Name: Date: Pd: Topic 1.2: The Mole Concept and Empirical Formula

Transcription

1 Name: Date: Pd: Topic 1.2: The Mole Concept and Empirical Formula Relative Atomic Mass- A r - the weighted average of one atom of an element relative to 1/12 of an atom of carbon-12. A r values do not have units. AKA: Average Atomic Mass Found in the data booklet on page 6 Practice: Use your data booklets to write the correct the A r values for each substance below. 1. Oxygen: 2. Sulfur: 3. Sodium: 4. Lead: Relative Formula Mass- M r - add the relative atomic masses of all the atoms or ions present in its formula. M r is also a relative term with no units. Practice: Name each substance and use your data booklets to write the correct the M r values for each substance below. Use the correct units. Substance Name M r KCl Al 2 (SO 3 ) 3 C 4 H 8 Molar Mass- M- is the relative atomic mass (A r ) or the relative formula mass (M r ) expressed in grams. It has the unit g mol -1. Practice: Use your data booklets to write the correct the M r values for each substance below. Use the correct units. Substance Name Molar Mass ( ) NaOH

2 Mg(OH) 2 H 3 PO 4 Hydrated Salts- Compounds that contain a fixed ratio of water molecules, known as water of crystallization, within the compound. The chemical formula reflects the ratio of water to salt. Practice: Molar Mass of a Hydrate Substance Molar Mass ( ) CoCl 2 2H 2 O (s) CuSO 4 5H 2 O (s) (NH 4 Fe(SO 4 6H 2 O (s) Mole Conversions Mole Conversion Chart

3 Practice 1. What volume (dm 3 ) would 3.96 moles of N 2 take up at STP? 2. What is the mass of 8.96 x molecules of C 6 H 12 O 2? 3. What is the total number of hydrogen atoms in 2.5 mol of P2P, phenyl-2-propanone, C 6 H 5 CH 2 COCH 3? 4. ** What is the mass, in g, of 0.500mol of 1,2-dibromoethane, CH 2, BrCH 2 Br? 5. ** What is the number of atoms of oxygen in mol of hydrated Zn(NO 3 ) 2 6H 2 O? 6. **How many moles of oxygen atoms are there in mol of hydrated iron (II) ammonium sulfate, (NH 4 Fe(SO 4 6H 2 O (s)?

4 7. ** Hydrazine is used to remove oxygen from water used to generate steam or hot water. N 2 H 4 (aq) + O 2 (aq) --> N 2(g) +2H 2 O (l) The concentration of dissolved oxygen in a sample of water is 8.0 x 10-3 g dm -3 a. Calculate, showing your working, the mass of hydrazine needed to remove all the dissolved oxygen from 1000 dm 3 of the sample. b. Calculate the volume, in dm 3, of nitrogen formed under STP conditions. Empirical and Molecular Formulas Percentage Composition by Mass When working with percentage figures, always check that they add up to 100. Sometimes an element is omitted from the data and you are expected to deduce its identify and percentage from the information. Empirical Formula- the simplest whole number ratio of elements in a compound Can be deduced from experimental results (usually involving combustion). Molecular Formula- shows all the atoms present in a molecule. It is a multiple of the empirical formula. Example: Determining the formula for magnesium oxide. Burn a known mass of magnesium and from this calculate the number of moles of Mg. Convert g of Mg to moles of Mg. Calculate the mass of oxygen reacted from the increase in mass and from this calculate the moles of oxygen. After burning, the mass of the product was g Determine the mass of oxygen reacted. Convert the mass of oxygen to moles of oxygen. Express the moles of oxygen and magnesium in simplest form.

5 The ratio of moles is the ratio of atoms, so we can deduce the simplest formula of magnesium oxide. The molecular mass was determined to be 40.3g mol -1. What is the molecular formula of the compound? Practice: 1. What are the percentages by mass of Cu, N, and O in Cu(NO 3? 2. Empirical Formula: 75% Carbon and 25% hydrogen? 3. Empirical Formula:22.1% aluminum, 25.4% phosphorus, 52.5% oxygen? 4. A bright orange, crystalline substance is analyzed and determined to have the following mass percentages: 17.5% Na, 39.7% Cr, and 42.8% O. Determine its empirical formula.

6 5. A sample of cisplatin is 65.02% platinum, 9.34% nitrogen, 2.02% hydrogen, and the rest is chlorine. Determine the empirical formula. 6. Consider the following: The mineral Celestine consists mostly of a compound of strontium, sulfur, and oxygen. It is found by combustion analysis to have the composition 47.70% by mass Sr, 17.46% S, and the remainder is O. What is the empirical formula for celestine? 7. The relative molecular mass of a gas is 112 and its empirical formula is CH 2. What is the molecular formula of the gas? 8. What are the empirical and molecular formulas for resorcinol? It is 65.44% Carbon, 5.49% Hydrogen, 29.06% Oxygen with a molar mass of 110 g mol -1.

7 9. A 1.50g sample of hydrocarbon undergoes complete combustion to produce 4.40g of CO 2 and 2.70g of H 2 O. a. What is the empirical formula of the hydrocarbon? b. The molar mass of the hydrocarbon is determined to be g mol -1. What is the molecular formula? 10. Quinone, which is used in the dye industry and in photography, is an organic compound containing only C, H, and O. a. What is the empirical formula of the compound if you find that g of the compound gives 0.257g of CO 2 and 0,0350g of H 2 O when burned completely? b. Given a molecular mass of approximately 108g mol -1, what is the molecular formula?

8 Formula of a Hydrate: Can be deduced using lab data Lab Procedure: Mass the Hydrate. Slowly dehydrate the salt in a crucible over a bunsen burner until you have driven off all water and left with only the anhydrous salt. Mass the Anhydrate. Determine the mass of water given off. Convert the mass of water and mass of anhydrate to moles. Determine the simplest ratio of moles of water to moles of the anhydrate. Practice: 1. A hydrate of magnesium sulfate has a mass of 13.52g. This sample is heated until no water remains. The anhydrous MgSO 4 has a mass of 6.60g. Find the formula of the hydrate. 2. A hydrate is determined to be 45.43% water and 54.57% CoCl 2. Find the chemical formula for the hydrate.