UNIT V Empirical & Molecular Formula

V.4 PERCENTAGE COMPOSITION Percentage composition is the percentage (by mass) of the species in a chemical formula. In the formula AB what is the percentage by mass of A? what is the percentage by mass of B?

V.4 PERCENTAGE COMPOSITION Example #1: What is the percent composition of...? a) FeCl 2 (what % of the mass is iron, what % of the mass is chloride?) b) MgO c) (NH 4 ) 3 PO 4

V.4 PERCENTAGE COMPOSITION c) (NH 4 ) 3 PO 4 Example #2: What is the percent composition of the bold species? NiSO 4 ·7 H 2 0 Questions: p. 91 # (a,ce,...)

V.5 EMPIRICAL & MOLECULAR FORMULA Empirical Formula is the SMALLEST ‘whole number ratio’ of atoms which represents the molecular make-up of a compound. Ex: CH 2, C 2 H 4, C 3 H 6, C 4 H 8, C 5 H all contain twice as many H’s as C’s Therefore, the empirical formula (or simplest ratio) is CH 2 In this case, all the formulae are whole-number multiples of CH 2 C 2 H 4 = 2 x CH 2 C 3 H 6 = 3 x CH 2 C 4 H 8 = 4 x CH 2

V.5 EMPIRICAL & MOLECULAR FORMULA Finding the Empirical Formula – you can work backwards from a percentage composition: Ex #1: What is empirical formula of a compound made of 70.0% Fe and 30.0% O? Assume that you have 100g of the compound.

V.5 EMPIRICAL & MOLECULAR FORMULA Ex #2: What is empirical formula of a compound containing 81.8% C and 18.2% H?

V.5 EMPIRICAL & MOLECULAR FORMULA ** Page 93 Common Fractions → Decimal Conversions Helpful to be able to recognize these! Saves you time! 2.67, 1.33, 5.67, 3.33, etc involve THIRDS ( x 3 to clear fraction) 1.75, 2.23, 3.75, etc involve QUARTERS ( x 4 to clear fraction) IMPORTANT: Don’t round intermediate values, keep 3 or 4 decimals... Questions: p. 93 # 46 (a,ce,...)

V.5 EMPIRICAL & MOLECULAR FORMULA The Molecular Formula Can be found using empirical mass. Empirical Mass is calculated from the molar mass of the empirical formula.

V.5 EMPIRICAL & MOLECULAR FORMULA Ex: Consider the empirical formula CH 2 The empirical mass would be 14.0 g C= 12.0 g; H = 1.0 g ; CH 2 = (1.0) = 14.0 g But remember, CH 2, C 2 H 4, C 3 H 6...have identical empirical formula (CH 2 ) *the molar mass of all the compounds are whole- number multiples of the empirical mass of CH 2

V.5 EMPIRICAL & MOLECULAR FORMULA Finding the molecular formula: We can find the molecular formula from the empirical formula, we just need to know what the whole-number multiple is! Let N = whole-number of the empirical mass multiple = N = _________________ Molecular Formula = N x (empirical mass)

V.5 EMPIRICAL & MOLECULAR FORMULA Finding Molar Mass *Page 94 summarizes ALL the methods for finding molar mass (gives examples too!)  FROM DENSITY STP)  FROM MASS & VOLUME STP)  IF GIVEN A # OF MOLES  IF GIVEN A MULTIPLE OF A KNOWN MOLAR MASS

V.5 EMPIRICAL & MOLECULAR FORMULA Example #1: The empirical formula of a compound is SiH 3. If mol of this compound has a mass of 1.71 g, what is the compound’s molecular formula? Questions: p. 95 # 47-55

V.6 MOLAR CONCENTRATION Concentration : how much of a substance is in a given amount of solution. symbol = [ ] ex: [NaCl] means “concentration of NaCl” Concentrated = high concentration Dilute = low concentration

V.6 MOLAR CONCENTRATION Molar concentration : how many moles of a substance in 1L of solution (mol /L) symbol = M ex: 3 M means “3 mol/L” can use the formula: Molarity (c) = moles ( n ) / volume (V) c = n V

V.6 MOLAR CONCENTRATION Example #1: What is the molarity of a NaBr solution when 3.5 L contains 4 moles of NaBr?

V.6 MOLAR CONCENTRATION Example #2: How many moles of CaCl 2 are there in 1.5 L of 0.5 M solution?

V.6 MOLAR CONCENTRATION Example #3: What is the mass of NaCl (in grams) in 5 L of a 0.75 M solution? Questions: p. 98 # (odd),