Basic Definition: How to determine emperical formula, molecular formula, molecular mass, formula mass (Part 3)

Question Write the empirical formulas for the compound containing carbon to hydrogen in the following ratios: (a) 1:4 (b) 2:6 (c) 2:2 (d) 6:6

Answer:

Molecular formula from above ratio

a. CH₄ b. C₂H₆ c. C₂H₂ d. C₆H₆

Empirical formula

a. CH₄ b. CH₃ c. CH d. CH

Self-Assessment Exercise 1.3 . 1. Aspirin is used as a mild pain killer. There are nine carbon atoms, eight hydrogen atoms and four oxygen atoms, in this compound. Write its empirical and molecular formulas.

2. Vinegar is 5% acetic acid. It contains 2 carbon atoms, four hydrogen atoms and 2 oxygen atoms. Write its empirical and molecular formulas.

3. Caffeine ( C8H₁₀N₄O₂ )is found in tea and coffee. Write the empirical formula for caffeine.

Solution

1. Molecular formula = C₂H₄O₂

Empirical formula = CH₂O

2. Molecular formula = C₂H₄O₂

Empirical formula = CH₂O

3. Molecular formula = C₈H₁₀N₄O₂

Empirical formula = C₄H₅N₂O

Molecular Mass: The sum of atomic masses of all the atoms present in the molecule is called molecular mass. Molecular mass of a molecule can be calculated by adding atomic masses of each atom present in the molecule. This term is used for molecular compounds.

For example, Molecular mass of water = 2(atomic mass of H) + atomic mass of O

= 2(1.008) + 16.00

= 2.016 + 16.00

= 18.016 amu

Example 1.3: Determining molecular mass

Question 1. Determine the molecular mass of glucose C₆H₁₂O₆ which is also known as blood sugar. 2. Determine the molecular mass of naphthalene C₁₀H₈, which is used in mothballs.

Solution:

1. Molecular mass of C₆H₁₂O₆ = 6(12.00) + 12(1.008) + 6(16.00)

= 180.096 amu

2. Molecular mass of C₁₀H₈ = 12 x 10 + 1 x 8

= 120 + 8 = 128 amu

Formula mass: The sum of the atomic masses of all the atoms in the formula unit of a substance is called formula mass. This term is used for ionic compounds.

For example, Formula mass of KCl = 1 ×atomic mass of K+ 1× atomic mass of Cl

= 39 + 35.5

= 74.5 amu

Example 1.4: Determining formula mass

1. Sodium Chloride, also called as table salt is used to flavor food, preserve meat, and in the preparation of large number of compounds. Determine its formula mass.

2. Milk of magnesia which contains Mg(OH)₂, is used to treat acidity. Determine its formula mass.

Solution:

1. Formula mass of NaCl = 1 x Atomic mass of Na + 1 x Atomic mass of Cl

= 1 x 23 + 1 x 35.5

= 58.5amu

2. Formula mass of Mg(OH)₂₂= 1 × Atomic mass of Mg + 2 × (Atomic mass of O + Atomic mass of H)

= 1 × 24 + 2(16 + 1)

= 24 + 32 + 2

= 58 amu

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