HW From textbook “CHEMISTRY a Contemporary Approach”

·         Read “Overview” and “Chemical Formulas” p145&146

·         Read “Molecular and Empirical Formulas” p150&151

·         Do exercises 7.1 & 7.2 p225 and exercise 7.10 p231

·         Read “Molecular Formulas” p234-236

·         Do exercise 7.13 p236

·         Do questions 2, 19,  p251-253

Types of chemical formulas include molecular, empirical, and structural.

The molecular formula of a compound is the actual ratio of atoms in a molecule of that compound.

Example

The molecular formula of hydrogen peroxide is H₂O₂. There are 2 hydrogen atoms for every 2 oxygen atoms.

The empirical formula of a compound is the simplest whole-number ratio of atoms of the elements in a compound. It may be different or the same as the molecular formula.

Molecular formula = (Empirical formula) _n

Example

The empirical formula of hydrogen peroxide, H₂O₂, is HO

H₂O₂ = (HO)₂

Example

The empirical formula of water, H₂O is H₂O

H₂O = (H₂O)₁

Problems

Molecular Formula	Empirical Formula	n
C6H6	CH	6
C3H6
P8O20
C2H4
CH3
P2O5
C10H22

The formula Molecular formula = (Empirical formula) _n can be changed to Molecular formula mass = (Empirical formula mass) _n and if the Molecular formula mass and the Empirical formula mass are known then n can be calculated. Then the molecular formula can be derived from Molecular formula = (Empirical formula) _n

Example

What is the molecular formula of a compound with empirical formula CH₃ if the molecular mass is 30g/mol?

Solution

Step 1: Determine the mass of the empirical formula.

            The empirical formula mass of CH₃ = 15g/mol

Step 2: Divide the molecular formula mass by the empirical formula mass to find n.

            Molecular formula mass = (empirical formula mass)_n 

            n = molecular formula mass / empirical formula mass = 30g/mol / 15g/mol = 2

Step 3: Substitute n for its value to find the molecular formula.

            Since n = 2

            molecular formula = (empirical formula)_n 

            C₂H₆  = (CH₃)₂

Problems

1. The empirical formula of a compound is NO₂. Its molecular mass is 92 g/mol. What is its molecular formula?

2. The empirical formula of a compound is CH₂. Its molecular mass is 70 g/mol. What is its molecular formula?

The structural formula of a compound is the arrangement of atoms in a molecule of that compound.

Example

The structural formula of methane CH₄ is

Mole calculations are done using the formula in  table T  (Mole Calculations)

number of moles = given mass (g) / gram-formula mass

Example

What is the number of moles  in a sample of 54g of H₂O?

given mass (g) = 54 g

gram-formula mass = 2 (1g) + 16g = 18g

number of moles = 54 g / 18g = 3

Example

What is the mass of 4 moles of H₂O₂?

number of moles = 4 mol

gram-formula mass of H₂O₂ = 34g/mol

        number of moles = given mass (g)/ gram-formula mass

given mass (g) = (number of moles) (gram-formula mass)

given mass (g) = (4 mol) (34g/mol) =  156g

Problems

What is the number of mole in a sample of 56g of H₂O₂?

What is the mass of 2 moles of H₂SO₄?

TEST YOUR UNDERSTANDING

1/07

35 The molar mass of Ba(OH)₂ is    (1) 154.3 g              (2) 155.3 g              (3) 171.3 g              (4) 308.6 g

6/07

38 Which pair consists of a molecular formula and its corresponding empirical formula?

(1) C₂H₂ and CH₃CH₃                          (2) C₆H₆ and C₂H₂               (3) P₄O₁₀ and P₂O₅               (4) SO₂ and SO₃

8/07

35 A compound has a molar mass of 90. grams per mole and the empirical formula CH₂O. What is the molecular formula of this compound?

(1) CH₂O                (2) C₂H₄O₂             (3) C₃H₆O₃             (4) C₄H₈O₄

1/06

7 Given the balanced equation representing a reaction: F₂(g) + H₂(g) -> 2HF(g) What is the mole ratio of H₂(g) to HF(g) in this reaction?    (1) 1:1 (2) 1:2 (3) 2:1(4) 2:3

1/05

34 Which pair of compounds has the same empirical formula?     (1) C₂H₂ and C₆H₆ (2) C₂H₆ and C₃H₈ (3) CH₃OH and C₂H₅OH (4) CH₃CHO and CH₃COOH

6/05

37 The molecular formula of glucose is C6H12O6. What is the empirical formula of glucose?	(1) CHO          (2) CH2O            (3) C6H12O6           (4) C12H24O12 

1/04

36 What is the empirical formula of a compound with the molecular formula N₂O₄ ? (1) NO (2) NO₂ (3) N₂O (4) N₂O₃

6/04

70 The empirical formula for naphthalene is C₅H₄ and the molecular mass of naphthalene is 128 grams/mole. What is the molecular formula for naphthalene?

38 What is the empirical formula for the compound C₆H₁₂O₆ ? (1) CH₂O (2) C₂H₄O₂ (3) C₃H₆O₃ (4) C₆H₁₂O₆

8/04

37 Which pair of formulas correctly represents a molecular formula and its corresponding empirical formula?     (1) C₂H₂ and CH (2) C₃H₄ and CH₂ (3) C₄H₆ and CH (4) C₅H₈ and C₂H₂

1/03

The empirical formula of a compound is CH₂ . Which molecular formula is correctly paired with a structural formula for this compound?

6/03

8. Which is an empirical formula? (1) P₂O₅  (2) P₄O₆ (3) C₂H₄ (4) C₃H₆

8/03

42 What is the molecular formula of a compound that has a molecular mass of 54 and the empirical formula C₂H₃ ?     (1) C₂H₃ (3) C₆H₉ (2) C₄H₆ (4) C₈H₁₂

6/02

1. The gram formula mass of NH₄Cl is (1) 22.4 g/mole     (2) 28.0 g/mole    (3) 53.5 g/mole     (4) 95.5 g/mole

2. What is the total number of oxygen atoms in the formula MgSO₄.7 H₂O? (The . represents seven units of H₂O attached to one unit of MgSO₄)  (1) 11    (2) 7   (3) 5    (4) 4

3. A compound has a gram formula mass of 56 grams per mole. What is the molecular formula for this compound? (1) CH₂     (2) C₂H₄      (3) C₃H₆     (4) C₄H₈

home