Transcript Slide 1
Chemical Quantities Dr. Schuerch Measuring Matter • What do the questions “how much” and “how many” have in common? – They are questions about the amount of a substance and are similar to questions scientist ask • There are three common ways to measure matter – Count the matter – Measure the mass or weight – Measure the volume What is a Mole • The SI unit for measuring the amount of a substance is the mole – A mole is a numerical amount of a substance • 1 mole = 6.022 x1023 Representative Units which is also known as Avogadro’s number • Representative units or particles are the smallest unit into which a substance can be broken down without a change in composition » Atoms, Molecules, Ions How Large is Avogadro's Number? • 6.02x1023 • 602,000,000,000,000,000,000,000 • If you could count two dollars every second, it would take you 9,544,647,387,113,140 (Nine Quadrillion, Five Hundred Forty Four Trillion, Six Hundred Forty Seven Billion, Three Hundred Eighty Seven Million, One Hundred Thirteen Thousand, One Hundred Forty) years to count one mole of dollars! – The earth is only 4.5 billion years old! Representative Units Moles • If you have 1.81x1024 atoms of gold, how many moles do you have? (Atoms Moles) • If you have 1.81x1024 molecules of glucose (C6H12O6), how many moles of glucose do you have? (Molecules Moles) Moles Representative Units • How many atoms of gold do you have if you have 3.0 moles of gold? (Moles Atoms) • How many molecules of glucose (C6H12O6) do you have if you have 3.0 moles of glucose? (Moles Molecules) • How many atoms are present in 3.0 moles of glucose (C6H12O6)? (Moles Molecules Atoms) The Mass of a Mole of Elements • All elements have mass – Larger elements are more massive than smaller elements • The mass of one mole of gold is 196.97 grams • The mass of one mole of hydrogen is 1.008 grams • The mass of one mole (6.02x1023) of an element is its molar mass – The molar mass of each element is equal to the atomic mass of an element in grams • The average atomic mass of each element can be found in the periodic table Examples Element Atomic Mass (AMU) Molar Mass (g/mol) Hydrogen (H) 1.0079 1.0079 Boron (B) 10.81 10.81 Calcium (Ca) 40.08 40.08 Bromine (Br) 79.904 79.904 Uranium (U) 238.03 238.03 Molar Mass Continued How many atoms of mercury are in the beaker? How many atoms of iron are on the plate? Problems using molar mass (Single Element) 1. What is the molar mass of gold (Au)? 2. What is the mass of 3 moles of gold (Au)? (Moles Mass) 3. How many moles of gold are in 591 grams of gold (Au)? (Mass Moles) The Mass of a Mole of a Compound 1. To calculate the molar mass of a compound, find the number grams of each element in one mole of the compound 2. Then add the masses of the elements in the compound Example – Determine the molar mass of acetic acid (CH3COOH). Molar mass of C 12.0 g/mol x 2 C atoms = 24 g/mol C Molar mass of H 1.00 g/mol x 4 H atoms Molar mass of O 16.0 g/mol x 2 atoms = 4 g/mol H = 32 g/mol O 24 g/mol C + 4 g/mol H + 32 g/mol O = 60 g/mol CH3COOH Problems using molar mass (compounds and molecules) 1. What is the molar mass of glucose (C6H12O6)? 2. What is the mass of 3 moles of glucose? (Moles Mass) 3. How many moles of glucose are in 540 grams of glucose? (Mass Moles) Problems using molar mass (compounds and molecules) 1. How many molecules of glucose are in 540 grams of glucose (C6H12O6 / 180g/mole)? (Mass Moles Molecules) 2. How many atoms are in 540 grams of glucose? (Mass Moles Molecules Atoms) 3. How many hydrogen atoms are in 540 grams of glucose (Mass Moles Molecules Atoms of Hydrogen) Problems using molar mass (compounds and molecules) • What is the mass of 1.81x1024 molecules of glucose (C6H12O6)? (Molecules Moles Mass) Percent Composition and Chemical Formulas • The relative amount of the elements in a compound are expressed as the percent composition or the percent by mass of each element in the compound • The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound % mass of element = mass of element x 100% Mass of compound Calculating percent composition from mass data • When a 13-g sample of a compound containing only magnesium and oxygen is decomposed, 5.40g of oxygen is obtained. What is the percent composition of the compound? Calculating percent composition from chemical formula % mass = Mass of element in one mole of compound X 100% Molar mass of a compound 1. The subscripts in the formula of the compound are used to calculate the mass of each element in a mole of that compound 2. The sum of these masses is the molar mass 3. Using the individual masses of the elements and the molar mass you can calculate the percent by mass of each element in one mole of the compound 4. Divide the mass of each element by the molar mass and multiply the results by 100% Calculating percent composition from chemical formula • Calculate the percent composition of ammonium nitrate NH4NO3 • What is the percent nitrogen in ammonium nitrate NH4NO3? Empirical Formulas • The empirical formula of a compound gives the lowest whole-number ratio of the atoms of the elements in a compound – Take Glucose for instance • Glucose has a molecular formula of C6H12O6 • Glucose has an empirical formula of CH2O Determining empirical formulas of a compound • 1,6-diaminohexane is used to make nylon. What is the empirical formula of this compound if it is 62.1% C, 13.8% H, 24.1% N? 1. Change the % to grams for each element 2. Divide the mass of each element by its molar mass 3. Divide the moles of each element by the element with the least amount of moles 4. The results of step 3 are the subscripts for each element in the compound Determining empirical formulas of a compound • 1,6-diaminohexane is used to make nylon. What is the empirical formula of this compound if it is 62.1% C, 13.8% H, 24.1% N? Molecular formulas • The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple wholenumber multiple of its empirical formula – To determine the molecular formula of a compound 1. Divide the experimentally determined molar mass by the molar mass of the empirical formula of the compound 2. Use the resulting multiplier to convert the empirical formula into a molecular formula Finding the molecular formula of a compound • Find the molecular formula of ethylene glycol, which is used as antifreeze. The molar mass is 62 g/mol and the empirical formulas is CH3O.