# TUTORIAL 5-1 HELP. Mass of sodium = 23.0 g/mol x 3 mol = 69.0 g of sodium. Percent mass of sodium = 69.0 g x 100% = 42.

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1 TUTORIAL 5-1 HELP ANSWER TO QUESTION 1 ON TUTORIAL 5-1: Question 1. Find the percent composition by mass of sodium phosphate, Na 3 PO 4. Step 1: Find the molar mass of Na 3 PO 4 (The mass of one mole of Na 3 PO 4 molecules) Molar Mass = 3(23.0) (16.0) = 164 g/mol. Step 2: Find the total mass of all the sodium atoms in one mole of the compound. To do this, multiply the atomic mass of sodium by the subscript of sodium in the formula.(na 3...) Mass of sodium = 23.0 g/mol x 3 mol = 69.0 g of sodium Step 3: Divide the mass of sodium by the molar mass of Na 3 PO 4 and multiply by 100% Percent mass of sodium = 69.0 g x 100% = 42.1 % Na 164 g Step 4: Find the total mass of all the phosphorus atoms in one mole of the compound. To do this, multiply the atomic mass of phosphorus by the subscript of phosphorus in the formula.(...p..) (no subscript, so call it 1 ) Mass of phosphorus = 31.0 g/mol x 1 mol = 31.0 g of phosphorus Step 5: Divide the mass of phosphorus by the molar mass of Na 3 PO 4 and multiply by 100% Percent mass of phosphorus = 31.0g x 100% = 18.9 % P 164 g Step 6: Find the total mass of all the oxygen atoms in one mole of the compound. To do this, multiply the atomic mass of oxygen by the subscript of oxygen in the formula.(...o 4 ) Mass of oxygen = 16.0 g/mol x 4 mol = 64.0 g of oxygen Step 7: Divide the mass of oxygen by the molar mass of Na 3 PO 4 and multiply by 100% Percent mass of oxygen = 64.0 g x 100% = 39.0 % O 164 g Now, we can summarize the percent composition by mass of Na 3 PO 4. So the percent composition by mass of Na 3 PO 4 is: 42.1 % Na, 18.9% P and 39.0 % O Chemistry 11-Unit 5-The Mole Concept - PAGE 1

2 ANSWER TO QUESTION 2 ON TUTORIAL 5-1 Question 2 Find the percent composition by mass of the compound ammonium phosphate, (NH 4 ) 3 PO 4. Step 1: Find the molar mass of (NH 4 ) 3 PO 4 (The mass of one mole of (NH 4 ) 3 PO 4 molecules) Molar Mass = 3(14.0) + 12(1.0) (16.0) = 149 g/mol. Step 2: Find the total mass of all the nitrogen atoms in one mole of the compound. To do this, multiply the atomic mass of nitrogen by the number of nitrogen atoms in the formula. (NH 4 ) 3... = 3 x 1 = 3 Mass of nitrogen = 14.0 g/mol x 3 mol = 42.0 g of nitrogen Step 3: Divide the mass of nitrogen by the molar mass of (NH 4 ) 3 PO 4 and multiply by 100% Percent mass of nitrogen = 42.0 g x 100% = 28.2 % N 149g Step 4: Find the total mass of all the hydrogen atoms in one mole of the compound. To do this, multiply the atomic mass of hydrogen by the number of hydrogen atoms in the formula. (NH 4 ) x 4 = 12 H atoms. Mass of hydrogen = 1.0 g/mol x 12 mol = 12.0 g of hydrogen Step 5: Divide the mass of hydrogen by the molar mass of (NH 4 ) 3 PO 4 and multiply by 100% Percent mass of hydrogen = 12.0g x 100% = 8.1% H 149 g Step 6: Find the total mass of all the phosphorus atoms in one mole of the compound. To do this, multiply the atomic mass of phosphorus by the subscript of phosphorus in the formula.(...p...) 1 atom of P. Mass of phosphorus = 31.0 g/mol x 1 mol = 31.0 g of phosphorus Step 7: Divide the mass of phosphorus by the molar mass of (NH 4 ) 3 PO 4 and multiply by 100% Percent mass of phosphorus = 31.0 g x 100% = 20.8 % P 149g Chemistry 11-Unit 5-The Mole Concept - PAGE 2

3 Step 8: Find the total mass of all the oxygen atoms in one mole of the compound. To do this, multiply the atomic mass of oxygen by the subscript of oxygen in the formula.(...o 4 ) Mass of oxygen = 16.0 g/mol x 4 mol = 64.0 g of oxygen Step 9: Divide the mass of oxygen by the molar mass of (NH 4 ) 3 PO 4 and multiply by 100% Percent mass of oxygen = 64.0 g x 100% = 43.0 % O 149 g Now, we can summarize the percent composition by mass of (NH 4 ) 3 PO 4 (NH 4 ) 3 PO 4 is 28.2 % N, 8.1% H, 20.8 % P and 43.0 % O by mass. NOTE: Add up all these percentages and we should get very close to 100%. (If you try it this time, you will come up with 100.1% which is close enough. Remember that each one of the percentages were rounded to 1 decimal place in this example.) ANSWER TO QUESTION 3 ON TUTORIAL 5-1 Question 3 Find the mass of Na in 568 g of Na 3 PO 4 First, the conversion factor is: 3(23.0) g of Na < -- (3 x the atomic mass of Na) 3(23.0) (16.0) g of Na 3 PO 4 < --(molar mass of Na 3 PO 4 ) = 69.0 g of Na 164 g of Na 3 PO 4 Final step: 568 g Na 3 PO 4 x 69.0 g of Na = 239 g of Na 164 g of Na 3 PO 4 ************************************************************ Chemistry 11-Unit 5-The Mole Concept - PAGE 3

4 ANSWER TO QUESTION 4 ON TUTORIAL 5-1 Question 4 Given the following molecular formulas, find the empirical formulas. Molecular Formula Empirical Formula P 4 O 10 P 2 O 5 C 10 H 22 C 5 H 11 C 6 H 18 O 3 C 2 H 6 O C 5 H 12 O C 5 H 12 O N 2 O 4 NO2 ************************************************************* ANSWER TO QUESTION 5 ON TUTORIAL 5-1. Question 5 The empirical formula for a compound is CH 2 O and the molar mass is Find the molecular formula. First we find the mass of the empirical formula: C H 2 O (1.0) = 30 g/mol is the mass of the empirical formula Now try to find a simple whole number that you multiply the mass of the empirical formula by to get the molar mass: mass of the empirical formula x? = molar mass 30 x? = 90 You can see that in this case the simple whole number is 3 Now, multiply all subscripts in the empirical formula by this whole number: ( 3 in this case) Empirical formula x 3 = Molecular Formula CH 2 O x 3 = C 3 H 6 O 3 Chemistry 11-Unit 5-The Mole Concept - PAGE 4

5 Empirical Molecular x 3 Formula CH 2 O C 3 H 6 O 3 x 3 Mass A good thing to do now is to figure out the molecular mass using your molecular formula (C 3 H 6 O 3 ) and make sure it is the same as the molar mass given (90.0 g/mol): C 3 H 6 O 3 3(12.0) + 6(1.0) +3(16.0) = 90 g/mol is the molar mass. So C 3 H 6 O 3 must be the correct molecular formula for this compound. ANSWERS TO THE SELF-TEST ON TUTORIAL Determine the percent composition of the compound calcium nitrate ( Ca(NO 3 ) 2 ) (That is, find the % calcium, the % nitrogen and the % oxygen in this compound.) First, find the molar mass of Ca(NO 3 ) 2. In this formula there is 1 Ca, 2 N s and 6 O s The molar mass is : (14.0) + 6(16.0) = g/mol The % Ca = 40.1 x 100% = 24.4 % The % N = 2(14.0) x 100% = 17.1 % The % O = 6(16.0) x 100% = 58.5 % So, the percent composition of calcium nitrate is 24.4% Ca, 17.1% N and 58.5% O. Chemistry 11-Unit 5-The Mole Concept - PAGE 5

6 2. Find the mass of carbon contained in grams of CO 2. There is 1 mole of C atoms in a mole of CO 2 The mass of 1 mole of C atoms is 12.0 grams (atomic mass of C) The mass of one mole of CO 2 (molar mass) is (16.0) = 44.0 grams. So we have a conversion factor: 12.0 g C 44.0 g CO 2 So we can now go: g CO 2 x 12.0 g C = grams of C = 91.7 grams of C 44.0 g CO 2 So there are grams of C in grams of CO 2. ************************************************************* 3. Find the mass of oxygen contained in grams of magnesium nitrate ( Mg(NO 3 ) 2 ) The molar mass of Mg(NO 3 ) 2 is (14.0) + 6(16.0) = g/mol. The mass of O in one mole of the compound is 6(16.0) = 96.0 g The conversion factor we can use is 96.0 g O g Mg(NO 3 ) 2 We can now find the answer: g Mg(NO 3 ) 2 x 96.0 g O = g O = 557 g O g Mg(NO 3 ) 2 Chemistry 11-Unit 5-The Mole Concept - PAGE 6

7 4. A compound used in photography is called potassium persulphate. A gram sample of the compound was analyzed and found to contain grams of potassium, grams of sulphur and the rest was oxygen. a) Find the mass of oxygen in the sample. The mass of oxygen = [Total mass of sample] - [(mass of K) + (mass of S)] = [ g] - [ ( ) + ( ) ] = [ g] - [ ] = grams of oxygen b) Determine the empirical formula for this compound. Element Mass Atomic Mass Moles Moles Smallest moles Simplest Whole # Ratio potassium g 39.1 g/mol mol = sulphur g 32.1 g/mol mol = oxygen g 16.0 g/mol mol = So the empirical formula is KSO 4 c) The molar mass of this compound is g/mol. Determine the molecular formula. The mass of the empirical formula (KSO 4 ) is (16.0) = g/mol Dividing the molar mass by the mass of the empirical formula: = We get the whole number 2. Chemistry 11-Unit 5-The Mole Concept - PAGE 7

8 Therefore, we multiply (all the subscripts in) the empirical formula by 2 : KSO 4 x 2 = K 2 S 2 O 8 So the molecular formula for the compound is K 2 S 2 O 8. NOTE: We can check by finding the molar mass of this formula and seeing if it adds up to the given molar mass (270.4 g/mol). Try it! Empirical Molecular x 2 Formula KSO 4 K 2 S 2 O 8 x 2 Mass *************************************************************** Chemistry 11-Unit 5-The Mole Concept - PAGE 8

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