in a molecule. Molecular formula can be determined from the simplest formula if we know the molecular mass of the substance. (Molar Mass of the Simplest Formula) x = Molecular Mass Molecular Formula = Simplest Formula X Sample Problem N on p. 248: 7

8 Practice Problem 2. on page 249: Formula of a Hydrate Hydrate: Anhydrous Compound: Waters of Hydration: Example Problem Washing soda is a hydrate of sodium carbonate. A student wanted to determine how many waters of hydration are in a formula unit of the hydrate. The student took a sample of the hydrate and heated it to evaporate the water. To make sure all of the water had evaporated she massed it after heating it several times, until the mass was constant. The student obtained the following data: Mass of the hydrate before heating: Mass of the compound after heating to a constant mass: g g Determine: 8

9 A. The mass % of water in the hydrate. B. The formula of the hydrate. 9

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