Chemistry St. Augustine Preparatory November 2, 2015

Molar Mass Recall from before: m = nM m: mass (grams, g) n: number of moles (moles, mol) M: molar mass (grams per mol, g/mol) *Molar mass comes from the periodic table

Example Problem Calculate the molar mass of water The formula for water is H 2 O, meaning that we have two hydrogen atoms and one oxygen atom connected. Molar mass = H + H + O = g/mol

Example Problem Calculate the molar mass of magnesium phosphate Step 1: Determine the formula of sodium phosphate Mg 2+ and PO 4 3- so Mg 3 (PO 4 ) 2 Step 2: Figure out how many of each type of atom that you have [distributive property for the number outside the ( ) ] 3 Mg, 2 P, 8 O Step 3: Create your equation M = 3(24.31 g/mol) + 2(30.97 g/mol) + 8(16.00 g/mol) M = g/mol g/mol g/mol M = g/mol

Practice Problem What mass would a 2.50 mol sample of potassium oxide have?

Answer Potassium Oxide is K 2 O Molar mass, M = 2(39.10) = 94.2 g/mol m = nM m = (2.50 mol)(94.2 g/mol) m = g m = 236 g (3 significant figures)

Practice Problem Ibuprofen, C 13 H 18 O 2 is the active ingredient in many nonprescription pain relievers. Its molar mass is g/mol. a.If the tablets in the bottle contain a total of 33g of ibuprofen, how many moles of ibuprofen are in the bottle b.How many molecules of ibuprofen are in the bottle (remember 6.022x10 23 moles in 1 mol) c.What is the total mass in grams of carbon in the 33 g of ibuprofen?

Answers a.n = m/M = (33 g)/( g/mol) = 0.16 mol b.Number of molecules = (0.16 mol)(6.022 x ) = 9.6 x molecules c. M c / M C13H18Os = ( g/mol) / ( g/mol) = … (percent of the total mass that’s carbon) m C = ( )(33 g) = g = 25g

Percent Composition Percent composition is the way of describing the mass of one element present in a compound relative to the others mass of element in 1 mol x 100% = % element in molar molar mass of compoundcompound

Example Find the percentage composition of copper(I) sulfide Step 1: Determine the formula Cu 2 S Step 2: Find the mass of each element - 1 mol if there is just 1 (sulfur), 2 mol if there are 2.. Cu: m = nM = (2 mol)(63.55g/mol) = g S: m = nM = (1 mol)(32.07 g/mol) = g Step 3: Determine the percentages Cu: (127.1g/mol) / (159.2 g/mol) x 100% = 79.85% S: (32.07 g/mol) / (159.2 g/mol) x 100% = 20.15%

Calculate the percentage composition of aluminum oxide.

Step 1: Determine the formula Al 2 O 3 Step 2: Find the mass of each element Cu: m = nM = (2 mol)(26.982g/mol) = g S: m = nM = (3 mol)(16.00g/mol) = g Step 3: Determine the percentages Al: (53.96g/mol) / ( g/mol) x 100% = 53.92% O: (48.00 g/mol) / ( g/mol) x 100% = 47.08%

Determining Chemical Formulas Read pages 233 and 234 Complete the examples on page 234 and 235 (they are completed in the book, however try to do them on your own as well). Complete the three practice problems at the bottom of page 235

Finding an empirical formula from percentages Step 1: Use the percentages to obtain the mass of each element in a 100g sample – Example: 49.32% oxygen, would be 49.32g of oxygen in a 100g sample % sodium, would be 32.38g. Step 2: Divide each of your answers by the molar mass of the respective element – Example: 49.32g / g/mol = mol – Example: 32.38g / g/mol = mol Step 3: Divide each of the elements by the least number of moles that you found in step 2. Step 4: Round each of your answers in step 3 either up or down using the rounding rules that you already know

Finding an empirical formula from mass Step 1: Find the mass of both elements in grams Step 2: Divide each element by the respective molar mass to calculate the number of moles of each element Step 3: Divide each of your answers in step 2 by the least number of moles from step 2. Step 4: If your numbers are close to whole numbers, you are finished. If they are not (ex. 1 and 2.497), you need to multiply each by 2 or whatever number is necessary to get two whole numbers. Step 5: record your answer correctly: (P 2 O 5 ), (Al 2 O 3 )