Question

1 .calculate the molecular masses (in amu) of the following compounds
(a) sulphur dioxide (SO2) and (b) caffeine ( C8H10 O2)
( Atomic mass of C=12 ,S=32, N=14,H=1, O=16)
2. Calculate the no.of moles in 5.68g of iron. (Atomic mass of Fe=55.8)

Answer 1

1 ans.

a. 64.066 g/mol

b. 194.19 g/mol

Answer 2

1 (a) 38.5 × 10²⁴ AMU

1 (b)83.13 × 10²⁴ AMU

2. 0.1 Mole

GIVEN

1. sulphur dioxide (SO₂) and (b) caffeine ( C₈H₁₀ O₂)

2. Mass of Iron = 5.68 grams

TO FIND

1. Molar masses of given compounds in AMU

2. Number of moles in 5.86 grams of Iron.

SOLUTION

We can simply solve the above problem as follows;

1. Molecular mass in AMU.

We know that AMU or Atomic Mass unit is defined as 1/12th the mass of one Carbon atom.

We know that,

1 AMU = 1.66 × 10⁻²⁴ grams.

We will first calculate molecular mass of given compounds in grams and then convert them into AMU.

(a) SO₂ = 32 + 32 = 64 grams.

Molecular mass in AMU =

$= \frac{1}{1.66 \times {10}^{ - 24} } \times 64$

= 38.5 × 10²⁴ AMU.

(b) Caffeine (C₈H₁₀O₂) = 12×8 + 1×10 + 16×2 = 138 grams

Molecular mass in AMU=

$= \frac{1}{1.66 \times {10}^{ - 24} } \times 138$

= 83.13 × 10²⁴ AMU.

(2) Number of moles in 5.68g of iron

We know that,

Molar mass of a substance = 1 Mole = 6.022 × 10²³

So,

55.8 grams of Iron = 1 mole

1 gram of Iron = 1/55.8 Moles

5.68 grams of Iron =

$\frac{1}{55.8} \times 5.68$

= 0.1 moles.

Hence, The answer is 0.1 moles.

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