Question

Q2. Calculate :-

(a) Percentage Composition of MgSO4.2H2O

(b) Emprical formula & Molecular formula of hydrocarbon , whose molecular mass is 30 and contain 80% carbon​.

Answer 1

Explanation:

a ) Elemental composition of MgSO4*2H2O

Element Symbol Mass percent

Magnesium Mg 15.5405

Sulfur S 20.5022

Oxygen O 61.3795

Hydrogen H 2.5779

b)Let the mass of unknown compound be 100 g.

Given that the compound contains 80% of carbon and 20% of hydrogen.

Therefore, weight of carbon in the compound =80g

weight of hydrogen in the compound =20g

Molecular weight of carbon =12g

Molecular weight of hydrogen =1g

∴ moles of carbon in the compound =

12

80

=6.67

Moles of hydrogen in the compound =

1

20

=20

Ratio of no. of moles of carbon to no. of moles of hydrogen-

No. of moles of carbon : no. of moles of hydrogen =6.67:20≈1:3

∴ 3 moles of hydrogen are present in the compound for 1 mole of carbon.

Hence, the empirical formula of the compound is CH

3

.

empirical mass=15 g/mol

Molecular formula =(CH

3

)

n

whereas,

n=

empirical mass

molecular mass

=

15

30

=2

Thus, the molecular formula of the compoud is C

2

H

6

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