1 / 20

Calculation of Empirical Formulas

Calculation of Empirical Formulas. Empirical Formula = consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole number ratio. Molar mass = the mass in grams of 1 mole of a substance

arich
Download Presentation

Calculation of Empirical Formulas

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Calculation of Empirical Formulas • Empirical Formula = consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole number ratio • Molar mass= the mass in grams of 1 mole of a substance • this is the mass indicated for each element on the periodic table

  2. Calculation of Empirical Formulas • treat percentages as a mass • Divide the given mass by the molar mass • Convert to whole numbers this can be done by dividing by the smallest number of moles If you still don’t have a whole number you can… Round to nearest whole number Multiple by 3 or 2 to get a whole number

  3. Simplest formula calculations Q- a compound is found to contain the following % by mass: 69.58% Ba, 6.090% C, 24.32% O. What is the simplest (i.e. empirical) formula? Step 1: imagine that you have 100 g of the substance. Thus, % will become mass in grams. E.g. 69.58 % Ba becomes 69.58 g Ba. (Some questionswillgivegramsrightoff, instead of %) Step 2:calculatethe#ofmoles(mol=g¸g/mol) Step 3: express moles as the simplest ratio by dividing through by the lowest number. Step 4: write the simplest formula from mol ratios.

  4. mol mol (reduced) Simplest formula: sample problem Q- 69.58% Ba, 6.090% C, 24.32% O. Whatistheempirical (a.k.a. simplest) formula? 1: 69.58 g Ba, 6.090 g C, 24.32 g O 2: Ba: 69.58 g ¸ 137.33 g/mol = 0.50666 mol Ba C: 6.090 g ¸ 12.01 g/mol = 0.50708 mol C O: 24.32 g ¸ 16.00 g/mol = 1.520 mol O 3: Ba C O 0.50666 0.50708 1.520 0.50666/ 0.50666 = 1 0.50708/ 0.50666 = 1.001 1.520/ 0.50666 = 3.000 4: the simplest formula is BaCO3

  5. Mole ratios and simplest formula Given the following mole ratios for the hypothetical compound AxBy, what would x and y be if the mol ratio of A and B were: A = 1 mol, B = 2.98 mol A = 1.337 mol, B = 1 mol A = 2.34 mol, B = 1 mol A = 1 mol, B = 1.48 mol AB3 A4B3 A7B3 A2B3 • A compound consists of 29.1%Na, 40.5% S, and30.4%O. Determinethesimplestformula. • A compound is composed of 7.20 g carbon, 1.20 g hydrogen, and 9.60 g oxygen. Find the empirical formula for this compound • 3. - 6. Try questions 3 - 6 on page 189.

  6. mol mol (reduced) Question 1 1: Assume 100 g: 29.1 g Na, 40.5 g S, 30.4 g O 2: Na: 29.1 g ¸ 22.99 g/mol = 1.266 mol Na S: 40.5 g ¸ 32.06 g/mol = 1.263 mol S O: 30.4 g ¸ 16.00 g/mol = 1.90 mol O 3: Na S O 1.266 1.263 1.90 1.266/ 1.263 = 1.00 1.263/ 1.263 = 1 1.90/ 1.263 = 1.50 4: the simplest formula is Na2S2O3 For instructor: prepare molecular models

  7. mol mol (reduced) Question 2 1: 7.20 g C, 1.20 g H, 9.60 g O 2: C: 7.20 g ¸ 12.01 g/mol = 0.5995 mol C H: 1.20 g ¸ 1.01 g/mol = 1.188 mol H O: 9.6 g ¸ 16.00 g/mol = 0.60 mol O 3: C H O 0.5995 1.188 0.60 0.5995/ 0.5995 = 1 1.188/ 0.5995 = 1.98 0.60/ 0.5995 = 1.0 4: the simplest formula is CH2O

  8. mol mol (reduced) Question 3 1: Assume 100 g: 28.9 g K, 23.7 g S, 47.7 g O 2: C: 7.20 g ¸ 12.01 g/mol = 0.5995 mol C H: 1.20 g ¸ 1.01 g/mol = 1.188 mol H O: 9.6 g ¸ 16.00 g/mol = 0.60 mol O 3: C H O 0.5995 1.188 0.60 0.5995/ 0.5995 = 1 1.188/ 0.5995 = 1.98 0.60/ 0.5995 = 1.0 4: the simplest formula is CH2O

  9. Molecular formula calculations • There is one additional step to solving for a molecular formula. First you need the molar massofthecompound.E.g. inQ2,themolecular formula can be determined if we know that the molar mass of the compound is 150 g/mol. • First, determine molar mass of the simplest formula. For CH2O it is 30 g/mol (12+2+16). • Divide the molar mass of the compound by this to get a factor: 150 g/mol ¸ 30 g/mol = 5 • Multiply each subscript in the formula by this factor: C5H10O5 is the molecular formula. (models) Q- For OF, give the molecular formula if the compound is 70 g/mol O2F270 ¸ 35 = 2

  10. Combustion analysis gives the following: 26.7% C, 2.2% hydrogen, 71.1% oxygen. If the molecular mass of the compound is 90 g/mol, determine its molecular formula. • What information must be known to determine a) the empirical formula of a substance? b) the molecular formula of a substance? • A compound’s empirical formula is CH, and it weighs 104g/mol. Give the molecular formula. • A substance is decomposed and found to consist of 53.2% C, 11.2% H, and 35.6% O by mass. Calculate the molecular formula of the unknown if its molar mass is 90 g/mol.

  11. Question 7 1: Assume 100 g total. Thus: 26.7 g C, 2.2 g H, and 71.1 g O 2: C: 26.7 g ¸ 12.01 g/mol = 2.223 mol C H: 2.2 g ¸ 1.01 g/mol = 2.18 mol H O: 71.1 g ¸ 16.00 g/mol = 4.444 mol O 3: 2.223 2.18 4.444 2.223/2.18 = 1.02 2.18/ 2.18 = 1 4.444/2.18 = 2.04 4: the simplest formula is CHO2 5: factor = 90/45=2. Molecular formula: C2H2O4

  12. Question 8, 9 • For the empirical formula we need to know the moles of each element in the compound (which can be derived from grams or %). For the molecular formula we need the above information & the molar mass of the compound • Molar mass of CH = 13 g/mol Factor = 104 g/mol ¸ 13 g/mol = 8 Molecular formula is C8H8

  13. Question 10 1: Assume 100 g total. Thus: 53.2 g C, 11.2 g H, and 35.6 g O 2: C: 53.2 g ¸ 12.01 g/mol = 4.430 mol C H: 11.2 g ¸ 1.01 g/mol = 11.09 mol H O: 35.6 g ¸ 16.00 g/mol = 2.225 mol O 3: 4.430 11.09 2.225 4.43/2.225 = 1.99 11.09/2.225 = 4.98 2.225/2.225 = 1 4: the simplest formula is C2H5O 5: factor = 90/45=2. Molecular formula: C4H10O2

  14. Assignment • Calculate the percentage composition of each substance: a) SiH4, b) FeSO4 • Calculate the simplest formulas for the compounds whose compositions are listed: a) carbon, 15.8%; sulfur, 84.2% b) silver,70.1%; nitrogen,9.1%; oxygen,20.8% c) K, 26.6%; Cr, 35.4%, O, 38.0% • The simplest formula for glucose is CH2O and its molar mass is 180 g/mol. What is its molecular formula?

  15. Determine the molecular formula for each compound below from the information listed. substancesimplest formulamolar mass(g/mol) a) octane C4H9 114 b) ethanol C2H6O 46 c) naphthalene C5H4 128 d) melamine CH2N2 126 • The percentage composition and approximate molar masses of some compounds are listed below. Calculate the molecular formula of each percentage compositionmolar mass(g/mol) 64.9% C, 13.5% H, 21.6% O 74 39.9% C, 6.7% H, 53.4 % O 60 40.3% B, 52.2% N, 7.5% H 80

  16. C S Mol 1.315 2.626 Mol reduced 1.315/1.315 = 1 2.626/1.315 = 2.00 2 a) Assume 100 g. Thus: 15.8 g C, 84.2 g S. C: 15.8 g ¸ 12.01 g/mol = 1.315 mol C S: 84.2 g ¸ 32.06 g/mol = 2.626 mol S 1 a) Si= 87.43% (28.09/32.13 x 100), H= 12.57% b) Fe= 36.77% (55.85/151.91 x 100), S= 21.10% (32.06/151.91 x 100), O= 42.13% the simplest formula is CS2

  17. 2 b) Ag: 70.1 g ¸ 107.87 g/mol = 0.6499 mol Ag N: 9.1 g ¸ 14.01 g/mol = 0.6495 mol N O: 20.8 g ¸ 16.00 g/mol = 1.30 mol O AgNO2 Ag N O Mol 0.6499 0.6495 1.30 Mol reduced .6499/.6495 = 1.0 .6495/.6495 = 1 1.30/.6495 = 2.00 K2Cr2O7 K Cr O Mol 0.6803 0.6808 2.375 Mol reduced .6803/.6803 = 1 .6808/.6803= 1.00 2.375/.6495 = 3.49 2 c) K: 26.6 g ¸ 39.10 g/mol = 0.6803 mol K Cr: 35.4 g ¸ 52.00 g/mol = 0.6808 mol Cr O: 38.0 g ¸ 16.00 g/mol = 2.375 mol O

  18. C4H10O C H O Mol 5.404 13.37 1.35 Mol reduced 5.404/1.35 = 4.00 13.37/1.35 = 9.90 1.35/1.35 = 1 4 a) C8H18 (C4H9 = 57 g/mol, 114/57 = 2) b) C2H6O (C2H6O = 46 g/mol, 46/46 = 1) c) C10H8 (C5H4 = 64 g/mol, 128/64 = 2) d) C3H6N6 (CH2N2 = 54 g/mol, 126/42 = 3) 3 C6H12O6(CH2O = 30 g/mol, 180/30 = 6) 5 a) C: 64.9 g ¸ 12.01 g/mol = 5.404 mol C H: 13.5 g ¸ 1.01 g/mol = 13.37 mol H O: 21.6 g ¸ 16.00 g/mol = 1.35 mol O C4H10O (C4H10O = 74 g/mol, 74/74 = 1)

  19. 5 b) C: 39.9 g ¸ 12.01 g/mol = 3.322 mol C H: 6.7 g ¸ 1.01 g/mol = 6.63 mol H O: 53.4 g ¸ 16.00 g/mol = 3.338 mol O CH2O C H O Mol 3.322 6.63 3.338 Mol reduced 3.322/3.322 = 1 6.63/3.322 = 2.0 3.338/3.322 = 1.00 B N H Mol 3.728 3.726 7.43 Mol reduced 3.728/3.726 = 1.00 3.726/3.726 = 1 7.43/3.726 = 2.0 C2H4O2 (CH2O = 30 g/mol, 60/30 = 2) 5 c) B3N3H6 (BNH2 = 26.84 g/mol, 80/26.84= 2.98) For more lessons, visit www.chalkbored.com

More Related